The Real-Time Specification
for Java™

The Real-Time Specification
for Java™

The Real-Time for Java Expert Group
http://www.rtj.org

Greg Bollella
Ben Brosgol
Steve Furr
David HardinPeter Dibble
James GoslingMark Turnbull

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CHANGES WILL BE INCORPORATED IN NEW EDITIONS OF THE PUBLICATION. THE REAL-TIMEFOR JAVA EXPERT GROUP MAY MAKE IMPROVEMENTS AND/OR CHANGES IN THE
PRODUCT(S) AND/OR THE PROGRAM(S) DESCRIBED IN THIS PUBLICATION AT ANY TIME. IN
PARTICULAR, THIS EDITION OF THE SPECIFICATION HAS NOT YET BEEN FINALIZED: THISSPECIFICATION IS BEING PRODUCED FOLLOWING THE JAVA COMMUNITY PROCESS AND
HENCE WILL NOT BE FINALIZED UNTIL THE REFERENCE IMPLEMENTATION IS COMPLETE.
THE EXPERIENCE OF BUILDING THAT REFERENCE IMPLEMENTATION MAY LEAD TO
CHANGES IN THE SPECIFICATION.
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To Paula and my daughter Alex, who forgave my extended absences
during critical phases of house construction — GB
To Deb, Abz, and Dan, for making it all worthwhile — BB
To Ken Kaplan and my family, who allowed me thetime and resources for this work — PD
To Linda, who has always been a true friend, cared for my home in my absences,welcomed me at the airport and generally shown patience and consideration — SF
To Judy, Kelsey, and Kate, who gave me theLove and Time to work on this book — JG
To Debbie, Sam, and Anna, who endured my frequent absences, and generalabsentmindedness, during the writing of this book — DH
To my daughters Christine, Heather, and Victoria, and especially to my wife Terry,
who all put up with my strange working hours — MT
To the Stanford Inn-by-the-Sea, the Chicago Hilton, and the Chateau Laurier forproviding space for a bunch of geeks to hang out; and to the Beaver Tail vendors by the
Rideau Canal for providing a yummy distraction.

Contents
Caveat ............................................................................................. xi
Authors ......................................................................................... xiii
Preface ............................................................................................ xv
Foreword ...................................................................................... xxi
1

Introduction ...................................................................................... 1

2

Design ................................................................................................ 5

3

Threads ............................................................................................ 21
RealtimeThread .................................................................................................... 22
NoHeapRealtimeThread ....................................................................................... 26

4

Scheduling ....................................................................................... 31
Semantics and Requirements ............................................................................... 32
Schedulable .......................................................................................................... 35
Scheduler .............................................................................................................. 36
PriorityScheduler .................................................................................................. 38
SchedulingParameters .......................................................................................... 40
PriorityParameters ................................................................................................ 41
ImportanceParameters .......................................................................................... 42
ReleaseParameters ................................................................................................ 43
PeriodicParameters ............................................................................................... 45
AperiodicParameters ............................................................................................ 47
SporadicParameters .............................................................................................. 49
ProcessingGroupParameters ................................................................................. 50

5 Memory Management .................................................................... 57
Semantics and Requirements ............................................................................... 57
MemoryArea ........................................................................................................ 60
HeapMemory ........................................................................................................ 61
ImmortalMemory ................................................................................................. 62
ScopedMemory .................................................................................................... 62
VTMemory ........................................................................................................... 65
LTMemory ........................................................................................................... 65
PhysicalMemoryFactory ...................................................................................... 68
ImmortalPhysicalMemory .................................................................................... 69
vii

viii
ScopedPhysicalMemory ....................................................................................... 71
RawMemoryAccess .............................................................................................. 72
RawMemoryFloatAccess ..................................................................................... 76
MemoryParameters ............................................................................................... 79
GarbageCollector .................................................................................................. 81
IncrementalCollectorExample .............................................................................. 82
MarkAndSweepCollectorExample ....................................................................... 83

6

Synchronization .............................................................................. 85
MonitorControl ..................................................................................................... 86
PriorityCeilingEmulation ..................................................................................... 87
PriorityInheritance ................................................................................................ 88
WaitFreeDequeue ................................................................................................. 88
WaitFreeReadQueue ............................................................................................. 90
WaitFreeWriteQueue ............................................................................................ 92

7 Time ................................................................................................. 95
HighResolutionTime ............................................................................................ 96
AbsoluteTime ....................................................................................................... 99
RelativeTime ...................................................................................................... 102
RationalTime ...................................................................................................... 105

8 Timers ............................................................................................ 109
Clock ................................................................................................................... 110
Timer .................................................................................................................. 112
OneShotTimer .................................................................................................... 113
PeriodicTimer ..................................................................................................... 114

9Asynchrony ................................................................................... 119
AsyncEvent ......................................................................................................... 125
AsyncEventHandler ............................................................................................ 127
BoundAsyncEventHandler ................................................................................. 132
Interruptible ........................................................................................................ 133
AsynchronouslyInterruptedException ................................................................ 134
Timed .................................................................................................................. 137

10 System and Options ...................................................................... 145
POSIXSignalHandler ......................................................................................... 146
RealtimeSecurity ................................................................................................ 149
RealtimeSystem .................................................................................................. 150

11 Exceptions ..................................................................................... 153
IllegalAssignmentError ...................................................................................... 154
MemoryAccessError ........................................................................................... 154
MemoryScopeException .................................................................................... 155
OffsetOutOfBoundsException ........................................................................... 155

CONTENTS
ResourceLimitError ............................................................................................ 156
SizeOutOfBoundsException .............................................................................. 156
ThrowBoundaryError ......................................................................................... 157
UnsupportedPhysicalMemoryException ............................................................ 157

Almanac ........................................................................................ 161
Bibliography ................................................................................. 183
Colophon ...................................................................................... 189
Index ............................................................................................. 191

ix

CaveatThis edition of The Real-Time Specification for Java™ (RTSJ) is preliminary. It isbeing developed under the Java Community Process (http://java.sun.com/aboutJava/
communityprocess). It will not be considered final until after the completion of thereference implementation. The experience gained from that implementation may
necessitate changes to the specification. Status information on the specification may
be obtained from the web site maintained by the expert group, http://www.rtj.org,
along with updates and samples.
Throughout the RTSJ, when we use the word code, we mean code written in theJava programming language. When we mention the Java language in the RTSJ, that
also refers to the Java programming language. The use of the term heap in the RTSJ
will refer to the heap used by the runtime of the Java language. If we refer to otherheaps, such as the heap used by the C language runtime or the operating system’sheap, we will explicitly state which heap.
Throughout the RTSJ we will use the term Thread to refer to the class Thread in
The Java Language Specification and thread to refer to a sequence of instructions or
to an instance of the class Thread. The context of uses of thread should be sufficient
to distinguish between the two meanings. We will be explicit where we thinknecessary.
In order to get this published and in your hands, we made some compromises incopyediting and proofreading for this first edition. It is our intention to provide thisbook for you to begin designing real-time applications with this specification. Please
send any and all comments to: comments@rtj.org.

xiAuthors
Greg Bollella, a Senior Architect at the IBM Corporation, is lead engineer of the RealTime for Java Expert Group. Previously, Greg designed and implemented
communications protocols for IBM. He holds a Ph.D. in computer science from the
University of North Carolina at Chapel Hill. His dissertation research is in real-timescheduling theory and real-time systems implementation.
Ben Brosgol is a senior technical staff member of Ada Core Technologies, Inc.
He has had a long involvement with programming language design andimplementation, focusing on Ada and real-time support, and has been providing Javarelated services since 1997. Ben holds a Ph.D. in applied mathematics from Harvard
University and a B.A. from Amherst College.
Peter Dibble, Senior Scientist at Microware Systems Corporation, has designed,
coded, and analyzed system software for real-time systems for more than ten years
with particular emphasis on real-time performance issues. As part of Microware’s
Java team, Peter has been involved with the Java Virtual Machine since early 1997.
Steve Furr currently works for QNX Software Systems, where he is responsible
for Java technologies for the QNX Neutrino Operating System. He graduated from
Simon Fraser University with a B.Sc. in computer science.
James Gosling, a Fellow at Sun Microsystems, is the originator of the Java
programming language. His career in programming started by developing real-time
software for scientific instrumentation. He has a Ph.D. and M.Sc. in computer sciencefrom Carnegie-Mellon University and a B.Sc. in computer science from the
University of Calgary.
David Hardin, Chief Technical Officer and co-founder of aJile Systems, has
worked in safety-critical computer systems architecture, formal methods, and custom
microprocessor design at Rockwell Collins, and was named a Rockwell Engineer of
the Year for 1997. He holds a Ph.D. in electrical and computer engineering from
Kansas State University.
Mark Turnbull has been an employee of Nortel Networks since 1983. Most of his
experience has been in the area of proprietary language design, compiler design, andreal-time systems.
xiiiPreface
DreamsIn 1997 the idea of writing real-time applications in the Java programming language
seemed unrealistic. Real-time programmers talk about wanting consistent timing
behavior more than absolute speed, but that doesn’t mean they don’t require excellent
overall performance. The Java runtime is sometimes interpreted, and almost always
uses a garbage collector. The early versions were not known for their blistering
performance.
Nevertheless, Java platforms were already being incorporated into real-timesystems. It is fairly easy to build a hybrid system that uses C for modules that havereal-time requirements and other components written to the Java platform. It is also
possible to implement the Java interpreter in hardware (for performance), andintegrate the system without a garbage collector (for consistent performance). aJile
Systems produces a Java processor with acceptable real-time characteristics.
Until the summer of 1998, efforts toward support for real-time programming on
the Java platform were fragmented. Kelvin Nilsen from NewMonics and Lisa
Carnahan from the National Institute for Standards and Technology (NIST) led one
effort, Greg Bollella from IBM led a group of companies that had a stake in Java
technology and real-time, and Sun had an internal real-time project based on the Javaplatform.
In the summer of 1998 the three groups merged. The real-time requirements
working group included Kelvin Nilsen from NewMonics, Bill Foote and Kevin
Russell from Sun, and the group of companies led by Greg Bollella. It also included adiverse selection of technical people from across the real-time industry and a few
representatives with a more marketing or management orientation.
The requirements group convened periodically until early 1999. Its final output
was a document, Requirements for Real-time Extensions for the Java Platform,
detailing the requirements the group had developed, and giving some rationale forthose requirements. It can be found on the web at http://www.nist.gov/rt-java.

xv

xvi

Realization
One of the critical events during this processess occurred in late 1998, when Sun
created the Java Community Process. Anyone who feels that the Java platform needs
a new facility can formally request the enhancement. If the request, called a Java
Specification Request (JSR), is accepted, a call for experts is posted. The
specification lead is chosen and then he or she forms the expert group. The result of
the effort is a specification, reference implementation, and test suite.
In late 1998, IBM asked Sun to accept a JSR, The Real-Time Specification for
Java, based partly on the work of the Requirements Working Group. Sun accepted therequest as JSR-000001. Greg Bollella was selected as the specification lead. He
formed the expert group in two tiers. The primary group:

Greg Bollella

IBM

Paul Bowman

Cyberonics

Ben Brosgol

Aonix/Ada Core Technologies

Peter Dibble

Microware Systems Corporation

Steve Furr

QNX System Software Lab

James Gosling

Sun Microsystems

David Hardin

Rockwell-Collins/aJile

Mark Turnbull

Nortel Networks

would actually write the specification, and the consultant group:

Rudy Belliardi
Alden Dima

Schneider Automation
NIST

E. Douglas Jensen

MITRE

Alexander Katz

NSIComMasahiro Kuroda

Mitsubishi Electric

C. Douglass Locke

Lockheed Martin/TimeSys

George Malek
Jean-Christophe Mielnik
Ragunathan Rajkumar

ApogeeThomson-CSF
CMU

PREFACE

xvii
Mike Schuette

Motorola

Chris Yurkoski

LucentSimon Waddington

Wind River Systems

would serve as a pool of readily available expertise and as initial reviewers of earlydrafts.
The effort commenced in March 1999 with a plenary meeting of the consultant
and primary groups at the Chicago Hilton and Towers. This was an educational
meeting where the consultants each presented selections of general real-time wisdom,and the specific requirements of their part of the real-time world.
The basis of the specification was laid down at the first primary group meeting. It
took place in one of the few civilized locations in the United States that is notaccessible to digital or analog cell phone traffic, Mendocino, California. This is also,
in the expert opinion of the primary group, the location of a restaurant that produces
the world’s most heavily cheesed pizza.
Through 1999 the primary group met slightly more than once a month, andmeetings for the joint primary and consultants groups were held slightly less thanonce a month. We worked hard and had glorious fun. Mainly, the fun was the joy ofsolving a welter of problems with a team of diverse and talented software architects,
but there were memorable nontechnical moments.
There was the seminal “under your butt” insight, when James told Greg that heshould stop looking over his head for the sense of an argument: “This is simple, Greg.
It’s not over your head, it’s going under your butt.” That was the same Burlington,
Massachusetts, meeting where a contingent of the expert group attended the 3:00 AM
second showing of the newly released Star Wars Phantom Menace. The only sane
reason for waking up at a time more suitable for going to sleep was that James hadgone back to California to attend the movie with his wife, who had purchased tickets
weeks in advance. It tickled our fancy to use the magic of time zones and early rising
to see the new release before them.
The cinnamon rolls in Des Moines, which David later claimed were bigger thanhis head. This was an exaggeration. Each roll was slightly less than half the size ofDavid’s head.
The “dead cat” meeting in Ottawa, where Greg claimed that when he took his
earache to the clinic, the doctor would probably remove a dead cat.
The “impolite phrase” meeting, also in Ottawa. The group made it into a
computer industry gossip column, and our feelings on the thrill of being treated like

xviiimovie stars simply cannot be expressed in this book. We are, however, impressed thata writer old enough to perceive Greg as IBM’s boy is still writing regularly.
In September 1999, the draft specification was published for formal review byparticipants in the Java Community Process and informal reading by anyone who
downloaded it from the group’s web site (http://www.rtj.org). In December 1999, therevised and extended document was published on the web site for public review.
Public review remained open until the 14th of February 2000 (yes, Valentine’s Day).
Then the specification was revised a final time to address the comments from thegeneral public.
The first result of this work is the document you are reading. IBM is also
producing a reference implementation and a test suite to accompany this specification.

Acknowledgments
The reader should consider this work truly a collaborative effort. Many people
contributed in diverse ways. Unlike most traditional published books this work is theresult of effort and contribution from engineers, executives, administrators, marketingand product managers, industry consultants, and university faculty members spread
across more than two dozen companies and organizations from around the globe. It isalso the result of a new and unique method for developing software, The Java
Community Process.
We’ll start at the beginning. Many of the technical contributors came together at
a series of forums conceived and hosted by Lisa Carnahan at the National Institute forStandards and Technology. One of the authors, Greg Bollella, was instrumental, alongwith Lisa, in the early establishment of the organization of the future authors. He
thanks his managers at IBM, Ruth Taylor, Rod Smith, and Pat Sueltz, for (in theirwords) being low-maintenance managers and for allowing Greg the freedom topursue his goal.
The Java Community Process was developed at Sun Microsystems by Jim
Mitchell, Ken Urquhart, and others to allow and promote the broad involvement of
the computer industry in the development of the Java™ platform. We thank them andall those at Sun and other companies who reviewed the initial proposals of theprocess. Vicki Shipkowitz the embedded Java product manager at Sun has also helped
the Real-Time for Java Expert Group with logistics concerning demonstrations andpresentations of the RTSJ.
The Real-Time for Java Expert Group comprises an engineering team and a
consultant team. The authors of this work are the primary engineers and we sincerelyPREFACE

thank the consultants, mentioned by name previously, for their efforts during the early
design phase and for reviewing various drafts. Along the way Ray Kamin, Wolfgang
Pieb, and Edward Wentworth replaced three of the original consultants and we thankthem for their effort as well.
We thank all those, but especially Kirk Reinholtz of NASA’s Jet Propulsion Lab,
who submitted comments during the participant and public reviews.
We thank Lisa Friendly, the Java Series editor at Sun Microsystems, and Mike
Hendrickson, and Julie DiNicola at Addison-Wesley for their effort in the preparationof this book.
We all thank Russ Richards at DISA for his support of our effort.
We thank Kevin Russell and Bill Foote of Sun Microsystems who worked hardduring the NIST sponsored requirements phase.
Although they have much left to do and will likely give us more work as they
implement the RTSJ, we thank the reference implementation team at IBM. Peter
Haggar leads the team of David Wendt and Jim Mickelson. Greg also thanks them fortheir efforts on the various robot demonstrations he used in his talks about the RTSJ.
Greg would like to personally thank his dissertation advisor Kevin Jeffay for his
guidance.
We thank Robin Coron and Feng Liu, administrative assistants at Sun
Microsystems and IBM, respectively, for their logistical support.

A Note on Format
We used javadoc on Java source files to produce most of this book (see the Colophon
for more details) and thus many references to class, interface, and method names use
the @link construct to produce a hyperlink in the (more typical) html formatted
output. Of course, clicking on the hyperlink in the html formatted version will display
the definition of the class. We tried to preserve this hyperlink characteristic in thebook by including on each occurrence of a name the page number of its definition as a
trailing subscript. Let us know if this is a useful feature (comments@rtj.org).

xix

Foreword
I expect The Real-Time Specification for Java to become the first real-time
programming language to be both commercially and technologically successful.
Other programming languages have been intended for use in the real-time
computing domain. However, none has been commercially successful in the sense ofbeing significantly adopted in that domain. Many were academic research projects.
Most did not focus on the core real-time issues of managing computing resources inorder to satisfy application timeliness requirements. Instead, they typicallyemphasized the orthogonal (albeit important) topic of concurrency and other topics
important to the whole field of embedded computing systems (of which real-time
computing systems are a subset).
Ada 95, including its Real-Time Systems Annex D, has probably been the mostsuccessful real-time language, in terms of both adoption and real-time technology.
One reason is that Ada is unusually effective (among real-time languages and also
operating systems) across the real-time computing system spectrum, fromprogramming-in-the-small in traditional device-level control subsystems, toprogramming-in-the-large in enterprise command and control systems. Despite that
achievement, a variety of nontechnical factors crippled Ada’s commercial success.
When James Gosling introduced the Java programming language in 1995, itappeared irrelevant to the real-time computing field, based on most of its initial
purposes and its design. Indeed, some of its fundamental principles were antitheticalto those of real-time computing. To facilitate its major goal of operating system andhardware independence, the language was deliberately given a weak vocabulary inareas such as thread behavior, synchronization, interrupts, memory management, andinput/output. However, these are among the critical areas needing explicit
management (by the language or the operating system) for meeting application
timeliness requirements.
Nevertheless, the Java platform’s promise of “Write Once, Run Anywhere,”
together with the Java language’s appeal as a programming language per se, offer far
greater cost-savings potential in the real-time (and more broadly, the embedded)
domain than in the desktop and server domains. Desktops are dominated by the“Wintel” duopoly; servers have only a few processor types and operating systems.
xxi

xxii

Real-time computing systems have tens of different processor types and many tens ofdifferent operating system products (not counting the custom-made ones thatcurrently constitute about half of the installations). The POSIX standard hasn’t
provided the intended real-time application portability because it permits widely
varying subsets to be implemented. The Java platform is already almost ubiquitous.
The real-time Java platform’s necessarily qualified promise of “Write Once Carefully,
Run Anywhere Conditionally” is nevertheless the best prospective opportunity for
application re-usability.
The overall challenge was to reconcile the intrinsically divergent natures of theJava language and most of real-time computing. Compatibility of the Real-Time
Specification for Java and the Java Language Specification had to be maintained,
while making the former cost-effective for real-time computing systems.
Most people involved in, and even aware of, the real-time Java effort, including
the authors of this book and me, were initially very skeptical about the feasibility ofadequately meeting this challenge.
The real-time Java community took two important and unusual initial steps
before forming the Real-Time for Java Expert Group under Sun’s Java Community
Process.
The first step was to convene many representatives of the real-time community anumber of times (under the auspices of the National Institute for Standards andTechnology), to achieve and document consensus on the requirements for the RealTime Specification for Java. Not surprisingly, when this consensus emerged, it
included mandatory requirements for building the kind of smaller scale, static, realtime subsystems familiar to current practitioners using C and C++.
More surprisingly, the consensus also included mandatory and optional
requirements for accommodating advanced dynamic and real-time resource
management technologies, such as asynchronous transfer of control and timelinessbased scheduling policies, and for building larger scale real-time systems. The
primary impetus for these dynamic and programming-in-the-large, real-time
requirements came from the communities already using the Java language, or using
the Ada language, or building defense (primarily command and control) systems.
The second, concomitant, step was to establish an agreed-upon lexicon of realtime computing concepts and terms to enable this dialog about, and consensus on, therequirements for the Real-Time Specification for Java. As unlikely as it may seem tothose outside of the real-time community, real-time computing concepts and terms arenormally not used in a well-defined way (except by most real-time researchers).
The next step toward the realization of the Java language’s potential for thepresent and the future of real-time computing is defining and writing the Real-Time

FOREWORD

Specification for Java, the first version of which is in this book. Understanding thisspecification will also improve the readers’ understanding of both the Java language
and real-time computing systems as well.
Greg Bollella was an ideal leader for this specification team. He recruited a wellbalanced group of real-time and Java language experts. His background in bothpractical and theoretical real-time computing prepared him for gently but resolutely
guiding the team’s rich and intense discussions into a coherent specification.
Of course, more work remains, including documenting use cases and examples;
performing implementations and evaluations; gaining experience from deployed
products; and iterations on The Real-Time Specification for Java. The Distributed
Real-Time Specification for Java also lies ahead.
The real-time Java platform is prepared not just to provide cost-reduced
functional parity with current mainstream real-time computing practice and products,
but also to play a leadership role as real-time computing practice moves forward in
the Internet age.
E. Douglas Jensen
Sherborn, MA

xxiii

CHAP TER

1Introduction
This book is a preliminary release of The Real-Time Specification for Java™ (RTSJ).
The final version will be available with the release of the reference implementation.
The Real-Time for Java Expert Group (RTJEG), convened under the Java
Community Process and JSR-000001, has been given the responsibility of producing
a specification for extending The Java Language Specification and The Java Virtual
Machine Specification and of providing an Application Programming Interface thatwill enable the creation, verification, analysis, execution, and management of Java
threads whose correctness conditions include timeliness constraints (also known as
real-time threads). This introduction describes the guiding principles that the RTJEG
created and used during our work, a description of the real-time Java requirements
developed under the auspices of The National Institute for Standards and Technology(NIST), and a brief, high-level description of each of the seven areas we identified as
requiring enhancements to accomplish our goal.

Guiding Principles
The guiding principles are high-level statements that delimit the scope of the work of
the RTJEG and introduce compatibility requirements for The Real-Time Specification
for Java.
Applicability to Particular Java Environments: The RTSJ shall not include
specifications that restrict its use to particular Java environments, such as a particularversion of the Java Development Kit, the Embedded Java Application Environment,or the Java™ 2 Platform, Micro Edition (J2ME™).

1

2

CHAPTER 1 INTRODUCTION

Backward Compatibility: The RTSJ shall not prevent existing, properly
written, non-real-time Java programs from executing on implementations of the
RTSJ.
Write Once, Run Anywhere™: The RTSJ should recognize the importance of“Write Once, Run Anywhere,” but it should also recognize the difficulty of achieving
WORA for real-time programs and not attempt to increase or maintain binary
portability at the expense of predictability.
Current Practice vs. Advanced Features: The RTSJ should address current
real-time system practice as well as allow future implementations to include advanced
features.
Predictable Execution: The RTSJ shall hold predictable execution as firstpriority in all tradeoffs; this may sometimes be at the expense of typical generalpurpose computing performance measures.
No Syntactic Extension: In order to facilitate the job of tool developers, andthus to increase the likelihood of timely implementations, the RTSJ shall not
introduce new keywords or make other syntactic extensions to the Java language.
Allow Variation in Implementation Decisions: The RTJEG recognizes that
implementations of the RTSJ may vary in a number of implementation decisions, suchas the use of efficient or inefficient algorithms, tradeoffs between time and space
efficiency, inclusion of scheduling algorithms not required in the minimum
implementation, and variation in code path length for the execution of byte codes. The
RTSJ should not mandate algorithms or specific time constants for such, but requirethat the semantics of the implementation be met. The RTSJ offers implementers theflexibility to create implementations suited to meet the requirements of theircustomers.

Overview of the Seven Enhanced Areas
In each of the seven sections that follow we give a brief statement of direction foreach area. These directions were defined at the first meeting of the eight primaryengineers in Mendocino, California, in late March 1999, and further clarified throughlate September 1999.
Thread Scheduling and Dispatching: In light of the significant diversity inscheduling and dispatching models and the recognition that each model has wide
applicability in the diverse real-time systems industry, we concluded that our direction
for a scheduling specification would be to allow an underlying scheduling mechanism
to be used by real-time Java threads but that we would not specify in advance theexact nature of all (or even a number of) possible scheduling mechanisms. The

OVERVIEW OF THE SEVEN ENHANCED AREAS

specification is constructed to allow implementations to provide unanticipatedscheduling algorithms. Implementations will allow the programmatic assignment ofparameters appropriate for the underlying scheduling mechanism as well as providing
any necessary methods for the creation, management, admittance, and termination ofreal-time Java threads. We also expect that, for now, particular thread scheduling anddispatching mechanisms are bound to an implementation. However, we provide
enough flexibility in the thread scheduling framework to allow future versions of thespecification to build on this release and allow the dynamic loading of schedulingpolicy modules.
To accomodate current practice the RTSJ requires a base scheduler in all
implementations. The required base scheduler will be familiar to real-time system
programmers. It is priority-based, preemptive, and must have at least 28 uniquepriorities.
Memory Management: We recognize that automatic memory management is aparticularly important feature of the Java programming environment, and we sought a
direction that would allow, as much as possible, the job of memory management to be
implemented automatically by the underlying system and not intrude on the
programming task. Additionally, we understand that many automatic memory
management algorithms, also known as garbage collection (GC), exist, and many of
those apply to certain classes of real-time programming styles and systems. In our
attempt to accommodate a diverse set of GC algorithms, we sought to define amemory allocation and reclamation specification that would:• be independent of any particular GC algorithm,• allow the program to precisely characterize a implemented GC algorithm’s effect on
the execution time, preemption, and dispatching of real-time Java threads, and• allow the allocation and reclamation of objects outside of any interference by any GC
algorithm.

Synchronization and Resource Sharing: Logic often needs to share serializable
resources. Real-time systems introduce an additional complexity: priority inversion.
We have decided that the least intrusive specification for allowing real-time safe
synchronization is to require that implementations of the Java keyword synchronizedinclude one or more algorithms that prevent priority inversion among real-time Java
threads that share the serialized resource. We also note that in some cases the use of
the synchronized keyword implementing the required priority inversion algorithm is
not sufficient to both prevent priority inverison and allow a thread to have an
execution eligibility logically higher than the garbage collector. We provide a set ofwait-free queue classes to be used in such situations.

3

4

CHAPTER 1 INTRODUCTION

Asynchronous Event Handling: Real-time sytems typically interact closelywith the real-world. With respect to the execution of logic, the real-world isasynchronous. We thus felt compelled to include efficient mechanisms for
programming disciplines that would accommodate this inherent asynchrony. The
RTSJ generalizes the Java language’s mechanism of asynchronous event handling.
Required classes represent things that can happen and logic that executes when those
things happen. A notable feature is that the execution of the logic is scheduled anddispatched by an implemented scheduler.
Asynchronous Transfer of Control: Sometimes the real-world changes sodrastically (and asynchronously) that the current point of logic execution should be
immediately and efficiently transferred to another location. The RTSJ includes amechanism which extends Java’s exception handling to allow applications toprogramatically change the locus of control of another Java thread. It is important tonote that the RTSJ restricts this asynchronous transfer of control to logic specifically
written with the assumption that its locus of control may asynchronously change.
Asynchronous Thread Termination: Again, due to the sometimes drastic andasynchronous changes in the real-world, application logic may need to arrange for areal-time Java thread to expeditiously and safely transfer its control to its outermost
scope and thus end in a normal manner. Note that unlike the traditional, unsafe, anddeprecated Java mechanism for stopping threads, the RTSJ’s mechanism forasynchronous event handling and transfer of control is safe.
Physical Memory Access: Although not directly a real-time issue, physicalmemory access is desirable for many of the applications that could productively makeuse of an implementation of the RTSJ. We thus define a class that allows
programmers byte-level access to physical memory as well as a class that allows theconstruction of objects in physical memory.

CHAP TER

2DesignThe RTSJ comprises eight areas of extended semantics. This chapter explains each infair detail. Further detail, exact requirements, and rationale are given in the opening
section of each relevant chapter. The eight areas are discussed in approximate order of
their relevance to real-time programming. However, the semantics and mechanisms ofeach of the areas — scheduling, memory management, synchronization,
asynchronous event handling, asynchronous transfer of control, asynchronous threadtermination, physical memory access, and exceptions — are all crucial to theacceptance of the RTSJ as a viable real-time development platform.

Scheduling
One of the concerns of real-time programming is to ensure the timely or predictable
execution of sequences of machine instructions. Various scheduling schemes namethese sequences of instructions differently. Typically used names include threads,
tasks, modules, and blocks. The RTSJ introduces the concept of a schedulable object.
Any instance of any class implementing the interface Schedulable is a schedulable
object and its scheduling and dispatching will be managed by the instance ofScheduler to which it holds a reference. The RTSJ requires three classes that areschedulable objects; RealtimeThread, NoHeapRealtimeThread, andAsyncEventHandler.

By timely execution of threads, we mean that the programmer can determine byanalysis of the program, testing the program on particular implementations, or bothwhether particular threads will always complete execution before a given timelinessconstraint. This is the essence of real-time programming: the addition of temporalconstraints to the correctness conditions for computation. For example, for a program
5

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CHAPTER 2 DESIGN

to compute the sum of two numbers it may no longer be acceptable to compute only
the correct arithmetic answer but the answer must be computed before a particulartime. Typically, temporal constraints are deadlines expressed in either relative or
absolute time.
We use the term scheduling (or scheduling algorithm) to refer to the productionof a sequence (or ordering) for the execution of a set of threads (a schedule). This
schedule attempts to optimize a particular metric (a metric that measures how well thesystem is meeting the temporal constraints). A feasibility analysis determines if aschedule has an acceptable value for the metric. For example, in hard real-timesystems the typical metric is “number of missed deadlines” and the only acceptablevalue for that metric is zero. So called soft real-time systems use other metrics (suchas mean tardiness) and may accept various values for the metric in use.
Many systems use thread priority in an attempt to determine a schedule. Priority
is typically an integer associated with a thread; these integers convey to the system theorder in which the threads should execute. The generalization of the concept ofpriority is execution eligibility. We use the term dispatching to refer to that portion of
the system which selects the thread with the highest execution eligibility from thepool of threads that are ready to run. In current real-time system practice, theassignment of priorities is typically under programmer control as opposed to undersystem control. The RTSJ’s base scheduler also leaves the assignment of priorities
under programmer control. However, the base scheduler also inherits methods fromits superclass to determine feasibility. The feasibility algorithms assume that the ratemonotonic priority assignment algorithm has been used to assign priorities. The RTSJ
does not require that implementations check that such a priority assignment is correct.
If, of course, the assignment is incorrect the feasibility analysis will be meaningless(note however, that this is no different than the vast majority of real-time operating
systems and kernels in use today).
The RTSJ requires a number of classes with names of the format<string>Parameters (such as SchedulingParameters). An instance of one of theseparameter classes holds a particular resource demand characteristic for one or moreschedulable objects. For example, the PriorityParameters subclass of
SchedulingParameters contains the execution eligibility metric of the basescheduler, i.e., priority. At some times (thread create-time or set (reset)), later
instances of parameter classes are bound to a schedulable object. The schedulable
object then assumes the characteristics of the values in the parameter object. For
example, if a PriorityParameter instance that had in its priority field the value
representing the highest priority available is bound to a schedulable object, then that
object will assume the characteristic that it will execute whenever it is ready inMEMORY MANAGEMENT

preference to all other schedulable objects (except, of course, those also with thehighest priority).
The RTSJ is written so as to allow implementers the flexibility to install arbitraryscheduling algorithms and feasibility analysis algorithms in an implementation of thespecification. We do this because the RTJEG understands that the real-time systems
industry has widely varying requirements with respect to scheduling. Programming to
the Java platform may result in code much closer toward the goal of reusing software
written once but able to execute on many different computing platforms (known as
Write Once, Run Anywhere) and that the above flexibility stands in opposition to that
goal, The Real-Time Specification for Java also specifies a particular schedulingalgorithm and semantic changes to the JVM that support predictable execution andmust be available on all implementations of the RTSJ. The initial default and requiredscheduling algorithm is fixed-priority preemptive with at least 28 unique priority
levels and will be represented in all implementations by the PriorityScheduler
subclass of Scheduler.

Memory Management
Garbage-collected memory heaps have always been considered an obstacle to realtime programming due to the unpredictable latencies introduced by the garbage
collector. The RTSJ addresses this issue by providing several extensions to thememory model, which support memory management in a manner that does notinterfere with the ability of real-time code to provide deterministic behavior. This goalis accomplished by allowing the allocation of objects outside of the garbage-collected
heap for both short-lived and long-lived objects.

Memory Areas
The RTSJ introduces the concept of a memory area. A memory area represents an areaof memory that may be used for the allocation of objects. Some memory areas exist
outside of the heap and place restrictions on what the system and garbage collector
may do with objects allocated within. Objects in some memory areas are nevergarbage collected; however, the garbage collector must be capable of scanning thesememory areas for references to any object within the heap to preserve the integrity of
the heap.
There are four basic types of memory areas:
1. Scoped memory provides a mechanism for dealing with a class of objects thathave a lifetime defined by syntactic scope (cf., the lifetime of objects on the heap).

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CHAPTER 2 DESIGN

2. Physical memory allows objects to be created within specific physical memory
regions that have particular important characteristics, such as memory that has
substantially faster access.
3. Immortal memory represents an area of memory containing objects that, once
allocated, exist until the end of the application, i.e., the objects are immortal.
4. Heap memory represents an area of memory that is the heap. The RTSJ does notchange the determinant of lifetime of objects on the heap. The lifetime is still
determined by visibility.

Scoped Memory
The RTSJ introduces the concept of scoped memory. A memory scope is used to givebounds to the lifetime of any objects allocated within it. When a scope is entered,every use of new causes the memory to be allocated from the active memory scope. A
scope may be entered explicitly, or it can be attached to a RealtimeThread which will
effectively enter the scope before it executes the thread’s run() method.
Every scoped memory area effectively maintains a count of the number ofexternal references to that memory area. The reference count for a ScopedMemory area
is increased by entering a new scope through the enter() method of MemoryArea, by
the creation of a RealtimeThread using the particular ScopedMemory area, or by the
opening of an inner scope. The reference count for a ScopedMemory area is decreased
when returning from the enter() method, when the RealtimeThread using the
ScopedMemory exits, or when an inner scope returns from its enter() method. Whenthe count drops to zero, the finalize method for each object in the memory is executed
to completion. The scope cannot be reused until finalization is complete and the RTSJ
requires that the finalizers execute to completion before the next use (calling enter()or in a constructor) of the scoped memory area.
Scopes may be nested. When a nested scope is entered, all subsequent allocations
are taken from the memory associated with the new scope. When the nested scope isexited, the previous scope is restored and subsequent allocations are again taken fromthat scope.
Because of the unusual lifetimes of scoped objects, it is necessary to limit thereferences to scoped objects, by means of a restricted set of assignment rules. A
reference to a scoped object cannot be assigned to a variable from an enclosing scope,or to a field of an object in either the heap or the immortal area. A reference to ascoped object may only be assigned into the same scope or into an inner scope. The
virtual machine must detect illegal assignment attempts and must throw anappropriate exception when they occur.

SYNCHRONIZATION

The flexibility provided in choice of scoped memory types allows the application
to use a memory area that has characteristics that are appropriate to a particularsyntactically defined region of the code.

Immortal Memory
ImmortalMemory is a memory resource shared among all threads in an application.
Objects allocated in ImmortalMemory are freed only when the Java runtime

environment terminates, and are never subject to garbage collection or movement.

Budgeted Allocation
The RTSJ also provides limited support for providing memory allocation budgets forthreads using memory areas. Maximum memory area consumption and maximum
allocation rates for individual real-time threads may be specified when the thread iscreated.

Synchronization
Terms
For the purposes of this section, the use of the term priority should be interpreted
somewhat more loosely than in conventional usage. In particular, the term highest
priority thread merely indicates the most eligible thread — the thread that the
dispatcher would choose among all of the threads that are ready to run — and doesn’tnecessarily presume a strict priority based dispatch mechanism.

Wait Queues
Threads waiting to acquire a resource must be released in execution eligibility order.
This applies to the processor as well as to synchronized blocks. If threads with thesame execution eligibility are possible under the active scheduling policy, suchthreads are awakened in FIFO order. For example:• Threads waiting to enter synchronized blocks are granted access to thesynchronized block in execution eligibility order.
• A blocked thread that becomes ready to run is given access to the processor in
execution eligibility order.
• A thread whose execution eligibility is explicitly set by itself or another thread is
given access to the processor in execution eligibility order.
• A thread that performs a yield will be given access to the processor after waiting
threads of the same execution eligibility.

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CHAPTER 2 DESIGN• Threads that are preempted in favor of a thread with higher execution eligibility
may be given access to the processor at any time as determined by a particular
implementation. The implementation is required to provide documentation
stating exactly the algorithm used for granting such access.

Priority Inversion Avoidance
Any conforming implementation must provide an implementation of thesynchronized primitive with default behavior that ensures that there is no unboundedpriority inversion. Furthermore, this must apply to code if it is run within theimplementation as well as to real-time threads. The priority inheritance protocol must
be implemented by default. The priority inheritance protocol is a well-known
algorithm in the real-time scheduling literature and it has the following effect. If
thread t1 attempts to acquire a lock that is held by a lower-priority thread t 2, then t2’s
priority is raised to that of t 1 as long as t2 holds the lock (and recursively if t2 is itself
waiting to acquire a lock held by an even lower-priority thread).
The specification also provides a mechanism by which the programmer canoverride the default system-wide policy, or control the policy to be used for aparticular monitor, provided that policy is supported by the implementation. The
monitor control policy specification is extensible so that new mechanisms can be
added by future implementations.
A second policy, priority ceiling emulation protocol (or highest locker protocol),is also specified for systems that support it. The highest locker protocol is also a wellknown algorithm in the literature, and it has the following effect:• With this policy, a monitor is given a priority ceiling when it is created, which is
the highest priority of any thread that could attempt to enter the monitor.
• As soon as a thread enters synchronized code, its priority is raised to themonitor’s ceiling priority, thus ensuring mutually exclusive access to the code
since it will not be preempted by any thread that could possibly attempt to enter
the same monitor.
• If, through programming error, a thread has a higher priority than the ceiling of
the monitor it is attempting to enter, then an exception is thrown.
One needs to consider the design point given above, the two new thread types,
RealtimeThread and NoHeapRealtimeThread, and regular Java threads and thepossible issues that could arise when a NoHeapRealtimeThread and a regular Java
thread attempt to synchronize on the same object. NoHeapRealtimeThreads have an
implicit execution eligibility that must be higher than that of the garbage collector.
This is fundamental to the RTSJ. However, given that regular Java threads may never
have an execution eligibility higher than the garbage collector, no known priority

ASYNCHRONOUS EVENT HANDLING

inversion avoidance algorithm can be correctly implemented when the shared object
is shared between a regular Java thread and a NoHeapRealtimeThread because thealgorithm may not raise the priority of the regular Java thread higher than the garbage
collector. Some mechanism other than the synchronized keyword is needed to ensure
non-blocking, protected access to objects shared between regular Java threads andNoHeapRealtimeThreads.
Note that if the RTSJ requires that the execution of NoHeapRealtimeThreads
must not be delayed by the execution of the garbage collector it is impossible for a
NoHeapRealtimeThread to synchronize, in the classic sense, on an object accessed byregular Java threads. The RTSJ provides three wait-free queue classes to provide
protected, non-blocking, shared access to objects accessed by both regular Java
threads and NoHeapRealtimeThreads. These classes are provided explicitly to enablecommunication between the real-time execution of NoHeapRealtimeThreads andregular Java threads.

Determinism
Conforming implementations shall provide a fixed upper bound on the time required
to enter a synchronized block for an unlocked monitor.

Asynchronous Event Handling
The asynchronous event facility comprises two classes: AsyncEvent andAsyncEventHandler. An AsyncEvent object represents something that can happen,
like a POSIX signal, a hardware interrupt, or a computed event like an airplane
entering a specified region. When one of these events occurs, which is indicated by
the fire() method being called, the associated handleAsyncEvent() methods of
instances of AsyncEventHandler are scheduled and thus perform the required logic.
An instance of AsyncEvent manages two things: 1) the unblocking of handlers
when the event is fired, and 2) the set of handlers associated with the event. This set
can be queried, have handlers added, or have handlers removed.
An instance of AsyncEventHandler can be thought of as something roughly
similar to a thread. It is a Runnable object: when the event fires, thehandleAsyncEvent() methods of the associated handlers are scheduled. Whatdistinguishes an AsyncEventHandler from a simple Runnable is that an
AsyncEventHandler has associated instances of ReleaseParameters,
SchedulingParameters and MemoryParameters that control the actual execution of
the handler once the associated AsyncEvent is fired. When an event is fired, thehandlers are executed asynchronously, scheduled according to the associatedReleaseParameters and SchedulingParameters objects, in a manner that looks

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CHAPTER 2 DESIGN

like the handler has just been assigned to its own thread. It is intended that the system
can cope well with situations where there are large numbers of instances ofAsyncEvent and AsyncEventHandler (tens of thousands). The number of fired (inprocess) handlers is expected to be smaller.
A specialized form of an AsyncEvent is the Timer class, which represents anevent whose occurrence is driven by time. There are two forms of Timers: the
OneShotTimer and the PeriodicTimer. Instances of OneShotTimer fire once, at the
specified time. Periodic timers fire off at the specified time, and then periodicallyaccording to a specified interval.
Timers are driven by Clock objects. There is a special Clock object,
Clock.getRealtimeClock(), that represents the real-time clock. The Clock class
may be extended to represent other clocks the underlying system might makeavailable (such as a soft clock of some granularity).

Asynchronous Transfer of Control
Many times a real-time programmer is faced with a situation where the computational
cost of an algorithm is highly variable, the algorithm is iterative, and the algorithm
produces successively refined results during each iteration. If the system, before
commencing the computation, can determine only a time bound on how long to
execute the computation (i.e., the cost of each iteration is highly variable and theminimum required latency to terminate the computation and receive the lastconsistent result is much less than about half of the mean iteration cost), thenasynchronously transferring control from the computation to the result transmission
code at the expiration of the known time bound is a convenient programming style.
The RTSJ supports this and other styles of programming where such transfer isconvenient with a feature termed Asynchronous Transfer of Control (ATC).
The RTSJ’s approach to ATC is based on several guiding principles, outlined in
the following lists.

Methodological Principles• A thread needs to explicitly indicate its susceptibility toATC. Since legacy codeor library methods might have been written assuming no ATC, by defaultATC
should be turned off (more precisely, it should be deferred as long as control is insuch code).
• Even if a thread allows ATC, some code sections need to be executed tocompletion and thus ATC is deferred in such sections. The ATC-deferred sections
are synchronized methods and statements.
• Code that responds to an ATC does not return to the point in the thread where theASYNCHRONOUS THREAD TERMINATION

ATC was triggered; that is, an ATC is an unconditional transfer of control.
Resumptive semantics, which returns control from the handler to the point ofinterruption, are not needed since they can be achieved through other mechanisms(in particular, an AsyncEventHandler).

Expressibility Principles• A mechanism is needed through which an ATC can be explicitly triggered in atarget thread. This triggering may be direct (from a source thread) or indirect(through an asynchronous event handler).
• It must be possible to trigger an ATC based on any asynchronous event including
an external happening or an explicit event firing from another thread. In
particular, it must be possible to base an ATC on a timer going off.
• Through ATC it must be possible to abort a thread but in a manner that does notcarry the dangers of the Thread class’s stop() and destroy() methods.

Semantic Principles• If ATC is modeled by exception handling, there must be some way to ensure thatan asynchronous exception is only caught by the intended handler and not, forexample, by an all-purpose handler that happens to be on the propagation path.
• Nested ATCs must work properly. For example, consider two, nestedATC-based
timers and assume that the outer timer has a shorter timeout than the nested, innertimer. If the outer timer times out while control is in the nested code of the innertimer, then the nested code must be aborted (as soon as it is outside an ATCdeferred section), and control must then transfer to the appropriate catch clause
for the outer timer. An implementation that either handles the outer timeout in thenested code, or that waits for the longer (nested) timer, is incorrect.

Pragmatic Principles• There should be straightforward idioms for common cases such as timer handlers
and thread termination.
• ATC must be implemented without inducing an overhead for programs that do
not use it.
• If code with a timeout completes before the timeout’s deadline, the timeout needs
to be automatically stopped and its resources returned to the system.

Asynchronous Thread Termination
Although not a real-time issue, many event-driven computer systems that tightlyinteract with external real-world noncomputer systems (e.g., humans, machines,
control processes, etc.) may require significant changes in their computational

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CHAPTER 2 DESIGN

behavior as a result of significant changes in the non-computer real-world system. It isconvenient to program threads that abnormally terminate when the external real-timesystem changes in a way such that the thread is no longer useful. Consider the
opposite case. A thread or set of threads would have to be coded in such a manner sothat their computational behavior anticipated all of the possible transitions among
possible states of the external system. It is an easier design task to code threads tocomputationally cooperate for only one (or a very few) possible states of the external
system. When the external system makes a state transition, the changes incomputation behavior might then be managed by an oracle, that terminates a set of
threads useful for the old state of the external system, and invokes a new set of threads
appropriate for the new state of the external system. Since the possible statetransitions of the external system are encoded in only the oracle and not in eachthread, the overall system design is easier.
Earlier versions of the Java language supplied mechanisms for achieving theseeffects: in particular the methods stop() and destroy() in class Thread. However,since stop() could leave shared objects in an inconsistent state, stop() has been
deprecated. The use of destroy() can lead to deadlock (if a thread is destroyed while
it is holding a lock) and although it has not yet been deprecated, its usage is
discouraged. A goal of the RTSJ was to meet the requirements of asynchronous threadtermination without introducing the dangers of the stop() or destroy() methods.
The RTSJ accommodates safe asynchronous thread termination through a
combination of the asynchronous event handling and the asynchronous transfer ofcontrol mechanisms. If the significantly long or blocking methods of a thread aremade interruptible the oracle can consist of a number of asynchronous event handlersthat are bound to external happenings. When the happenings occur the handlers can
invoke interrupt() on appropriate threads. Those threads will then clean up by
having all of the interruptible methods transfer control to appropriate catch clauses ascontrol enters those methods (either by invocation or by the return bytecode). This
continues until the run() method of the thread returns. This idiom provides a quick (ifcoded to be so) but orderly clean up and termination of the thread. Note that the oracle
can comprise as many or as few asynchronous event handlers as appropriate.

Physical Memory Access
The RTSJ defines classes for programmers wishing to directly access physicalmemory from code. RawMemoryAccess defines methods that allow the programmer toconstruct an object that represents a range of physical addresses and then access thephysical memory with byte, short, int, long, float, and double granularity. No
semantics other than the set<type>() and get<type>() methods are implied. The
ScopedPhysicalMemory and ImmortalPhysicalMemory classes allow programmers

PHYSICAL MEMORY ACCESS

to create objects that represent a range of physical memory addresses and in whichJava objects can be located. The RTSJ requires a PhysicalMemoryFactory in each
implementation. Methods on the factory object are the only way to create instances ofphysical memory objects. On each physical memory class create() methods invoke
appropriate methods on the PhysicalMemoryFactory class to create the required
instance. The factory also enforces security policies.

Raw Memory AccessAn instance of RawMemoryAccess models a “raw storage” area as a fixed-size
sequence of bytes. Factory methods allow RawMemoryAccess objects to be createdfrom memory at a particular address range or using a particular type of memory. The
implementation must provide a factory that interprets these requests correctly. The
factory may be set by applications based on documentation from the implementation
provider. A full complement of set<type>() and get<type>() methods allow thecontents of the physical memory area to be accessed through offsets from the base,interpreted as byte, short, int, long or float data values, and copied to/from arrays of
those types.
The byte-ordering interpretation of the data is based on the value of the
BYTE_ORDER static variable in class RealtimeSystem.

The RawMemoryAccess class allows a real-time program to implement device
drivers, memory-mapped I/O, flash memory, battery-backed RAM, and similar lowlevel software.
A raw memory access object cannot contain objects or references to objects.
Such a capability would be unsafe (since it could be used to defeat Java’s typechecking) and error-prone (since it is sensitive to the specific representational choices
made by the Java compiler). This capability is provided by physical memory areas,
which do not provide raw access to the memory.

Physical Memory Areas
In many cases systems, needing the predictable execution of the RTSJ will also needto access various kinds of memory at particular addresses for performance or otherreasons. Consider a system in which very fast static RAM was programmaticallyavailable. A design that could optimize performance might wish to place various
frequently used Java objects in the fast static RAM. The ScopedPhysicalMemory andImmortalPhysicalMemory classes allow the programmer this flexibility. The
programmer would construct a physical memory object on the memory addresses
occupied by the fast RAM.

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CHAPTER 2 DESIGNIn order to maintain safety, a factory object constructs all physical memory
objects. The factory ensures that physical memory areas don’t overlap other memoryareas or raw memory access objects.

Exceptions
The RTSJ introduces several new exceptions, and some new treatment of exceptions
surrounding asynchronous transfer of control and memory allocators.
The new exceptions introduced are:• AsynchronouslyInterruptedException: Generated when a thread isasynchronously interrupted.
• MemoryAccessError: Thrown by the JVM when a thread attempts to accessmemory that is not in scope.
• ThrowBoundaryError: A throwable tried to propagate into a scope where it was
not accessible.
• MemoryScopeException: Thrown by the wait-free queue implementation whenan object is passed that is not compatible with both ends of the queue.
• OffsetOutOfBoundsException: Generated by the physical memory classes when
the given offset is out of bounds.
• SizeOutOfBoundsException: Generated by the physical memory classes when thegiven size is out of bounds.
• UnsupportedPhysicalMemoryException: Generated by the physical memory
classes when the requested physical memory is unsupported.
• IllegalAssignmentError: Thrown on an attempt to make an illegal assignment.
• ResourceLimitError: Thrown if an attempt is made to exceed a system resource
limit, such as the maximum number of locks.

Minimum Implementations of the RTSJ
The flexibility of the RTSJ indicates that implementations may provide different
semantics for scheduling, synchronization, and garbage collection. This section
defines what minimum semantics for these areas and other semantics and APIs
required of all implementations of the RTSJ. In general, the RTSJ does not allow any
subsetting of the APIs in the javax.realtime package (except those noted as
optionally required); however, some of the classes are specific to certain well-known
scheduling or synchronization algorithms and may have no underlying support in aminimum implementation of the RTSJ. The RTSJ provides these classes as standard
parent classes for implementations supporting such algorithms.
The minimum scheduling semantics that must be supported in all
implementations of the RTSJ are fixed-priority preemptive scheduling and at least 28

MINIMUM IMPLEMENTATIONS OF THE RTSJ

unique priority levels. By fixed-priority we mean that the system does not change thepriority of any RealtimeThread or NoHeapRealtimeThread except, temporarily, forpriority inversion avoidance. Note, however, that application code may change suchpriorities. What the RTSJ precludes by this statement is scheduling algorithms thatchange thread priorities according to policies for optimizing throughput (such asincreasing the priority of threads that have been receiving few processor cycles
because of higher priority threads (aging)). The 28 unique priority levels are required
to be unique to preclude implementations from using fewer priority levels of
underlying systems to implement the required 28 by simplistic algorithms (such as
lumping four RTSJ priorities into seven buckets for an underlying system that only
supports seven priority levels). It is sufficient for systems with fewer than 28 priority
levels to use more sophisticated algorithms to implement the required 28 uniquelevels as long as RealtimeThreads and NoHeapRealtimeThreads behave as though
there were at least 28 unique levels. (e.g. if there were 28 RealtimeThreads (t1,...,t28)
with priorities (p1,...,p28), respectively, where the value of p1 was the highest priority
and the value of p2 the next highest priority, etc., then for all executions of threads t1
through t28 thread t1 would always execute in preference to threads t2, ..., t28 andthread t2 would always execute in preference to threads t3,..., t28, etc.)
The minimum synchronization semantics that must be supported in all
implementations of the RTSJ are detailed in the above section on synchronization andrepeated here.
All implementations of the RTSJ must provide an implementation of thesynchronized primitive with default behavior that ensures that there is no unboundedpriority inversion. Furthermore, this must apply to code if it is run within theimplementation as well as to real-time threads. The priority inheritance protocol must
be implemented by default.
All threads waiting to acquire a resource must be queued in priority order. Thisapplies to the processor as well as to synchronized blocks. If threads with the same
exact priority are possible under the active scheduling policy, threads with the samepriority are queued in FIFO order. (Note that these requirements apply only to therequired base scheduling policy and hence use the specific term “priority”). In
particular:• Threads waiting to enter synchronized blocks are granted access to thesynchronized block in priority order.
• A blocked thread that becomes ready to run is given access to the processor inpriority order.
• A thread whose execution eligibility is explicitly set by itself or another thread is
given access to the processor in priority order.

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CHAPTER 2 DESIGN• A thread that performs a yield() will be given access to the processor after
waiting threads of the same priority.
• However, threads that are preempted in favor of a thread with higher priority may
be given access to the processor at any time as determined by a particular
implementation. The implementation is required to provide documentation
stating exactly the algorithm used for granting such access.
The RTSJ does not require any particular garbage collection algorithm. All
implementations of the RTSJ must, however, support the class GarbageCollector
and implement all of its methods.

Optionally Required Components
The RTSJ does not, in general, support the concept of optional components of thespecification. Optional components would further complicate the already difficult
task of writing WORA (Write Once Run Anywhere) software components for realtime systems. However, understanding the difficulty of providing implementations ofmechanisms for which there is no underlying support, the RTSJ does provide for a
few exceptions. Any components that are considered optional will be listed as such inthe class definitions.
The most notable optional component of the specification is the
POSIXSignalHandler. A conformant implementation must support POSIX signals if

and only if the underlying system supports them. Also, the class
RawMemoryFloatAccess is required to be implemented if and only if the JVM itself

supports floating point types.

Documentation Requirements
In order to properly engineer a real-time system, an understanding of the costassociated with any arbitrary code segment is required. This is especially importantfor operations that are performed by the runtime system, largely hidden from theprogrammer. (An example of this is the maximum expected latency before thegarbage collector can be interrupted.)
The RTSJ does not require specific performance or latency numbers to bematched. Rather, to be conformant to this specification, an implementation must
provide documentation regarding the expected behavior of particular mechanisms.
The mechanisms requiring such documentation, and the specific data to be provided,
will be detailed in the class and method definitions.

PARAMETER OBJECTSParameter Objects
A number of constructors in this specification take objects generically named
feasibility parameters (classes named <string>Parameters where <string>
identifies the kind of parameter). When a reference to a Parameters object is given as
a parameter to a constructor the Parameters object becomes bound to the object
being created. Changes to the values in the Parameters object affect the constructedobject. For example, if a reference to a SchedulingParameters object, sp, is given to
the constructor of a RealtimeThread, rt, then calls to sp.setPriority() will change
the priority of rt. There is no restriction on the number of constructors to which areference to a single Parameters object may be given. If a Parameters object isgiven to more than one constructor, then changes to the values in the Parameters
object affect all of the associated schedulable objects. Note that this is a one-to-many
relationship, not a many-to-many relationship, that is, a schedulable object (e.g., an
instance of RealtimeThread) must have zero or one associated instance of eachParameter object type.
Caution: <string>Parameter objects are explicitly unsafe in multithreaded
situations when they are being changed. No synchronization is done. It is assumed
that users of this class who are mutating instances will be doing their own
synchronization at a higher level.

Java Platform Dependencies
In some cases the classes and methods defined in this specification are dependent on
the underlying Java platform.
1. The Comparable interface is available in Java™ 2 v1.2 and 1.3 and not in what
was formally known as JDK’s 1.0 and 1.1. Thus, we expect implementations of
this specification which are based on JDK’s 1.0 or 1.1 to include a Comparable
interface.
2. The class RawMemoryFloatAccess is required if and only if the underlying Java
Virtual Machine supports floating point data types.

19

CHAP TER

3

Threads
Chapter 3 Threads

This section contains classes that:• Provide for the creation of threads that have more precise scheduling semantics
than java.lang.Thread.
• Allow the use of areas of memory other than the heap for the allocation of
objects.
• Allow the definition of methods that can be asynchronously interrupted.
• Provide the scheduling semantics for handling asynchronous events.
The RealtimeThread class extends java.lang.Thread. The ReleaseParameters,
SchedulingParameters, and MemoryParameters provided to the RealtimeThread
constructor allow the temporal and processor demands of the thread to becommunicated to the system.
The NoHeapRealtimeThread class extends RealtimeThread. A
NoHeapRealtimeThread is not allowed to allocate or even reference objects from theJava heap, and can thus safely execute in preference to the garbage collector.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. The default scheduling policy must manage the execution of instances ofRealtimeThread and NoHeapRealtimeThread.
21

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Chapter 3 Threads

2. Any scheduling policy present in an implementation must be available toinstances of RealtimeThread and NoHeapRealtimeThread.
3. The function of allocating objects in memory in areas defined by instances of
ScopedMemory or its subclasses shall be available only to logic within instances ofRealtimeThread and NoHeapRealtimeThread.
4. The invocation of methods that throw AsynchronouslyInterruptedException
has the indicated effect only when the invocation occurs in the context of
instances of RealtimeThread and NoHeapRealtimeThread.
5. Instances of the NoHeapRealtimeThread class have an implicit execution
eligibility logically higher than the garbage collector.
6. Instances of the RealtimeThread class may have an execution eligibility
logically lower than the garbage collector.
7. Changing values in SchedulingParameters, ProcessingParameters,
ReleaseParameters, ProcessingGroupParameters, or use of
Thread.setPriority() must not affect the correctness of any implemented
priority inversion avoidance algorithm.

Rationale
The Java platform’s priority-preemptive dispatching model is very similar to thedispatching model found in the majority of commercial real-time operating systems.
However, the dispatching semantics were purposefully relaxed in order to allow
execution on a wide variety of operating systems. Thus, it is appropriate to specify
real-time threads by merely extending java.lang.Thread. The RealtimeParametersand MemoryParameters provided to the RealtimeThread constructor allow for anumber of common real-time thread types, including periodic threads.
The NoHeapRealtimeThread class is provided in order to allow time-criticalthreads to execute in preference to the garbage collector. The memory access andassignment semantics of the NoHeapRealtimeThread are designed to guarantee thatthe execution of such threads does not lead to an inconsistent heap state.

3.1

RealtimeThread

Syntax: public class RealtimeThread extends java.lang.Thread implements
Schedulable35

Direct Known Subclasses: NoHeapRealtimeThread26
All Implemented Interfaces: java.lang.Runnable, Schedulable35

REALTIMETHREAD
RealtimeThread extends java.lang.Thread and includes classes and methods
to get and set parameter objects, manage the execution of those threads with a
ReleaseParameters43 type of PeriodicParameters45 , and waiting. A
RealtimeThreadobject must be placed in a memory area such that thread logic may
unexceptionally access instance variables and such that Java methods on
java.lang.Thread (e.g., enumerate and join) complete normally except where suchexecution would cause access violations. (Implementation hint: They could beallocated in HeapMemory61 .)3.1.1

Constructors

public RealtimeThread()

Create a real-time thread. All parameter values are null. The default values
for null parameter objects are dependent on the value of the default
Scheduler36 at the time the thread is created.
public RealtimeThread(SchedulingParameters40 scheduling)
Create a real-time thread with the given SchedulingParameters40 .

Parameters:
scheduling - The SchedulingParameters40 associated with this(and possibly other RealtimeThread).
public RealtimeThread(SchedulingParameters40 scheduling,
ReleaseParameters43 release)
Create a real-time thread with the given SchedulingParameters40 andReleaseParameters43 .

Parameters:
scheduling - The SchedulingParameters40 associated with this(and possibly other RealtimeThread).
release - The ReleaseParameters43 associated with this (andpossibly other RealtimeThread).
public RealtimeThread(SchedulingParameters40 scheduling,
ReleaseParameters43 release,
MemoryParameters79 memory, MemoryArea60 area,
ProcessingGroupParameters50 group,java.lang.Runnable logic)

Create a real-time thread with the given characteristics and ajava.lang.Runnable .
Parameters:
scheduling - The SchedulingParameters40 associated with this(and possibly other RealtimeThread).

23

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Chapter 3 Threads
release - The ReleaseParameters43 associated with this (andpossibly other RealtimeThread ).
memory - The MemoryParameters79 associated with this (andpossibly other RealtimeThread ).
area - The MemoryArea60 associated with this.
group - The ProcessingGroupParameters50 associated with this(and possibly other RealtimeThread ).

3.1.2

Methods

public void addToFeasibility()

Inform the scheduler and cooperating facilities that this thread’s feasibility
parameters should be considered in feasibility analysis until further
notified.
public static RealtimeThread22 currentRealtimeThread()This will throw a ClassCastException if the current thread is not a
RealtimeThread.
public synchronized void deschedulePeriodic()
Stop unblocking public boolean waitForNextPeriod()26 for a periodic
schedulable object. If this does not have a type of PeriodicParameters45
as it ReleaseParameters43 nothing happens.
public MemoryArea60 getMemoryArea()
Get the current MemoryArea60 .

Returns: The current memory area in which allocations occur.
public MemoryParameters79 getMemoryParameters()
Return a reference to the MemoryParameters79 object.
public ProcessingGroupParameters50 getProcessingGroupParameters()
Return a reference to the ProcessingGroupParameters50 object.
public ReleaseParameters43 getReleaseParameters()
Returns a reference to the ReleaseParameters43 object.
public Scheduler36 getScheduler()

Get the scheduler for this thread.
public SchedulingParameters40 getSchedulingParameters()
Return a reference to the SchedulingParameters40 object.
public synchronized void interrupt()
Set the state of the generic AsynchronouslyInterruptedException134 topending.
Overrides: java.lang.Thread.interrupt() in class java.lang.Thread

REALTIMETHREAD

25

public void removeFromFeasibility()

Inform the scheduler and cooperating facilities that this thread’s feasibility
parameters should not be considered in feasibility analysis until further
notified.
public synchronized void schedulePeriodic()
Begin unblocking public boolean waitForNextPeriod()26 for aperiodic thread. Typically used when a periodic schedulable object is in an
overrun condition. The scheduler should recompute the schedule andperform admission control. If this does not have a type of
PeriodicParameters45 as it ReleaseParameters43 nothing happens.
public void setMemoryParameters(MemoryParameters79 parameters)
Set the reference to the MemoryParameters79 object.
public void setProcessingGroupParameters(ProcessingGroupParameters50
parameters)
Set the reference to the ProcessingGroupParameters50 object.
public void setReleaseParameters(ReleaseParameters43 parameters)
Set the reference to the ReleaseParameters43 object.
public void setScheduler(Scheduler36 scheduler)

Set the scheduler. This is a reference to the scheduler that will manage theexecution of this thread.
Throws: IllegalThreadStateException - Thrown when((Thread.isAlive() && Not Blocked) == true). (Where
blocked means waiting in Thread.wait(), Thread.join(), or
Thread.sleep())
public void setSchedulingParameters(SchedulingParameters40
scheduling)
Set the reference to the SchedulingParameters40 object.
public static void sleep(Clock110 clock, HighResolutionTime97 time)

An accurate timer with nanosecond granularity. The actual resolution
available for the clock must be queried from somewhere else. The timebase is the given Clock110 . The sleep time may be relative or absolute. If
relative, then the calling thread is blocked for the amount of time given by
the parameter. If absolute, then the calling thread is blocked until theindicated point in time. If the given absolute time is before the current time,
the call to sleep returns immediately.
Throws: InterruptedException

26

Chapter 3 Threads
public static void sleep(HighResolutionTime97 time)

An accurate timer with nanosecond granularity. The actual resolution
available for the clock must be queried from somewhere else. The timebase is the default Clock110 . The sleep time may be relative or absolute. If
relative, then the calling thread is blocked for the amount of time given by
the parameter. If absolute, then the calling thread is blocked until theindicated point in time. If the given absolute time is before the current time,
the call to sleep returns immediately.
Throws: InterruptedException
public boolean waitForNextPeriod()

Used by threads that have a reference to a ReleaseParameters43 type of
PeriodicParameters45 to block until the start of each period. Periods
start at either the start time in PeriodicParameters45 or when
this.start() is called. This method will block until the start of the next
period unless the thread is in either an overrun or deadline miss condition.
If both overrun and miss handlers are null and the thread has overrun its
cost or missed a deadline public boolean waitForNextPeriod()26 will
immediately return false once per overrun or deadline miss. It will thenagain block until the start of the next period (unless, of course, the thread
has overrun or missed again). If either the overrun or deadline misshandlers are not null and the thread is in either an overrun or deadline misscondition public boolean waitForNextPeriod()26 will block until thehandler corrects the situation (possibly by calling public synchronizedvoid schedulePeriodic()25 ). public boolean
waitForNextPeriod()26 throws IllegalThreadStateException if thisdoes not have a reference to a ReleaseParameters43 type of
PeriodicParameters45 .
Returns: True when the thread is not in an overrun or deadline misscondition and unblocks at the start of the next period.
Throws: IllegalThreadStateException

3.2

NoHeapRealtimeThread

Syntax: public class NoHeapRealtimeThread extends RealtimeThread22
All Implemented Interfaces: java.lang.Runnable, Schedulable35

A NoHeapRealtimeThread is a specialized form of RealtimeThread22 . Because
an instance of NoHeapRealtimeThread may immediately preempt any implemented

NOHEAPREALTIMETHREAD

garbage collector logic contained in its run() is never allowed to allocate or reference
any object allocated in the heap nor it is even allowed to manipulate the references toobjects in the heap. For example, if a and b are objects in immortal memory, b.p isreference to an object on the heap, and a.p is type compatible with b.p, then a
NoHeapRealtimeThread is not allowed to execute anyting like the following:
a.p = b.p; b.p = null;Thus, it is always safe for a NoHeapRealtimeThread to interrupt the garbage
collector at any time, without waiting for the end of the garbage collection cycle or adefined preemption point. Due to these restrictions, a NoHeapRealtimeThread object
must be placed in a memory area such that thread logic may unexceptionally accessinstance variables and such that Java methods on java.lang.Thread (e.g.,enumerate and join) complete normally except where execution would cause accessviolations. (Implementation hint: They could be allocated in ImmortalMemory62 .)The constructors of NoHeapRealtimeThread require a reference to ScopedMemory62
or ImmortalMemory62 . When the thread is started, all execution occurs in the scope ofthe given memory area. Thus, all memory allocation performed with the “new”
operator is taken from this given area.

3.2.1

Constructors

public NoHeapRealtimeThread(SchedulingParameters40 scheduling,
MemoryArea60 area)
Create a NoHeapRealtimeThread.

Parameters:
scheduling - A SchedulingParameters40 object that will be

associated with this. A null value means this will not have anassociated SchedulingParameters40 object.
area - A MemoryArea60 object. Must be a ScopedMemory62 or
ImmortalMemory62 type. A null value causes an
IllegalArgumentException to be thrown.
Throws: IllegalArgumentException
public NoHeapRealtimeThread(SchedulingParameters40 scheduling,
ReleaseParameters43 release, MemoryArea60 area)
Create a NoHeapRealtimeThread.

Parameters:
scheduling - A SchedulingParameters40 object that will be

associated with this. A null value means this will not have anassociated SchedulingParameters40 object.

27

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Chapter 3 Threads
release - A ReleaseParameters43 object that will be associatedwith this. A null value means this will not have an associatedReleaseParameters43 object.
area - A MemoryArea60 object. Must be a ScopedMemory62 or
ImmortalMemory62 type. A null value causes an
IllegalArgumentException to be thrown.
Throws: IllegalArgumentException
public NoHeapRealtimeThread(SchedulingParameters40 scheduling,
ReleaseParameters43 release,
MemoryParameters79 memory, MemoryArea60 area,
ProcessingGroupParameters50 group,java.lang.Runnable logic)
Create a NoHeapRealtimeThread.

Parameters:
scheduling - A SchedulingParameters40 object that will be

associated with this. A null value means this will not have anassociated SchedulingParameters40 object.
release - A ReleaseParameters43 object that will be associatedwith this. A null value means this will not have an associatedReleaseParameters43 object.
memory - A MemoryParameters79 object that will be associated withthis. A null value means this will not have a
MemoryParameters79 object.
area - A MemoryArea60 object. Must be a ScopedMemory62 or
ImmortalMemory62 type. A null value causes an
IllegalArgumentException to be thrown.
group - A ProcessingGroupParameters50 object that will be
associated with this. A null value means this will not have anassociated ProcessingGroupParameters50 object.
logic - A Runnable whose run() method will be executed for this.
Throws: IllegalArgumentException

RealtimeThread Example
The simplest way to create a thread is to accept the default parameters from theconstructor and override the run method with the desired behavior for the thread. Thiscan be done with a new class definition:
public class ReceiveThread extends RealtimeThread {public void run() {//logic for receive thread}

NOHEAPREALTIMETHREAD

The thread can then be created with:
RealtimeThread rt = new ReceiveThread();Thread rt will have normal priority, the value returned byPrioritySchedule.getNormPriority(). Thread rt will have no release or memory
parameters. Before starting the thread, use the isFeasible() method on Scheduler
to determine if there is a feasible schedule.
if (!rt.getScheduler().isFeasible())
throw new Exception(“Whatever...”);
rt.start();An alternative for creating the thread would be to use an anonymous inner class basedon RealtimeThread, overriding the run method. Here is an anonymous classimplementation:
RealtimeThread rt2 = new RealtimeThread() {public void run() {//logic for receive thread}

A thread can be created with just priority information:
SchedulingParameters sp =new PriorityParameters(PriorityScheduler.getNormPriority(null));
RealtimeThread t2 = new RealtimeThread(sp) {public void run() {//thread logic}

A real-time thread that is created with scheduling parameters but without releaseparameters will have no cost information available for feasibility analysis. The
scheduler doesn't perform admission control on these nonscheduled threads. When
doing static priority analysis, it's important to use a disjoint set of priorities for thestatically analyzed (scheduled) threads from the ones assigned to nonscheduledthreads, with the scheduled threads executing in preference to nonscheduled threads.
A logical division might be at PriorityScheduler.getNormPriority(), as
employed above, although this may unduly limit the range available for scheduledthreads in systems that provide the minimum number of real-time priorities.

29

CHAP TER

4

Scheduling
Chapter 4 SchedulingThis section contains classes that:• Allow the definition of schedulable objects.
• Manage the assignment of execution eligibility to schedulable objects.
• Perform feasibility analysis for sets of schedulable objects.
• Control the admission of new schedulable objects.
• Manage the execution of instances of the AsyncEventHandler andRealtimeThread classes.
• Assign release characteristics to schedulable objects.
• Assign execution eligibility values to schedulable objects.
• Define temporal containers used to enforce correct temporal behavior of multiple
schedulable objects.
The scheduler required by this specification is fixed-priority preemptive with 28
unique priority levels. It is represented by the class PriorityScheduler and is called
the base scheduler.
The schedulable objects required by this specification are defined by the classes
RealtimeThread, NoHeapRealtimeThread, and AsyncEventHandler. Each of these isassigned processor resources according to their release characteristics, execution
eligibility, and processing group values. Any subclass of these objects or any classimplementing the Schedulable interface are schedulable objects and behave as theserequired classes.
An instance of the SchedulingParameters class contains values of execution
eligibility. A schedulable object is considered to have the execution eligibility in the
SchedulingParameters object used in the constructor of the schedulable object. For
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CHAPTER 4 SCHEDULING

implementations providing only the base scheduling policy, the previous statement
holds for the specific type PriorityParameters (a subclass of
SchedulingParameters). If an implementation provides additional schedulingpolicies or execution eligibility assignment policies which require an application
visible field to contain the execution eligibility value, then SchedulingParamtersmust be subclassed and the previous statement holds for the specific subclass type. If,
however, additionally provided scheduling policies or execution eligibility
assignment policies do not require application visibility of execution eligibility or it
appears in another parameter object (e.g., the earliest deadline first scheduling uses
deadline as the execution eligibility metric and would thus be visible inReleaseParameters), then SchedulingParameters need not be subclassed.
An instance of the ReleaseParameters class or its subclasses,
PeriodicParameters, AperiodicParameters, and SporadicParameters, contains
values that define a particular release discipline. A schedulable object is considered tohave the release characteristics of a single associated instance of theReleaseParameters class. In all cases the Scheduler uses these values to perform its
feasibility analysis over the set of schedulable objects and admission control for theschedulable object. Additionally, for those schedulable objects whose associatedinstance of ReleaseParameters is an instance of PeriodicParameters, thescheduler manages the behavior of the object’s waitForNextPeriod() method andmonitors overrun and deadline-miss conditions. In the case of overrun or deadlinemiss the scheduler changed the behavior of the waitForNextPeriod()and schedules
the appropriate handler.
An instance of the ProcessingGroupParameters class contains values thatdefine a temporal scope for a processing group. If a schedulable object has an
associated instance of the ProcessingGroupParameters class, it is said to execute
within the temporal scope defined by that instance. A single instance of theProcessingGroupParameters class can be (and typically is) associated with manyschedulable objects. The combined processor demand of all of the schedulable objects
associated with an instance of the ProcessingParameters class must not exceed thevalues in that instance (i.e., the defined temporal scope). The processor demand isdetermined by the Scheduler.
Semantics and Requirements

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section and the required scheduling algorithm. Semantics that apply toparticular classes, constructors, methods, and fields will be found in the classdescription and the constructor, method, and field detail sections.
1. The base scheduler must support at least 28 unique values in the priorityLevel

SEMANTICS AND REQUIREMENTS

field of an instance of PriorityParameters.
2. Higher values in the priorityLevel field of an instance of PriorityParameters
have a higher execution eligibility.
3. In (1) unique means that if two schedulable objects have different values in thepriorityLevel field in their respective instances of PriorityParameters, theschedulable object with the higher value will always execute in preference to theschedulable object with the lower value when both are ready to execute.
4. An implementation must make available some native priorities which are lower
than the 28 required real-time priorities. These are to be used for regular Java
threads (i.e., instances of threads which are not instances of RealtimeThread,
NoHeapRealtimeThread, or AsyncEventHandler classes or subclasses). The ten
traditional Java thread priorities may have an arbitrary mapping into the native
priorities. These ten traditional Java thead priorities and the required minimum 28
unique real-time thread priorities shall be from the same space. Assignment of
any of these (minimum) 38 priorities to real-time threads or traditional Java
threads is legal. It is the responsibility of application logic to make rational
priority assignments.
5. The dispatching mechanism must allow the preemption of the execution ofschedulable objects at a point not governed by the preempted object.
6. For schedulable objects managed by the base scheduler no part of the system maychange the execution eligibility for any reason other than implementation of apriority inversion algorithm. This does not preclude additional schedulers fromchanging the execution eligibility of schedulable objects — which they manage— according to the scheduling algorithm.
7. Threads that are preempted in favor of a higher priority thread may be placed in
the appropriate queue at any position as determined by a particular
implementation. The implementation is required to provide documentation
stating exactly the algorithm used for such placement.
8. If an implementation provides any schedulers other than the base scheduler it
shall provide documentation explicitly stating the semantics expressed by 8
through 11 in language and constructs appropriate to the provided schedulingalgorithms.
9. All instances of RelativeTime used in instances of ProcessingParameters,
SchedulingParameters, and ReleaseParameters are measured from the time atwhich the associated thread (or first such thread) is started.
10. PriorityScheduler.getNormPriority() shall be set to((PriorityScheduler.getMaxPriority() PriorityScheduler.getMinPriority())/3) +PriorityScheduler.getMinPriority().

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CHAPTER 4 SCHEDULING

11. If instances of RealtimeThread or NoHeapRealtimeThread are constructedwithout a reference to a SchedulingParameters object a SchedulingParamtersobject is created and assigned the values of the current thread. This does not
imply that other schedulers should follow this rule. Other schedulers are free todefine the default scheduling parameters in the absence of a given
SchedulingParameters object.
12. The policy and semantics embodied in 1 through 15 above and by thedescriptions of the refered to classes, methods, and their interactions must be
available in all implementations of this specification.
13. This specification does not require any particular feasibility algorithm be
implemented in the Scheduler object. Those implementations that choose to not
implement a feasibility algorithm shall return success whenever the feasibility
algorithm is executed.
14. Implementations that provide a scheduler with a feasibility algorithm are required
to clearly document the behavior of that algorithm
The following hold for the PriorityScheduler:
1. A blocked thread that becomes ready to run is added to the tail of any runnable
queue for that priority.
2. For a thread whose effective priority is changed as a result of explicitly setting
priorityLevel this thread or another thread is added to the tail of the runnable
queue for the new priorityLevel.
3. A thread that performs a yield() goes to the tail of the runnable queue for its
priorityLevel.

Rationale
As specified the required semantics and requirements of this section establish ascheduling policy that is very similar to the scheduling policies found on the vast
majority of real-time operating systems and kernels in commercial use today. By
requirement 16, the specification accommodates existing practice, which is a statedgoal of the effort.
The semantics of the classes, constructors, methods, and fields within allow for
the natural extension of the scheduling policy by implementations that provide
different scheduler objects.
Some research shows that, given a set of reasonable common assumptions, 32
unique priority levels are a reasonable choice for close-to-optimal schedulingefficiency when using the rate-monotonic priority assignment algorithm (256 priority
levels better provide better efficiency). This specification requires at least 28 unique

SCHEDULABLE

35

priority levels as a compromise noting that implementations of this specification will
exist on systems with logic executing outside of the Java Virtual Machine and mayneed priorities above, below, or both for system activities.

4.1

Schedulable

Syntax: public interface Schedulable extends java.lang.Runnable
All Superinterfaces: java.lang.Runnable
All Known Implementing Classes: AsyncEventHandler127, RealtimeThread22

Handlers and other objects can be run by a Scheduler36 if they provide a run()method and the methods defined below. The Scheduler36 uses this information tocreate a suitable context to execute the run() method.

4.1.1

Methods

public void addToFeasibility()
Inform the Scheduler36 and cooperating facilities that this thread’s

feasibility parameters should be considered in feasibility analysis until
further notified.
public MemoryParameters79 getMemoryParameters()
Return the MemoryParameters79 of this schedulable object.
public ReleaseParameters43 getReleaseParameters()
Return the ReleaseParameters43 of this schedulable object.
public Scheduler36 getScheduler()
Return the Scheduler36 for this schedulable object.
public SchedulingParameters40 getSchedulingParameters()
Return the SchedulingParameters40 of this schedulable object.
public void removeFromFeasibility()
Inform the Scheduler36 and cooperating facilities that this thread’s

feasibility parameters should not be considered in feasibility analysis until
further notified.
public void setMemoryParameters(MemoryParameters79 memory)
Set the MemoryParameters79 of this schedulable object.

Parameters:
memory - The MemoryParameters79 object. If null nothing happens.

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CHAPTER 4 SCHEDULING
public void setReleaseParameters(ReleaseParameters43 release)
Set the ReleaseParameters43 for this schedulable object.

Parameters:
release - The ReleaseParameters43 object. If null nothing

happens.
public void setScheduler(Scheduler36 scheduler)
Set the Scheduler36 for this schedulable object.

Parameters:
scheduler - The Scheduler36 object. If null nothing happens.
public void setSchedulingParameters(SchedulingParameters40
scheduling)
Set the SchedulingParameters40 of this scheduable object.

Parameters:
scheduling - The SchedulingParameters40 object. If null nothing

happens.

4.2

Scheduler

Syntax: public abstract class Scheduler
Direct Known Subclasses: PriorityScheduler38

An instance of Scheduler manages the execution of schedulable objects and may
implement a feasibility algorithm. The feasibility algorithm may determine if the
known set of schedulable objects, given their particular execution ordering (or priority
assignment), is a feasible schedule. Subclasses of Scheduler are used for alternative
scheduling policies and should define an instance() class method to return thedefault instance of the subclass. The name of the subclass should be descriptive of thepolicy, allowing applications to deduce the policy available for the scheduler obtained
via public static Scheduler36 getDefaultScheduler()37 (e.g., EDFScheduler).

4.2.1

Constructors

public Scheduler()

4.2.2

Methods

protected abstract void addToFeasibility(Schedulable35 schedulable)
Inform the scheduler that this thread’s ReleaseParameters43 should beconsidered in feasibility analysis until further notified.

SCHEDULER

37

public boolean changeIfFeasible(Schedulable35 schedulable,
ReleaseParameters43 release,
MemoryParameters79 memory)
Returns true if, after changing the Schedulable35 ’s release and GC

parameters isFeasible would return true. The parameters wil be changed. If
the resulting system would not be feasible, this method returns false and nochanges are made.
Parameters:
schedulable - The Schedulable35 object for which to check

admittance. If null nothing happens.
release - The proposed ReleaseParameters43 . If null, no change
is made.
memory - The proposed MemoryParameters79 . If null, no change ismade.
public static Scheduler36 getDefaultScheduler()

Return a reference to the default scheduler.
public abstract java.lang.String getPolicyName()
Used to determine the policy of the Scheduler.

Returns: A String object which is the name of the scheduling policy usedby this.
public abstract boolean isFeasible()

Returns true if and only if the system is able to satisfy the constraints
expressed in the release parameters of the existing schedulable objects.
protected abstract void removeFromFeasibility(Schedulable35
schedulable)
Inform the scheduler that this thread’s ReleaseParameters43 should not

be considered in feasibility analysis until further notified.
public static void setDefaultScheduler(Scheduler36 scheduler)

Set the default scheduler. This is the scheduler given to instances ofRealtimeThread22 when they are constructed. The default scheduler is set
to the required PriorityScheduler38 at startup.
Parameters:
scheduler - The Scheduler that becomes the default scheduler

assigned to new threads. If null nothing happens.

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CHAPTER 4 SCHEDULING

4.3

PriorityScheduler

Syntax: public class PriorityScheduler extends Scheduler36
Class for priority-based scheduling. The default instance is the required priority
scheduler which does fixed priority, preemptive scheduling.

4.3.1

Constructors

public PriorityScheduler()

4.3.2

Methods

protected void addToFeasibility(Schedulable35 s)
Inform the scheduler that this thread’s ReleaseParameters43 should beconsidered in feasibility analysis until further notified.
Overrides: protected abstract void
addToFeasibility(Schedulable35 schedulable)36 in class
Scheduler36
public boolean changeIfFeasible(Schedulable35 schedulable,
ReleaseParameters43 release,
MemoryParameters79 memory)

Returns true if, after changing the Schedulable35 ’s release and GC
parameters isFeasible would return true. The parameters wil be changed.
If the resulting system would not be feasible, this method returns false andno changes are made.
Overrides: public boolean changeIfFeasible(Schedulable35
schedulable, ReleaseParameters43 release,
MemoryParameters79 memory)37 in class Scheduler36

Parameters:
schedulable - The Schedulable35 object for which to check

admittance. If null nothing happens.
release - The proposed ReleaseParameters43 . If null, no change

is made.
memory - The proposed MemoryParameters79 . If null, no change is

made.

PRIORITYSCHEDULER
public void fireSchedulable(Schedulable35 schedulable)
Triggers the execution of a Schedulable35 object (like an
AsyncEventHandler127 ).

Parameters:
schedulable - The Schedulable35 object to make active.
public int getMaxPriority()

Returns the maximum priority available for a thread managed by thisscheduler.
public static int getMaxPriority(java.lang.Thread thread)
If the given thread is scheduled by the required PriorityScheduler the
maximum priority of the PriorityScheduler is returned otherwise

Thread.MAX_PRIORITY is returned.
Parameters:
thread - An instance of Thread. If null the maximum priority of therequired PriorityScheduler is returned.
public int getMinPriority()

Returns the minimum priority available for a thread managed by thisscheduler.
public static int getMinPriority(java.lang.Thread thread)
If the given thread is scheduled by the required PriorityScheduler theminimum priority of the PriorityScheduler is returned otherwise

Thread.MIN_PRIORITY is returned.
Parameters:
thread - An instance of Thread. If null the minimum priority of therequired PriorityScheduler is returned.
public int getNormPriority()

Returns the normal priority available for a thread managed by thisscheduler.
public static int getNormPriority(java.lang.Thread thread)
If the given thread is scheduled by the required PriorityScheduler thenormal priority of the PriorityScheduler is returned otherwise

Thread.NORM_PRIORITY is returned.
Parameters:
thread - An instance of Thread. If null the normal priority of therequired PriorityScheduler is returned.

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CHAPTER 4 SCHEDULING
public java.lang.String getPolicyName()

Used to determine the policy of the Scheduler.
Overrides: public abstract java.lang.String getPolicyName()37 inclass Scheduler36
Returns: A String object which is the name of the scheduling policy usedby this.
public static PriorityScheduler38 instance()
Return a pointer to an instance of PriorityScheduler.
public boolean isFeasible()

Returns true if the system is able to satisfy the constraints expressed in therelease parameters of the existing schedulable objects.
Overrides: public abstract boolean isFeasible()37 in class
Scheduler36
protected void removeFromFeasibility(Schedulable35 s)
Inform the scheduler that this thread’s ReleaseParameters43 should not

be considered in feasibility analysis until further notified.
Overrides: protected abstract void
removeFromFeasibility(Schedulable35 schedulable)37 inclass Scheduler36

4.4

SchedulingParameters

Syntax: public abstract class SchedulingParameters
Direct Known Subclasses: PriorityParameters41

Subclasses of SchedulingParameters (PriorityParameters41 ,ImportanceParameters42 , and any others defined for particular schedulers) provide
the parameters to be used by the Scheduler36 . Changes to the values in a parameters
object affects the scheduling behaviour of all the Schedulable35 objects to which it
is bound.
Caution: Subclasses of this class are explicitly unsafe in multithreaded situations
when they are being changed. No synchronization is done. It is assumed that users of
this class who are mutating instances will be doing their own synchronization at ahigher level.

PRIORITYPARAMETERS

4.4.1

Constructors

public SchedulingParameters()

4.5

PriorityParameters

Syntax: public class PriorityParameters extends SchedulingParameters40
Direct Known Subclasses: ImportanceParameters42

Instances of this class should be assigned to threads that are managed byschedulers which use a single integer to determine execution order. The basescheduler required by this specification and represented by the class
PriorityScheduler38 is such a scheduler.

4.5.1

Constructors

public PriorityParameters(int priority)
Create an instance of SchedulingParameters40 with the given priority.

Parameters:
priority - The priority assigned to a thread. This value is used inplace of the value returned byjava.lang.Thread.setPriority(int) .

4.5.2

Methods

public int getPriority()

Get the priority.
public void setPriority(int priority)

Set the priority.
Parameters:
priority - The new value of priority.

Throws: IllegalArgumentException - Thrown if the given priority value
is less than the minimum priority of the scheduler of any of theassociated threads or greater then the maximum priority of thescheduler of any of the associated threads.
public java.lang.String toString()

Overrides: java.lang.Object.toString() in class java.lang.Object

41

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CHAPTER 4 SCHEDULING

4.6

ImportanceParameters

Syntax: public class ImportanceParameters extends PriorityParameters41
Importance is an additional scheduling metric that may be used by some prioritybased scheduling algorithms during overload conditions to differentiate execution
order among threads of the same priority.
In some real-time systems an external physical process determines the period ofmany threads. If rate-monotonic priority assignment is used to assign priorities manyof the threads in the system may have the same priority because their periods are thesame. However, it is conceivable that some threads may be more important thanothers and in an overload situation importance can help the scheduler decide which
threads to execute first. The base scheduling algorithm represented byPriorityScheduler38 is not required to use importance. However, the RTSJ strongly
suggests to implementers that a fairly simple subclass of PriorityScheduler38 thatuses importance can offer value to some real-time applications.

4.6.1

Constructors

public ImportanceParameters(int priority, intimportance)
Create an instance of ImportanceParameters.

Parameters:
priority - The priority assigned to a thread. This value is used inplace of java.lang.Thread.priority.
importance - The importance value assigned to a thread.

4.6.2

Methods

public int getImportance()

Get the importance value.
public void setImportance(int importance)

Set the importance.
public java.lang.String toString()

Overrides: public java.lang.String toString()41 in class
PriorityParameters41

RELEASEPARAMETERS

4.7

ReleaseParameters

Syntax: public abstract class ReleaseParameters
Direct Known Subclasses: AperiodicParameters47, PeriodicParameters45

The abstract top-level class for release characteristics of threads. When areference to a ReleaseParameters object is given as a parameter to a constructor, theReleaseParameters object becomes bound to the object being created. Changes tothe values in the ReleaseParameters object affect the constructed object. If given to
more than one constructor, then changes to the values in the ReleaseParameters
object affect all of the associated objects. Note that this is a one-to-many relationshipand not a many-to-many.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.
Caution: The cost parameter time should be considered to be measured against
the target platform.

4.7.1

Constructors

protected ReleaseParameters(RelativeTime102 cost,
RelativeTime102 deadline,
AsyncEventHandler127 overrunHandler,
AsyncEventHandler127 missHandler)

Subclasses use this constructor to create a ReleaseParameters type object.
Parameters:
cost - Processing time units per interval. On implementations which
can measure the amount of time a schedulable object is
executed, this value is the maximum amount of time aschedulable object receives per interval. On implementations
which cannot measure execution time, this value is used as ahint to the feasibility algorithm. On such systems it is notpossible to determine when any particular object exceeds cost.
Equivalent to RelativeTime(0,0) if null.
deadline - The latest permissible completion time measured fromthe release time of the associated invocation of the schedulable
object. Changing the deadline might not take effect after the
expiration of the current deadline. More detail provided in the
subclasses.

43

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CHAPTER 4 SCHEDULING
overrunHandler - This handler is invoked if an invocation of theschedulable object exceeds cost. Not required for minimum
implementation. If null, nothing happens on the overrun
condition, and waitForNextPeriod returns false immediately andupdates the start time for the next period.
missHandler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed. Although minimum implementations do not consider
deadlines in feasibility calculations, they must recognize
variable deadlines and invoke the miss handler as appropriate. Ifnull, nothing happens on the miss deadline condition.

4.7.2

Methods

public RelativeTime102 getCost()

Get the cost value.
public AsyncEventHandler127 getCostOverrunHandler()

Get the cost overrun handler.
public RelativeTime102 getDeadline()

Get the deadline.
public AsyncEventHandler127 getDeadlineMissHandler()

Get the deadline miss handler.
public void setCost(RelativeTime102 cost)

Set the cost value.
Parameters:
cost - Processing time units per period or per minimum interarrival
interval. On implementations which can measure the amount oftime a schedulable object is executed, this value is the maximum
amount of time a schedulable object receives per period or per
minimum interarrival interval. On implementations whichcannot measure execution time, this value is used as a hint to thefeasibility algorithm. On such systems it is not possible todetermine when any particular object exceeds or will exceed
cost time units in a period or interval. Equivalent toRelativeTime(0,0) if null.

PERIODICPARAMETERS
public void setCostOverrunHandler(AsyncEventHandler127 handler)

Set the cost overrun handler.
Parameters:
handler - This handler is invoked if an invocation of theschedulable object exceeds cost. Not required for minimum
implementation. See comments in setCost().
public void setDeadline(RelativeTime102 deadline)

Set the deadline value.
Parameters:
deadline - The latest permissible completion time measured fromthe release time of the associated invocation of the schedulable
object. For a minimum implementation for purposes of
feasibility analysis, the deadline is equal to the period orminimum interarrival interval. Other implementations may use
this parameter to compute execution eligibility.
public void setDeadlineMissHandler(AsyncEventHandler127 handler)

Set the deadline miss handler.
Parameters:
handler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed. Although minimum implementations do not consider
deadlines in feasibility calculations, they must recognize
variable deadlines and invoke the miss handler as appropriate.

4.8

PeriodicParameters

Syntax: public class PeriodicParameters extends ReleaseParameters43
This release parameter indicates that the public boolean
waitForNextPeriod()26 method on the associated Schedulable35 object will be
unblocked at the start of each period. When a reference to a PeriodicParameters
object is given as a parameter to a constructor the PeriodicParameters object
becomes bound to the object being created. Changes to the values in the
PeriodicParameters object affect the constructed object. If given to more than one
constructor then changes to the values in the PeriodicParameters object affect all of

the associated objects. Note that this is a one-to-many relationship and not a many-tomany.

45

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CHAPTER 4 SCHEDULING

Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

4.8.1

Constructors

public PeriodicParameters(HighResolutionTime97 start,
RelativeTime102 period, RelativeTime102 cost,
RelativeTime102 deadline,
AsyncEventHandler127 overrunHandler,
AsyncEventHandler127 missHandler)
Create a PeriodicParameters object.

Parameters:
start - Time at which the first period begins. If a RelativeTime102 ,this time is relative to the first time the schedulable object
becomes schedulable (schedulable time) (e.g., when start() iscalled on a thread). If an AbsoluteTime99 and it is before theschedulable time, start is equivalent to the schedulable time.
period - The period is the interval between successive unblocks ofpublic boolean waitForNextPeriod()26 . Must be greaterthan zero when entering feasibility analysis.
cost - Processing time per period. On implementations which canmeasure the amount of time a schedulable object is executed,
this value is the maximum amount of time a schedulable object
receives per period. On implementations which cannot measureexecution time, this value is used as a hint to the feasibility
algorithm. On such systems it is not possible to determine when
any particular object exceeds or will exceed cost time units in aperiod. Equivalent to RelativeTime(0,0) if null.
deadline - The latest permissible completion time measured fromthe release time of the associated invocation of the schedulable
object. For a minimum implementation for purposes of
feasibility analysis, the deadline is equal to the period. Other
implementations may use this parameter to compute execution
eligibility. If null, deadline will equal the period.
overrunHandler - This handler is invoked if an invocation of theschedulable object exceeds cost in the given period. Not
required for minimum implementation. If null, nothing happens
on the overrun condition.

APERIODICPARAMETERS
missHandler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed. Although minimum implementations do not consider
deadlines in feasibility calculations, they must recognize
variable deadlines and invoke the miss handler as appropriate. Ifnull, nothing happens on the miss deadline condition.

4.8.2

Methods

public RelativeTime102 getPeriod()Get the period.
public HighResolutionTime97 getStart()

Get the start time.
public void setPeriod(RelativeTime102 period)

Set the period.
Parameters:
period - The period is the interval between successive unblocks ofpublic boolean waitForNextPeriod()26 . Also used in thefeasibility analysis and admission control algorithms.
public void setStart(HighResolutionTime97 start)

Set the start time.
Parameters:
start - Time at which the first period begins.

4.9

AperiodicParameters

Syntax: public class AperiodicParameters extends ReleaseParameters43
Direct Known Subclasses: SporadicParameters49

This release parameter object characterizes a schedulable object that may
become active at any time. When a reference to a AperiodicParameters47 object isgiven as a parameter to a constructor the AperiodicParameters47 object becomesbound to the object being created. Changes to the values in the
AperiodicParameters47 object affect the constructed object. If given to more than
one constructor then changes to the values in the AperiodicParameters47 object
affect all of the associated objects. Note that this is a one-to-many relationship and nota many-to-many.

47

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CHAPTER 4 SCHEDULING

Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

4.9.1

Constructors

public AperiodicParameters(RelativeTime102 cost,
RelativeTime102 deadline,
AsyncEventHandler127 overrunHandler,
AsyncEventHandler127 missHandler)
Create an AperiodicParameters47 object.

Parameters:
cost - Processing time per invocation. On implementations which
can measure the amount of time a schedulable object is
executed, this value is the maximum amount of time aschedulable object receives. On implementations which cannot
measure execution time, this value is used as a hint to thefeasibility algorithm. On such systems it is not possible todetermine when any particular object exceeds cost. Equivalent
to RelativeTime(0,0) if null.
deadline - The latest permissible completion time measured fromthe release time of the associated invocation of the schedulable
object. Not used in feasibility analysis for minimum
implementation. If null, the deadline will be
RelativeTime(Long.MAX_VALUE,999999).
overrunHandler - This handler is invoked if an invocation of theschedulable object exceeds cost. Not required for minimum
implementation. If null, nothing happens on the overrun
condition.
missHandler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed. Although minimum implementations do not consider
deadlines in feasibility calculations, they must recognize
variable deadlines and invoke the miss handler as appropriate. Ifnull, nothing happens on the miss deadline condition.

SPORADICPARAMETERS

4.10 SporadicParameters
Syntax: public class SporadicParameters extends AperiodicParameters47
A notice to the scheduler that the associated schedulable object’s run method will
be released aperiodically but with a minimum time between releases. When areference to a SporadicParameters object is given as a parameter to a constructor,
the SporadicParameters object becomes bound to the object being created. Changes
to the values in the SporadicParameters object affect the constructed object. If given
to more than one constructor, then changes to the values in the SporadicParameters
object affect all of the associated objects. Note that this is a one-to-many relationshipand not a many-to-many.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

4.10.1 Constructors
public SporadicParameters(RelativeTime102 minInterarrival,
RelativeTime102 cost, RelativeTime102 deadline,
AsyncEventHandler127 overrunHandler,
AsyncEventHandler127 missHandler)
Create a SporadicParameters object.

Parameters:
minInterarrival - The release times of the schedulable object will

occur no closer than this interval. Must be greater than zero
when entering feasibility analysis.
cost - Processing time per minimum interarrival interval. On
implementations which can measure the amount of time aschedulable object is executed, this value is the maximum
amount of time a schedulable object receives per interval. On
implementations which cannot measure execution time, thisvalue is used as a hint to the feasibility algorithm. On such
systems it is not possible to determine when any particular
object exceeds cost. Equivalent to RelativeTime(0,0) if null.
deadline - The latest permissible completion time measured fromthe release time of the associated invocation of the schedulable
object. For a minimum implementation for purposes of
feasibility analysis, the deadline is equal to the minimum
interarrival interval. Other implementations may use this

49

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CHAPTER 4 SCHEDULING

parameter to compute execution eligibility. If null, deadline will
equal the minimum interarrival time.
overrunHandler - This handler is invoked if an invocation of theschedulable object exceeds cost. Not required for minimum
implementation. If null, nothing happens on the overrun
condition.
missHandler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed. Although minimum implementations do not consider
deadlines in feasibility calculations, they must recognize
variable deadlines and invoke the miss handler as appropriate. Ifnull, nothing happens on the miss deadline condition.

4.10.2 Methods
public RelativeTime102 getMinimumInterarrival()

Get the minimum interarrival time.
public void setMinimumInterarrival(RelativeTime102 minimum)

Set the minimum interarrival time.
Parameters:
minimum - The release times of the schedulable object will occur nocloser than this interval. Must be greater than zero whenentering feasibility analysis.

4.11 ProcessingGroupParameters
Syntax: public class ProcessingGroupParameters
This is associated with one or more schedulable objects for which the systemguarantees that the associated objects will not be given more time per period than
indicated by cost. For all threads with a reference to an instance ofProcessingGroupParameters p and a reference to an instance ofAperiodicParameters47 no more than p.cost will be allocated to the execution ofthese threads in each interval of time given by p.period after the time indicated byp.start. When a reference to a ProcessingGroupParameters object is given as aparameter to a constructor the ProcessingGroupParameters object becomes boundto the object being created. Changes to the values in theProcessingGroupParameters object affect the constructed object. If given to morethan one constructor, then changes to the values in the ProcessingGroupParameters

PROCESSINGGROUPPARAMETERS

object affect all of the associated objects. Note that this is a one-to-many relationshipand not a many-to-many.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.
Caution: The cost parameter time should be considered to be measured against
the target platform.

4.11.1 Constructors
public ProcessingGroupParameters(HighResolutionTime97 start,
RelativeTime102 period, RelativeTime102 cost,
RelativeTime102 deadline,
AsyncEventHandler127 overrunHandler,
AsyncEventHandler127 missHandler)
Create a ProcessingGroupParameters object.

Parameters:
start - Time at which the first period begins.
period - The period is the interval between successive unblocks of
waitForNextPeriod().
cost - Processing time per period.
deadline - The latest permissible completion time measured fromthe start of the current period. Changing the deadline might not
take effect after the expiration of the current deadline.
overrunHandler - This handler is invoked if the run() method of
the schedulable object of the previous period is still executing at
the start of the current period.
missHandler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed.

4.11.2 Methods
public RelativeTime102 getCost()

Get the cost value.
public AsyncEventHandler127 getCostOverrunHandler()

Get the cost overrun handler.
Returns: An AsyncEventHandler127 object that is cost overrun handler of
this.

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CHAPTER 4 SCHEDULING
public RelativeTime102 getDeadline()

Get the deadline value.
Returns: A RelativeTime102 object that represents the deadline of this.
public AsyncEventHandler127 getDeadlineMissHandler()

Get the deadline missed handler.
Returns: An AsyncEventHandler127 object that is deadline miss handlerof this.
public RelativeTime102 getPeriod()Get the period.
Returns: A RelativeTime102 object that represents the period of time ofthis.
public HighResolutionTime97 getStart()

Get the start time.
Returns: A HighResolutionTime97 object that represents the start time ofthis.
public void setCost(RelativeTime102 cost)

Set the cost value.
Parameters:
cost - The schedulable objects with a reference to this receive
cumulatively no more than cost time per period on
implementations that can collect execution time per thread.
public void setCostOverrunHandler(AsyncEventHandler127 handler)

Set the cost overrun handler.
Parameters:
handler - This handler is invoked if the run() method of theschedulable object of the previous period is still executing at thestart of the current period.
public void setDeadline(RelativeTime102 deadline)

Set the deadline value.
Parameters:
deadline - The latest permissible completion time measured fromthe start of the current period. Not used in a minimum
implementation. Other implmentations may use this parameterto compute execution eligibility. The default value is the same asperiod.

PROCESSINGGROUPPARAMETERS
public void setDeadlineMissHandler(AsyncEventHandler127 handler)

Set the deadline miss handler.
Parameters:
handler - This handler is invoked if the run() method of theschedulable object is still executing after the deadline has
passed.
public void setPeriod(RelativeTime102 period)

Set the period.
Parameters:
period - Interval used to enforce allocation of processing resources

to the associated schedulable objects. Also used in the feasibility
analysis and admission control algorithms.
public void setStart(HighResolutionTime97 start)

Set the start time.
Parameters:
start - Time at which the first period begins.

Scheduler Example
An implementation may provide a scheduler other than the required minimum
scheduler. If you wish to use that scheduler to manage your threads, you need to findout about the alternative scheduler. In some cases, the alternative scheduler may beinstalled as the default scheduler for the implementation. In others, it may benecessary to locate the scheduler in order to use it to schedule threads. The following
method shows how a scheduler implementing a policy can be located and the instanceto the singleton object obtained:

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public static Scheduler findScheduler(String policy) {String className = System.getProperty(“javax.realtime.scheduler.
” +policy);Class clazz;try {if (className != null&& (clazz = Class.forName(className)) != null) {return (Scheduler)clazz.getMethod(“instance”,null).invoke(null,null);}} catch (ClassNotFoundException notFound) {} catch (NoSuchMethodException noSuch) {} catch (SecurityException security) {} catch (IllegalAccessException access) {} catch (IllegalArgumentException arg) {} catch (InvocationTargetException target) {}return null;}

To find, say, an EDF scheduler, the above method requires that the system property
javax.realtime.scheduler.EDF have been set to the fully qualified class name for
the EDF scheduler class. Thus, to get an EDF scheduler and use it to schedule aperiodic thread, t1, we do:
Scheduler scheduler = findScheduler(“EDF”);
if (scheduler != null) {RealtimeThread t1 =new RealtimeThread(null, /* default scheduling parameters */new PeriodicParameters(
null, /*start immediately*/
new RelativeTime(100, 0), /* period */new RelativeTime(5, 0), /* cost */new RelativeTime(50, 0), /* deadline */null,null),null,
null,null) {
public void run() {//thread processing}

PROCESSINGGROUPPARAMETERS

Once the scheduler is found, it is also possible to set it as the default scheduler for all
subsequent thread creations. This is done with a call to
Scheduler.setDefaultScheduler():try {Scheduler.setDefaultScheduler(scheduler);} catch (SecurityException security) {};Finally, you can test the current default scheduler to see if it implements thescheduling policy you want:boolean useEDF = false;try {if (Scheduler.getDefaultScheduler().getPolicyName().equals(“EDF”)) {Life is grand, use EDF to your heart’s content.
useEDF = true;

ProcessingGroup Example
Processing groups are used to provide information to the scheduler about aperiodic orsporadic activities — either threads or asynchronous event handlers — for the
purposes of the feasibility analysis. The processing group carries information about
the cost, period and deadline associated with aperiodic or sporadic activities that havebeen grouped together for the purposes of completing the analysis. The following will
identify a processing group that allows for up to 10 milliseconds of execution during
each 100 millisecond interval:
SchedulingParameters pp =new PriorityParameters(PriorityScheduler.getNormPriority());
ProcessingGroupParameters group =new ProcessingGroupParameters(null, /* start when released */new RelativeTime(100, 0), /* period */new RelativeTime(10, 0), /* cost */null, /* deadline == period */null, /* cost overrun handler */null); /* deadline miss handler */Every thread that is created within this processing group should have a reference tothe same processing group parameters object. The identity of the object is important
to convey to the feasibility algorithm what group it is, in addition to the information

55

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CHAPTER 4 SCHEDULING

about the group itself, so that the cost and period aren’t accounted for more than once.
Thus, after the first thread is added:
RealtimeThread t1 = new RealtimeThread(pp, /* priority parameters */new AperiodicParameters(
new RelativeTime(10,0), /* cost */new RelativeTime(300, 0), /* deadline */null, /* cost overrun handler */null), /* deadline miss handler */null, /* memory parameters */group,null) {
public void run() {//do thread task}

We can add a second thread that goes in the same group:
RealtimeThread t2 = new RealtimeThread(pp, /* priority parameters */new AperiodicParameters(
new RelativeTime(5,0), /* cost */new RelativeTime(200, 0), /* deadline */null, /* cost overrun handler */null), /* deadline miss handler */null, /* memory parameters */group,null) {
public void run() {//do thread task}

The priority of the PriorityParameters should be assigned according to the periodof the processing group (relative to the periods of other periodic activities). This willaffect both threads:
sp.setPriority(GROUP_PRIORITY);
t1.start();t2.start();Both threads are now running. If the implementation can accumulate execution time
per thread then, if either of these threads consumes more than 10ms in any period of
this group, the cost overrun handler will be invoked. On the other hand, if theimplementation cannot accumulate execution time per thread, then the deadline misshandler will be invoked if either thread is active at the end of the period of the group.

CHAP TER

5

Memory Management
Chapter 5 Memory Management

This section contains classes that:• Allow the definition of regions of memory outside of the traditional Java heap.
• Allow the definition of regions of scoped memory, that is, memory regions with alimited lifetime.
• Allow the definition of regions of memory containing objects whose lifetime
matches that of the application.
• Allow the definition of regions of memory mapped to specific physical addresses.
• Allow the specification of maximum memory area consumption and maximum
allocation rates for individual real-time threads.
• Allow the programmer to query information characterizing the behavior of thegarbage collection algorithm, and to some limited ability, alter the behavior ofthat algorithm.
Semantics and Requirements

Semantics and Requirements
The following list establishes the semantics and requirements that are applicable
across the classes of this section. Semantics that apply to particular classes,
constructors, methods, and fields will be found in the class description and theconstructor, method, and field detail sections.
1. Some MemoryArea classes are required to have linear (in object size) allocation
time. The linear time attribute requires that, ignoring performance variations due
to hardware caches or similar optimizations and execution of any static
initializers, the execution time of new must be bounded by a polynomial, f(n),where n is the size of the object and for all n>0, f(n) <= Cn for some constant C.
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CHAPTER 5 MEMORY MANAGEMENT

2. Execution time of object constructors is explicitly not considered in any bounds.
3. A memory scope is represented by an instance of the ScopedMemory class.
When a new scope is entered, by calling the enter() method of the instance or bystarting an instance of RealtimeThread or NoHeapRealtimeThread whose
constructors were given a reference to an instance of ScopedMemory, allsubsequent uses of the new keyword within the program logic of the scope will
allocate the memory from the memory represented by that instance of
ScopedMemory. When the scope is exited by returning from the enter() methodof the instance of ScopedMemory, all subsequent uses of the new operation will
allocate the memory from the area of memory associated with the enclosing
scope.
4. Each instance of the class ScopedMemory or its subclasses must contain areference count of the number of scopes in which it is being used.
5. The reference count for an instance of ScopedMemory or one of its subclasses isincreased by one each time a reference to the instance is given to the constructorof a RealtimeThread or a NoHeapRealtimeThread, when a scope is opened forthe instance (by calling the enter() method of the instance), and for each scope
opened within its scope (whether for this instance or another instance).
6. The reference count for a ScopedMemory area is decreased by one when returningfrom an invocation of its enter() method, when an instance of RealtimeThreador NoHeapRealtimeThread to which the area is associated through a reference inthe thread’s MemoryParameters object exits, or when an inner scope returns fromits enter() method (whether for this instance or another instance).
7. When the reference count for an instance of the class ScopedMemory or its
subclasses is decremented from one to zero, all objects within that area areconsidered unreachable and as candidates for reclamation. The finalizers for eachobject in the memory associated with an instance of ScopedMemory are executed
to completion before any statement in any thread attempts to access the memoryarea.
8. Scopes may be nested. When a nested scope is entered, all subsequent allocations
are taken from the memory associated with the new scope. When the nested scopeis exited, the previous scope is restored and subsequent allocations are again
taken from that scope.
9. Any MemoryArea that is associated with a NoHeapRealtimeThread may not move
any objects.
10. Objects created in any immortal memory area live for the duration of theapplication. The finalizers are only run when the application is terminated.
11. Each instance of the virtual machine will have exactly one instance of the class
ImmortalMemory.

SEMANTICS AND REQUIREMENTS

59

12. Each instance of the virtual machine will have exactly one instance of the class
HeapMemory.
13. Each instance of the virtual machine will behave as if there is an area of memory
into which all Class objects are placed and which is unexceptionallyreferenceable by NoHeapRealtimeThreads.
14. Strict assignment rules placed on assignments to or from memory areas preventthe creation of dangling pointers, and thus maintain the pointer safety of Java.
The restrictions are listed in the following table:
Reference toHeap

Reference toImmortal

Reference to
Scoped

Heap

Yes

Yes

No

Immortal

Yes

Yes

No

ScopedYes

Yes

Yes, if same, outer,or shared scope

Local
Variable

Yes

Yes

Yes, if same, outer,or shared scope

15. An implementation must ensure that the above checks are performed before thestatement is executed. (This includes the possibility of static analysis of theapplication logic).

Rationale
Languages that employ automatic reclamation of blocks of memory allocated in what
is traditionally called the heap by program logic also typically use an algorithm called
a garbage collector. Garbage collection algorithms and implementations vary in theamount of non-determinancy they add to the execution of program logic. To date, theexpert group believes that no garbage collector algorithm or implementation is known
that allows preemption at points that leave the inter-object pointers in the heap in aconsistent state and are sufficiently close in time to minimize the overhead added totask switch latencies to a sufficiently small enough value which could be consideredappropriate for all real-time systems.
Thus, this specification provides the above described areas of memory to allow
program logic to allocate objects in a Java-like style, ignore the reclamation of thoseobjects, and not incur the latency of the implemented garbage collection algorithm.

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5.1

MemoryArea

Syntax: public abstract class MemoryArea
Direct Known Subclasses: HeapMemory61, ImmortalMemory62,
ImmortalPhysicalMemory69, ScopedMemory62
MemoryArea is the abstract base class of all classes dealing with representationsof allocatable memory areas, including the immortal memory area, physical memoryand scoped memory areas.

5.1.1

Constructors

protected MemoryArea(long sizeInBytes)

Parameters:
sizeInBytes - The size of the MemoryArea to allocate, in bytes.

5.1.2

Methods

public void enter(java.lang.Runnable logic)

Associate this memory area to the current real-time thread for the durationof the execution of the run() method of the given java.lang.Runnable .
During this bound period of execution, all objects are allocated from thememory area until another one takes effect, or the enter() method isexited. A runtime exception is thrown if this method is called from threadother than a RealtimeThread22 or NoHeapRealtimeThread26 .
Parameters:
logic - The runnable object whose run() method should beexecuted.
public static MemoryArea60 getMemoryArea(java.lang.Object object)
Return the MemoryArea in which the given object is located.
public long memoryConsumed()

An exact count, in bytes, of the all of the memory currently used by thesystem for the allocated objects.
Returns: The amount of memory consumed in bytes.
public long memoryRemaining()

An approximation to the total amount of memory currently available for
future allocated objects, measured in bytes.
Returns: The amount of remaining memory in bytes.

HEAPMEMORY
public synchronized java.lang.Object newArray(java.lang.Class type,
int number)

Allocate an array of T in this memory area.
Parameters:
type - The class of the elements of the new array.
number - The number of elements in the new array.
Returns: A new array of class type, of number elements.
Throws: IllegalAccessException - The class or initializer is
inaccessible.
InstantiationException - The array cannot be instantiated.
OutOfMemoryError - Space in the memory area is exhausted.
public synchronized java.lang.Object newInstance(java.lang.Class
type)

Allocate an object in this memory area.
Parameters:
type - The class of which to create a new instance.
Returns: A new instance of class type.
Throws: IllegalAccessException - The class or initializer is
inaccessible.
InstantiationException - The specified class object could not be
instantiated. Possible causes are: it is an interface, it is abstract,
it is an array, or an exception was thrown by the constructor.
OutOfMemoryError - Space in the memory area is exhausted.
public long size()

Query the size of the memory area. The returned value is the current size.
Current size may be larger than initial size for those areas that are allowed
to grow.
Returns: The size of the memory area in bytes.

5.2

HeapMemory

Syntax: public final class HeapMemory extends MemoryArea60
The HeapMemory class is a singleton object that allows logic within other scoped
memory to allocate objects in the Java heap.

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5.2.1

Methods

public static HeapMemory61 instance()
Return a pointer to the singleton HeapMemory space

Returns: The singleton HeapMemory object.

5.3

ImmortalMemory

Syntax: public final class ImmortalMemory extends MemoryArea60
ImmortalMemory is a memory resource that is shared among all threads. Objects
allocated in the immortal memory live until the end of the application. Objects inimmortal memory are never subject to garbage collection, although some GC
algorithms may require a scan of the immortal memory. An immortal object may only
contain reference to other immortal objects or to heap objects. Unlike standard Java
heap objects, immortal objects continue to exist even after there are no otherreferences to them.

5.3.1

Methods

public static ImmortalMemory62 instance()
Return a pointer to the singleton ImmortalMemory space.

5.4

ScopedMemory

Syntax: public abstract class ScopedMemory extends MemoryArea60
Direct Known Subclasses: LTMemory65, ScopedPhysicalMemory71, VTMemory65
ScopedMemory is the abstract base class of all classes dealing withrepresentations of memory spaces with a limited lifetime. The ScopedMemory area isvalid as long as there are real-time threads with access to it. A reference is created foreach accessor when either a real-time thread is created with the ScopedMemory object
as its memory area, or a real-time thread runs the public voidenter(java.lang.Runnable logic)64 method for the memory area. When the lastreference to the object is removed, by exiting the thread or exiting the enter()method, finalizers are run for all objects in the memory area, and the area is emptied.

A ScopedMemory area is a connection to a particular region of memory andreflects the current status of it. The object does not necessarily contain directreferences to the region of memory that is implementation dependent.

SCOPEDMEMORY

When a ScopedMemory area is instantiated, the object itself is allocated from thecurrent memory allocation scheme in use, but the memory space that object represents
is not. Typically, the memory for a ScopedMemory area might be allocated using native
method implementations that make appropriate use of malloc() and free() orsimilar routines to manipulate memory. The enter() method of ScopedMemory is themechanism used to activate a new memory scope. Entry into the scope is done bycalling the method:
public void enter(Runnable r)

Where r is a Runnable object whose run() method represents the entry point tothe code that will run in the new scope. Exit from the scope occurs when the r.run()completes. Allocations of objects within r.run() are done with the ScopedMemory
area. When r.run() is complete, the scoped memory area is no longer active. Its
reference count will be decremented and if it is zero all of the objects in the memoryarea finalized and collected.
Objects allocated from a ScopedMemory area have a unique lifetime. They cease
to exist on exiting a public void enter(java.lang.Runnable logic)64 method orupon exiting the last real-time thread referencing the area, regardless of any
references that may exist to the object. Thus, to maintain the safety of Java and avoid
dangling references, a very restrictive set of rules apply to ScopedMemory area objects:
1. A reference to an object in ScopedMemory can never be stored in an Object
allocated in the Java heap.
2. A reference to an object in ScopedMemory can never be stored in an Object
allocated in ImmortalMemory62 .
3. A reference to an object in ScopedMemory can only be stored in Objects allocatedin the same ScopedMemory area, or into a — more inner — ScopedMemory area
nested by the use of its enter() method.
4. References to immortal or heap objects may be stored into an object allocated in a
ScopedMemory area.

5.4.1

Constructors

public ScopedMemory(long size)
Create a new ScopedMemory area with a particular size.

Parameters:
size - The size of the new ScopedMemory area in bytes. If size is lessthan or equal to zero nothing happens.

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5.4.2

Methods

public void enter(java.lang.Runnable logic)
Associate this ScopedMemory area to the current real-time thread for theduration of the execution of the run() method of the given
java.lang.Runnable . During this bound period of execution, all objects
are allocated from the ScopedMemory area until another one takes effect, orthe enter() method is exited. A runtime exception is thrown if this method
is called from a thread other than a RealtimeThread22 or
NoHeapRealtimeThread26 .

Overrides: public void enter(java.lang.Runnable logic)60 in class
MemoryArea60

Parameters:
logic - The runnable object which contains the code to execute.
public int getMaximumSize()

Get the maximum size this memory area can attain. If this is a fixed size
memory area, the returned value will be equal to the initial size.
public MemoryArea60 getOuterScope()
Find the ScopedMemory area in effect, for the current RealtimeThread22 ,prior to the current invocation of a ScopedMemoryenter} method.

Returns: The containing scope. If this is the outermost scoped memory
then the MemoryArea60 associated with the thread.
public java.lang.Object getPortal()

Return a reference to the portal object in this instance of ScopedMemory.
Returns: The portal object or null if there is no portal object.
public void setPortal(java.lang.Object object)

Set the argument to the portal object in the memory area represented bythis instance of ScopedMemory.
Parameters:
object - The object which will become the portal for this. If null theprevious portal object remains the portal object for this or if
there was no previous portal object then there is still no portal
object for this.

VTMEMORY

5.565

VTMemory

Syntax: public class VTMemory extends ScopedMemory62
The execution time of an allocation from a VTMemory area may take a variable
amount of time. However, since VTMemory areas are not subject to garbage collection
and objects within may not be moved, these areas can be used by instances of
NoHeapRealtimeThread26 .

5.5.1

Constructors

public VTMemory(int initial, intmaximum)
Create a VTMemory of the given size.

Parameters:
initial - The size in bytes of the memory to initially allocate forthis area.
maximum - The maximum size in bytes this memory area can grow to.

5.6

LTMemory

Syntax: public class LTMemory extends ScopedMemory62
LTMemory represents a memory area, allocated per RealtimeThread22 , or for agroup of real-time threads, guaranteed by the system to have linear time allocation.
The memory area described by a LTMemory instance does not exist in the Java heap,
and is not subject to garbage collection. Thus, it is safe to use a LTMemory object as thememory area associated with a NoHeapRealtimeThread26 , or to enter the memoryarea using the public void enter(java.lang.Runnable logic)64 method within a
NoHeapRealtimeThread26 . An LTMemory area has an initial size. Enough memory
must be committed by the completion of the constructor to satisfy this initialrequirement. (Committed means that this memory must always be available forallocation). The initial memory allocation must behave, with respect to successful
allocation, as if it were contiguous; i.e., a correct implementation must guarantee thatany sequence of object allocations that could ever succeed without exceeding a
specified initial memory size will always succeed without exceeding that initialmemory size and succeed for any instance of LTMemory with that initial memory size.
(Note: It is important to understand that the above statement does not require that if
the initial memory size is N and (sizeof(object1) + sizeof(object2) + ... +sizeof(objectn) = N) the allocations of objects 1 through n will necessarily succeed.)
Execution time of an allocator allocating from this initial area must be linear in the

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size of the allocated object. Execution time of an allocator allocating from memory
between initial and maximum is allowed to vary. Furthermore, the underlying system
is not required to guarantee that memory between initial and maximum will always
be available. (Note: to ensure that all requested memory is available set inital andmaximum to the same value) See also: MemoryArea60 ScopedMemory62
RealtimeThread22 NoHeapRealtimeThread26

5.6.1

Constructors

public LTMemory(long initialSizeInBytes, longmaxSizeInBytes)
Create a LTMemory of the given size.

Parameters:
initialSizeInBytes - The size in bytes of the memory to allocate

for this area. This memory must be committed before thecompletion of the constructor.
maxSizeInBytes - The size in bytes of the memory to allocate forthis area.

ScopedMemory Example
A real-time thread — including the primordial thread will perform allocations fromwithin the memory area assigned to the thread. The default memory area is the Java
heap. Allocations can be performed from a different memory area in one of two ways:
entering a new scope, or calling newInstance() or newArray() on a different
memory area. To enter a new scope that has constant time allocation:
final ScopedMemory scope = new CTMemory(16 * 1024);

The enter() method will call the run method of the given object with memory area as
the object pool for allocations. All new operations will come from the constant-time
pool until a new scope is entered, or the run() method completes.
scope.enter(new Runnable() {public void run() {//do some time-critical operations//to allocate from the heap within this scope:try {HeapMemory.instance().newInstance(Class.forName(“Foo”));//to allocate from the previous scope within this one
scope.getOuterScope().newInstance(Class.forName(“Foo”));} catch (ClassNotFoundException e) {} catch (IllegalAccessException ia) {} catch (InstantiationException ie) {}}});LTMEMORY

ScopedMemory Example 2
A real-time thread may be associated with a memory area when it is created. All new
operations will allocate objects for the thread from the object pool provided by thememory area.
final ScopedMemory scope = new CTMemory(16 * 1024);
RealtimeThread t1 = new RealtimeThread(null, null,
new MemoryParameters(scope), null,
new Runnable() {public void run() {//do some stuff}

Additional threads can share the same memory area, and the reference count will be
incremented.
RealtimeThread t2 = new RealtimeThread(null, null,
new MemoryParameters(scope), null,
new Runnable() {public void run() {//do some other stuff}

Wait for the threads to finish
boolean interrupted = false;
do {
try {
t1.join();} catch (InterruptedException ie) {interrupted = true;}} while (interrupted);interrupted = false;
do {
try {
t2.join();} catch (InterruptedException ie) {interrupted = true;}} while (interrupted);

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After this point, the threads are dead, and the reference count will have dropped tozero so finalizers may be run. If we now try to create a new thread using the memory
area:
RealtimeThread t3 = new RealtimeThread(null, null,
new MemoryParameters(scope), null,
new Runnable() {public void run() {//do some other stuff}

The constructor will block until the finalizers have completed. It will then be safe tostart the thread:t3.start();

5.7

PhysicalMemoryFactory

Syntax: public class PhysicalMemoryFactory
The PhysicalMemoryFactory is available for use by the various physicalmemory accessor objects to create objects of the correct type that are bound to areasof physical memory with the appropriate characteristics — or with appropriate
accessor behavior. Examples of characteristics that might be specified are: DMA
memory, accessors with byte swapping, etc. The implementation will provide a
default factory. OEMs may provide derived factories that allow additional
characteristics to be specified.

5.7.1

Fieldspublic static final java.lang.String ALIGNED

Specify this to identify aligned memory.
public static final java.lang.String BYTESWAPSpecify this if byte swapping should be used.
public static final java.lang.String DMASpecify this to identify DMA memory.
public static final java.lang.String SHARED

Specify this to identify shared memory.

IMMORTALPHYSICALMEMORY

5.7.2

Methods

protected synchronized java.lang.Object create(java.lang.Object
memoryType, java.lang.ClassphysMemType,long base, longsize)

Used to actually create the physical memory accessor.
Parameters:
memoryType - Description of the memory type required.
physMemType - Indicates the type of physical memory object toconstruct.
base - The physical address of the start of the region.
size - The size of the region in bytes.
protected synchronized long getTypedMemoryBase(java.lang.Object
memoryType, longsize)

Get the base address of a range of memory of the correct type that is at least
the size specified.
Parameters:
size - The desired size of the memory range.

5.8

ImmortalPhysicalMemory

Syntax: public class ImmortalPhysicalMemory extends MemoryArea60
An instance of ImmortalPhysicalMemory allows objects to be allocated from a
range of physical memory with particular attributes, determined by their memory type.
This memory area has the same restrictive set of assignment rules as
ImmortalMemory62 memory areas and may be used in any constructor where
ImmortalMemory62 is appropriate. Objects allocated in immortal physical memory
have a lifetime greater than the application as do objects allocated in immortal
memory.

5.8.1

Constructors

protected ImmortalPhysicalMemory(ImmortalPhysicalMemory69 memory,long base, longsize)

Constructor for use by the memory object factory.
protected ImmortalPhysicalMemory(long base, long size)

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5.8.2

Methods

public static ImmortalPhysicalMemory69 create(java.lang.Object
type, long size)

Parameters:
type - An object representing the type of memory required (e.g.,dma, shared) - used to define the base address and control the
mapping. The passed object is typically provided by the vendor
of the physical memory or the implementation vendor.
size - The size of the memory area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given type of memory.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public static ImmortalPhysicalMemory69 create(java.lang.Object
type, long base, long size)

Parameters:
type - An object representing the type of memory required (e.g.,dma, shared). The passed object is typically provided by thevendor of the physical memory or the implementation vendor.
base - The physical memory address of the region
size - The size of the memory area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given range of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public static void setFactory(PhysicalMemoryFactory68 factory)

Set the physical memory factory to the given argument.
Parameters:
factory - A physical memory factory which will be the factory for
PhysicalMemoryFactory68 at the completion of this method.

SCOPEDPHYSICALMEMORY

5.9

ScopedPhysicalMemory

Syntax: public class ScopedPhysicalMemory extends ScopedMemory62
An instance of ScopedPhysicalMemory allows objects to be allocated from a
range of physical memory with particular attributes, determined by their memory
type. This memory area has the same restrictive set of assignment rules as
ScopedMemory62 memory areas.

5.9.1

Constructors

protected ScopedPhysicalMemory(long base, lo n gsize)

Constructor for use by the memory object factory.
protected ScopedPhysicalMemory(ScopedPhysicalMemory71 memory,long base, longsize)

Constructor for use by the memory object factory.

5.9.2

Methods

public static ScopedPhysicalMemory71 create(java.lang.Object type,long base, longsize)

Parameters:
type - An Object representing the type of memory required (e.g.,dma, shared)
base - The physical memory address of the area.
size - The size of the area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given range of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public static void setFactory(PhysicalMemoryFactory68 factory)
Sets the factory that will be used to generate ScopedPhysicalMemory

instances.
Parameters:
factory - The PhysicalMemoryFactory68 which will become thefactory for this. If null the previous factory remains as thefactory for this.

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5.10 RawMemoryAccess
Syntax: public class RawMemoryAccessDirect Known Subclasses: RawMemoryFloatAccess76

An instance of RawMemoryAccess models a range of physical memory as a fixedsize sequence of bytes. A full complement of accessor methods allow the contents ofthe physical memory area to be accessed through offsets from the base, interpreted asbyte, short, int, or long data values or as arrays of these types.
Whether the offset addresses the high-order or low-order byte is based on thevalue of the BYTE_ORDER static boolean variable in class RealtimeSystem150 .
The RawMemoryAccess class allows a real-time program to implement device
drivers, memory-mapped I/O, flash memory, battery-backed RAM, and similar lowlevel software.
A raw memory area cannot contain references to Java objects. Such a capability
would be unsafe (since it could be used to defeat Java’s type checking) and errorprone (since it is sensitive to the specific representational choices made by the Javacompiler).
Many of the constructors and methods in this class throw
OffsetOutOfBoundsException155 . This exception means that the value given in theoffset parameter is either negative or outside the memory area.
Many of the constructors and methods in this class throw
SizeOutOfBoundsException156 . This exception means that the value given in thesize parameter is either negative, larger than an allowable range, or would cause an
accessor method to access an address outside of the memory area.

5.10.1 Constructors
protected RawMemoryAccess(long base, longsize)
protected RawMemoryAccess(RawMemoryAccess72 memory, long base,long size)

Constructor reserved for use by the memory object factory.

5.10.2 Methods
public static RawMemoryAccess72 create(java.lang.Object type,long size)

RAWMEMORYACCESS

73

Parameters:
type - An Object representing the type of memory required (e.g.,dma, shared) - used to define the base address and control the
mappingsize - The size of the area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given type of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public static RawMemoryAccess72 create(java.lang.Object type,long base, longsize)

Parameters:
type - An Object representing the type of memory required (e.g.,dma, shared)
base - The physical memory address of the region
size - The size of the area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given range of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public byte getByte(long offset)
Get the byte at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getBytes(long offset, byte[]bytes, intlow,
int number)
Get number bytes starting at the given offset in this and assign them into thebyte array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public int getInt(long offset)
Get the int at the given offset.

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Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getInts(long offset, int[] ints, int low, i n tnumber)
Get number int values, starting at the given offset in this, and assign
them into the int array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public long getLong(long offset)
Get the long value at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getLongs(long offset, long[]longs, intlow,
int number)
Get number long values, starting at the given offset in this, and assign
them into the long array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public long getMappedAddress()

Return the virtual memory location at which the memory region is mapped.
Returns: The virtual address to which this is mapped (for reference
purposes). Same as the base address if virtual memory isn’t
supported.
public short getShort(long offset)
Get the short at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getShorts(long offset, short[]shorts, intlow,
int number)
Get number shorts starting at the given offset in this from the short arraystarting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public long map()

Map the physical address range into virtual memory. No-op if the systemdoesn’t support virtual memory.
Returns: The virtual address to which this is mapped (for reference
purposes).

RAWMEMORYACCESS

75

public long map(long base)

Map the physical address range into virtual memory at the specified
location. No-op if the system doesn’t support virtual memory.
Parameters:base - The location to map to in the virtual address space.
Returns: The virtual address to which this is mapped (for reference
purposes).
public long map(long base, longsize)

Map the physical address range into virtual memory at the specified
location. No-op if the system doesn’t support virtual memory.
Parameters:base - The location to map to in the virtual address space.
size - The size of the block to map in.
Returns: The virtual address to which this is mapped (for reference
purposes).
public void setByte(long offset, byte value)
Set the byte at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setBytes(long offset, byte[]bytes, intlow,
int number)
Set number bytes starting at the given offset in this from the byte arraystarting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setInt(long offset, i n tvalue)
Set the int value at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setInts(long offset, int[] ints, int low, i n tnumber)
Set number int values starting at the given offset in this, from the int
array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setLong(long offset, long value)
Set the long value at the given offset starting at position low.

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Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setLongs(long offset, long[]longs, intlow ,intn)
Set number long values starting at the given offset in this, from the longarray starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setShort(long offset, shortvalue)
Set the short at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setShorts(long offset, short[]shorts, intlow,
int number)
Set number shorts starting at the given offset in this, from the short arraystarting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void unmap()

Unmap the physical address range from virtual memory. No-op if thesystem doesn’t support virtual memory.

5.11 RawMemoryFloatAccess
Syntax: public class RawMemoryFloatAccess extends RawMemoryAccess72
This class holds the accessor methods for accessing a raw memory area by float
and double types. Implementations are required to implement this class if and only if
the underlying Java Virtual Machine supports floating point data types.
Many of the constructors and methods in this class throw
OffsetOutOfBoundsException155 . This exception means that the value given in theoffset parameter is either negative or outside the memory area.
Many of the constructors and methods in this class throw
SizeOutOfBoundsException156 . This exception means that the value given in thesize parameter is either negative, larger than an allowable range, or would cause an
accessor method to access an address outside of the memory area.

RAWMEMORYFLOATACCESS

5.11.1 Constructors
protected RawMemoryFloatAccess(long base, lo n gsize)
protected RawMemoryFloatAccess(RawMemoryAccess72 memory, long base,long size)

Constructor reserved for use by the memory object factory.

5.11.2 Methods
public static RawMemoryFloatAccess76
createFloatAccess(java.lang.Object type,long size)

Parameters:
type - An Object representing the type of memory required (e.g.,dma, shared) - used to define the base address and control the
mappingsize - The size of the area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given type of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public static RawMemoryFloatAccess76
createFloatAccess(java.lang.Object type,long base, longsize)

Parameters:
type - An Object representing the type of memory required (e.g.,dma, shared)
base - The physical memory address of the area.
size - The size of the area in bytes.
Throws: SecurityException - The application doesn’t have permissions
to access physical memory or the given range of memory.
OffsetOutOfBoundsException155 - The address is invalid.
SizeOutOfBoundsException156 - The size is negative or extends
into an invalid range of memory.
UnsupportedPhysicalMemoryException157 - Thrown if theunderlying hardware does not support the given type.
public byte getDouble(long offset)
Get the double at the given offset.

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Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getDoubles(long offset, double[]doubless, in tlow,
int number)
Get number double values starting at the given offset in this, and assigns
them into the double array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public byte getFloat(long offset)
Get the float at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void getFloats(long offset, float[]floats, intlow,
int number)
Get number float values starting at the given offset in this and assign
them into the byte array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setDouble(long offset, doublevalue)
Set the double at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setDoubles(long offset, double[]doubles ,intlow,
int number)
Set number double values starting at the given offset in this, from the
double array starting at position low.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setFloat(long offset, floatvalue)
Set the float at the given offset.

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155
public void setFloats(long offset, float[]floats, intlow,
int number)
Set number float values starting at the given offset in this from the byte
array starting at position low.

MEMORYPARAMETERS

Throws: SizeOutOfBoundsException156,
OffsetOutOfBoundsException155

5.12 MemoryParameters
Syntax: public class MemoryParameters
Memory parameters can be given on the constructor of RealtimeThread22 andAsyncEventHandler127 . These can be used both for the purposes of admission
control by the scheduler and for the purposes of pacing the garbage collector to satisfy
all of the thread allocation rates. When a reference to a MemoryParameters object isgiven as a parameter to a constructor, the MemoryParameters object becomes boundto the object being created. Changes to the values in the MemoryParameters object
affect the constructed object. If given to more than one constructor, then changes tothe values in the MemoryParameters object affect all of the associated objects. Notethat this is a one-to-many relationship and not a many-to-many.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

5.12.1 Fields
public static final long NO_MAX

5.12.2 Constructors
public MemoryParameters(long maxMemoryArea, longmaxImmortal)
Create a MemoryParameters object with the given values.

Parameters:
maxMemoryArea - A limit on the amount of memory the thread may

allocate in the memory area. Units are in bytes. If zero, no
allocation allowed in the memory area. To specify no limit, use
NO_MAX or a value less than zero.
maxImmortal - A limit on the amount of memory the thread may
allocate in the immortal area. Units are in bytes. If zero, no
allocation allowed in immortal. To specify no limit, use
NO_MAX or a value less than zero.
Throws: IllegalArgumentException

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public MemoryParameters(long maxMemoryArea, longmaxImmortal,
long allocationRate)
Create a MemoryParameters object with the given values.

Parameters:
maxMemoryArea - A limit on the amount of memory the thread may

allocate in the memory area. Units are in bytes. If zero, no
allocation allowed in the memory area. To specify no limit, use
NO_MAX or a value less than zero.
maxImmortal - A limit on the amount of memory the thread may
allocate in the immortal area. Units are in bytes. If zero, no
allocation allowed in immortal. To specify no limit, use
NO_MAX or a value less than zero.
allocationRate - A limit on the rate of allocation in the heap. Units
are in bytes per second. If zero, no allocation is allowed in theheap. To specify no limit, use NO_MAX or a value less thanzero.
Throws: IllegalArgumentException

5.12.3 Methods
public long getAllocationRate()

Get the allocation rate. Units are bytes per second.
public long getMaxImmortal()

Get the limit on the amount of memory the thread may allocate in theimmortal area. Units are in bytes.
public long getMaxMemoryArea()

Get the limit on the amount of memory the thread may allocate in thememory area. Units are in bytes.
public void setAllocationRate(long rate)

A limit on the rate of allocation in the heap.
Parameters:
rate - Units are in bytes per second. If zero, no allocation is allowedin the heap. To specify no limit, use NO_MAX or a value lessthan zero.

GARBAGECOLLECTOR

81

public boolean setMaxImmortal(long maximum)

Set the limit on the amount of memory the thread may allocate in theimmortal area.
Parameters:
maximum - Units are in bytes. If zero, no allocation is allowed in theimmortal area. To specify no limit, use NO_MAX or a value lessthan zero.
Returns: False if any of the threads have already allocated more than thegiven value. In this case the call has no effect.
public boolean setMaxMemoryArea(long maximum)

Set the limit on the amount of memory the thread may allocate in thememory area.
Parameters:
maximum - Units are in bytes. If zero, no allocation allowed in thememory area. To specify no limit, use NO_MAX or a value lessthan zero.
Returns: False if any of the threads have already allocated more than thegiven value. In this case the call has no effect.

5.13 GarbageCollector
Syntax: public abstract class GarbageCollector
Direct Known Subclasses: IncrementalCollectorExample82,
MarkAndSweepCollectorExample83

The system shall provide dynamic and static information characterizing the
temporal behavior and imposed overhead of any garbage collection algorithm
provided by the system. This impormation shall be made available to applications via
methods on subclasses of GarbageCollector. Implementations are allowed toprovide any set of methods in subclasses as long as the temporal behavior andoverhead are sufficiently categorized. The implementations are also required to fully
document the subclasses. In addition, the method(s) in GarbageCollector shall bemade available by all implementations. See: IncrementalCollectorExample82 andMarkAndSweepCollectorExample83

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5.13.1 Constructors
public GarbageCollector()

5.13.2 Methods
public abstract RelativeTime102 getPreemptionLatency()
Instances of RealtimeThread22 are allowed to preempt the execution of
the garbage collector (instances of NoHeapRealtimeThread26 preempt
immediately but instances of RealtimeThread22 must wait until thecollector reaches a preemption-safe point). Preemption latency is ameasure of the maximum time a RealtimeThread22 may have to wait forthe collector to reach a preemption-safe point.
Returns: The preempting latency of this if applicable. May return zero if
there is no collector avaiable.

5.14 IncrementalCollectorExample
Syntax: public class IncrementalCollectorExample extends
GarbageCollector81

This class is provided as an example only and is not required on any
implementation, even ones which employ an incremental collector.

5.14.1 Constructors
public IncrementalCollectorExample()

5.14.2 Methods
public long getMaximumReclamationRate()

Maximum reclamation rate the garbage collector can sustain. This is a
dynamically assigned value dependent on schedule.
Returns: Return value is measured in kilobytes per second.
public RelativeTime102 getPreemptionLatency()

The instantiation of the abstract method in GarbageCollector.
Overrides: public abstract RelativeTime102
getPreemptionLatency()82 in class GarbageCollector81

MARKANDSWEEPCOLLECTOREXAMPLE
public int getReadBarrierOverhead()

Overhead of the read barrier. Given in percentage of the cost of a field
access.
public int getWriteBarrierOverhead()

Overhead of the write barrier. Given in percentage of the cost of anassignment.
public void setReclamationRate(int rate)

The reclamation rate as a ratio: 1 / number of kilobytes scanned perkilobyte allocated. Used by incremental collection algorithms to pace theirreclamation rate.
Parameters:
rate - The new reclamation rate. Ignored if collector does not

5.15 MarkAndSweepCollectorExample
Syntax: public class MarkAndSweepCollectorExample extends
GarbageCollector81

This class is provided as an example only and is not required on any
implementation, even ones which employ an incremental collector.

5.15.1 Constructors
public MarkAndSweepCollectorExample()

5.15.2 Methods
public RelativeTime102 getPreemptionLatency()

The instantiation of the abstract method in GarbageCollector.
Overrides: public abstract RelativeTime102
getPreemptionLatency()82 in class GarbageCollector81

83

CHAP TER

6

Synchronization
Chapter 6 Synchronization

This section contains classes that:• Allow the application of the priority ceiling emulation algorithm to individual
objects.
• Allow the setting of the system default priority inversion algorithm.
• Allow wait-free communication between real-time threads and regular Java
threads.
The specification strengthens the semantics of Java synchronization for use in realtime systems by mandating monitor execution eligibility control, commonly referred
to as priority inversion control. A MonitorControl class is defined as the superclassof all such execution eligibility control algorithms. PriorityInheritance is thedefault monitor control policy; the specification also defines a
PriorityCeilingEmulation option.
The wait-free queue classes provide protected, concurrent access to data shared
between instances of java.lang.Thread and NoHeapRealtimeThread.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. Threads waiting to enter synchronized blocks are priority queue ordered. If
threads with the same priority are possible under the active scheduling policy
such threads are queued in FIFO order.
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2. Any conforming implementation must provide an implementation of thesynchronized primitive with default behavior that ensures that there is nounbounded priority inversion. Furthermore, this must apply to code if it is run
within the implementation as well as to real-time threads.
3. The Priority Inheritance monitor control policy must be implemented.
4. Implementations that provide a monitor control algorithm in addition to thosedescribed herein are required to clearly document the behavior of that algorithm.

RationaleJava monitors, and especially the synchronized keyword, provide a very elegant
means for mutual exclusion synchronization. Thus, rather than invent a new real-timesynchronization mechanism, this specification strengthens the semantics of Java
synchronization to allow its use in real-time systems. In particular, this specification
mandates priority inversion control. Priority inheritance and priority ceiling emulation
are both popular priority inversion control mechanisms; however, priority inheritanceis more widely implemented in real-time operating systems and so is the default
mechanism in this specification.
By design the only mechanism required by this specification which can enforce
mutual exclusion in the traditional sense is the keyword synchronized. Noting thatthe specification allows the use of synchronized by both instances ofjava.lang.Thread, RealtimeThread, and NoHeapRealtimeThread and that suchflexibility precludes the correct implementation of any known priority inversion
algorithm when locked objects are accessed by instances of java.lang.Thread andNoHeapRealtimeThread, it is incumbent on the specification to provide alternatemeans for protected, concurrent data access by both types of threads (protected means
access to data without the possibility of corruption). The three wait-free queue classes
provide such access.

6.1

MonitorControl

Syntax: public abstract class MonitorControl
Direct Known Subclasses: PriorityCeilingEmulation87, PriorityInheritance88

Abstract superclass for all monitor control policy objects.

PRIORITYCEILINGEMULATION

6.1.1

Constructors

public MonitorControl()

6.1.2

Methods

public static void setMonitorControl(MonitorControl86 policy)

Control the default monitor behavior for object monitors used bysynchronized statements and methods in the system. The type of the policy
object determines the type of behavior. Conforming implementations must
support priority ceiling emulation and priority inheritance for fixed priority
preemptive threads.
Parameters:
policy - The new monitor control policy. If null nothing happens.
public static void setMonitorControl(java.lang.Object monitor,
MonitorControl86 policy)
Has the same effect as setMonitorControl(), except that the policy only

affects the indicated object monitor.
Parameters:monitor - The monitor for which the new policy will be in use. The

policy will take effect on the first attempt to lock the monitor
after the completion of this method. If null nothing will happen.
policy - The new policy for the object. If null nothing will happen.

6.2

PriorityCeilingEmulation

Syntax: public class PriorityCeilingEmulation extends MonitorControl86
Monitor control class specifying use of the priority ceiling emulation protocol formonitor objects. Objects under the influence of this protocol have the effect that athread entering the monitor has its effective priority — for priority-based dispatching— raised to the ceiling on entry, and is restored to its previous effective priority when
it exits the monitor. See also MonitorControl86 and PriorityInheritance88 .

6.2.1

Constructors

public PriorityCeilingEmulation(int ceiling)
Create a PriorityCeilingEmulation object with a given ceiling.

Parameters:
ceiling - Priority ceiling value.

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6.2.2

Methods

public int getDefaultCeiling()

Get the priority ceiling for this PriorityCeilingEmulation object.

6.3

PriorityInheritance

Syntax: public class PriorityInheritance extends MonitorControl86
Monitor control class specifying use of the priority inheritance protocol for
object monitors. Objects under the influence of this protocol have the effect that athread entering the monitor will boost the effective priority of the thread in themonitor to its own effective priority. When that thread exits the monitor, its effective
priority will be restored to its previous value. See also MonitorControl86 andPriorityCeilingEmulation87

6.3.1

Constructors

public PriorityInheritance()

6.3.2

Methods

public static PriorityInheritance88 instance()
Return a pointer to the singleton PriorityInheritance.

6.4

WaitFreeDequeue

Syntax: public class WaitFreeDequeue
The wait-free queue classes facilitate communication and synchronization
between instances of RealtimeThread22 and java.lang.Thread . See
WaitFreeWriteQueue92 or WaitFreeReadQueue90 for more details. Instances of thisclass create a WaitFreeWriteQueue92 and a WaitFreeReadQueue90 and make callson the respective read() and write() methods.

6.4.1

Constructors

public WaitFreeDequeue(java.lang.Thread writer,
java.lang.Thread reader, intmaximum,
MemoryArea60 area)
A queue with unsynchronized and nonblocking read() and write()
methods and synchronized and blocking read()and write() methods.

WAITFREEDEQUEUE

89

Parameters:
writer - An instance of Thread.
reader - An instance of Thread.
maximum - Then maximum number of elements in the both the
WaitFreeReadQueue90 and the WaitFreeWriteQueue92 .
area - The MemoryArea60 in which this object and internal elements

are allocated.
Throws: InstantiationException, ClassNotFoundException,
IllegalAccessException, IllegalArgumentException

6.4.2

Methods

public java.lang.Object blockingRead()A synchronized call of the read() method of the underlying
WaitFreeWriteQueue92 . This call blocks on queue empty and will wait

until there is an element in the queue to return.
Returns: An java.lang.Object from this.
public boolean blockingWrite(java.lang.Object object)
A synchronized call of the write() method of the underlying
WaitFreeReadQueue90 . This call blocks on queue full and waits until thereis space in this.
Parameters:
object - The java.lang.Object to place in this.

Returns: True if object is now in this.
Throws: MemoryScopeException155
public boolean force(java.lang.Object object)

If this is full then this call overwrites the last object written to this with thegiven object. If this is not full this call is equivalent to the
nonBlockingWrite() call.
Parameters:
object - The java.lang.Object which will overwrite the last
object if this is full. Otherwise object will be placed in this.
public java.lang.Object nonBlockingRead()
An unsynchronized call of the read() method of the underlying
WaitFreeReadQueue90 .

Returns: A java.lang.Object object read from this. If there are noelements in this then null is returned.

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CHAPTER 6 SYNCHRONIZATION
public boolean nonBlockingWrite(java.lang.Object object)
An unsynchronized call of the write() method of the underlying
WaitFreeWriteQueue92 . This call does not block on queue full.

Parameters:
object - The java.lang.Object to attempt to place in this.

Returns: True if the object is now in this, otherwise returns false.
Throws: MemoryScopeException155

6.5

WaitFreeReadQueue

Syntax: public class WaitFreeReadQueue
The wait-free queue classes facilitate communication and synchronization
between instances of RealtimeThread22 and java.lang.Thread . The problem isthat synchronized access objects shared between real-time threads and threads mightcause the real-time threads to incur delays due to execution of the garbage collector.
The read() method of this class does not block on an imagined queue-empty
condition variable. If the read() is called on an empty queue null is returned. If two
real-time threads intend to read from this queue they must provide their own
synchronization.
The write method of this queue is synchronized and may be called by more than
one writer and will block on queue empty.

6.5.1

Constructors

public WaitFreeReadQueue(java.lang.Thread writer,
java.lang.Thread reader, intmaximum,
MemoryArea60 memory)
A queue with an unsynchronized and nonblocking read() method and asynchronized and blocking write() method. The memory areas of thegiven threads are found. If these memory areas are the same the queue iscreated in that memory area. If these memory areas are different the queueis created in the memory area accessible by the most restricted thread type.
Parameters:
writer - An instance of java.lang.Thread .
reader - An instance of java.lang.Thread .
maximum - The maximum number of elements in the queue.
memory - The MemoryArea60 in which this object and internal

elements are stored.

WAITFREEREADQUEUE

Throws: IllegalAccessException, ClassNotFoundException,
InstantiationException, IllegalArgumentException
public WaitFreeReadQueue(java.lang.Thread writer,
java.lang.Thread reader, intmaximum,
MemoryArea60 memory, boolean notify)

A queue with an unsynchronized and nonblocking read() method and asynchronized and blocking write() method.
Parameters:
writer - An instance of java.lang.Thread .
reader - An instance of java.lang.Thread .
maximum - The maximum number of elements in the queue.
memory - The MemoryArea60 in which this object and internal

elements are stored.
notify - Whether or not the reader is notified when data is added.

Throws: IllegalAccessException, ClassNotFoundException,
InstantiationException, IllegalArgumentException

6.5.2

Methods

public void clear()Set this to empty.
public boolean isEmpty()

Used to determine if this is empty.
Returns: True if this is empty and false if this is not empty.
public boolean isFull()Used to determine if this is full.
Returns: True if this is full and false if this is not full.
public java.lang.Object read()

Returns the next element in the queue unless the queue is empty. If thequeue is empty null is returned.
public int size()

Used to determine the number of elements in this.
Returns: An integer which is the number of empty positions in this.
public void waitForData()
If this is empty waitForData() waits on the event until the writer inserts

data. Note that true priority inversion does not occur since the writer locks
a different object and the notify is executed by the AsyncEventHandler127
which has noHeap characteristics.

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public synchronized boolean write(java.lang.Object object)

The synchronized and blocking write. This call blocks on queue full andwill wait until there is space in the queue.
Parameters:
object - The java.lang.Object that is placed in this.

Throws: MemoryScopeException155

6.6

WaitFreeWriteQueue

Syntax: public class WaitFreeWriteQueue
The wait-free queue classes facilitate communication and synchronization
between instances of RealtimeThread22 and java.lang.Thread . The problem isthat synchronized access objects shared between real-time threads and threads mightcause the real-time threads to incur delays due to execution of the garbage collector.
The write method of this class does not block on an imagined queue-fullcondition variable. If the write() method is called on a full queue false is returned. If
two real-time threads intend to read from this queue they must provide their own
synchronization.
The read() method of this queue is synchronized and may be called by morethan one writer and will block on queue empty.

6.6.1

Constructors

public WaitFreeWriteQueue(java.lang.Thread writer,
java.lang.Thread reader, intmaximum,
MemoryArea60 memory)
A queue with an unsynchronized and nonblocking write() method and asynchronized and blocking read() method.

Parameters:
writer - An instance of java.lang.Thread .
reader - An instance of java.lang.Thread .
maximum - The maximum number of elements in the queue.
memory - The MemoryArea60 in which this object and internal

elements are allocated.
Throws: InstantiationException, ClassNotFoundException,
IllegalAccessException, IllegalArgumentException

WAITFREEWRITEQUEUE

6.6.2

Methods

public void bind(java.lang.Thread writer, java.lang.Thread reader,
MemoryArea60 memory)

Binds two threads together for the purpose of using this in each thread. If
two unrelated (by common fixed memory area) threads are bound together,
only immortal objects can be placed in the queue.
Parameters:
writer - The java.lang.Thread object which will write to this.
reader - The java.lang.Thread object which will read from this.
memory - The new MemoryArea60 to use to test against the memoryarea of objects placed into this.
Throws: InstantiationException, IllegalAccessException,
IllegalArgumentException
public void clear()Set this to empty.
public boolean force(java.lang.Object object)
Force this java.lang.Object to replace the last one. If the reader should
happen to have just removed the other java.lang.Object just as we were

updating it, we will return false. False may mean that it just saw what we
put in there. Either way, the best thing to do is to just write again — which
will succeed, and check on the readers side for consecutive identical readvalues.
public boolean isEmpty()

Used to determine if this is empty.
Returns: True if this is empty and false if this is not empty.
public boolean isFull()Used to determine if this is full.
Returns: True if this is full and false if this is not full.
public synchronized java.lang.Object read()A synchronized read on the queue.
Returns: The java.lang.Object read or null if this is empty.
public int size()

Used to determine the number of elements in this.
Returns: An integer which is the number of empty positions in this.

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public boolean write(java.lang.Object object)

Try to insert an element into the queue.
Parameters:
object - The java.lang.Object to insert.

Returns: True if the insert succeeded, false if not.
Throws: MemoryScopeException155

CHAP TER

7

Time
Chapter 7 Time

This section contains classes that:• Allow description of a point in time with up to nanosecond accuracy andprecision (actual accuracy and precision is dependent on the precision of theunderlying system).
• Allow distinctions between absolute points in time, times relative to some starting
point, and a new construct, rational time, which allows the efficient expression of
occurrences per some interval of relative time.
The time classes required by the specification are HighResolutionTime,
AbsoluteTime, RelativeTime, and RationalTime.
Instances of HighResolutionTime are not created, as the class exists to provide
an implementation of the other three classes. An instance of AbsoluteTime
encapsulates an absolute time expressed relative to midnight January 1, 1970 GMT.
An instance of RelativeTime encapsulates a point in time that is relative to someother time value. Instances of RationalTime express a frequency by a numerator oftype long (the frequency) and a denominator of type RelativeTime. If instances ofRationalTime are given to certain constructors or methods the activity occurs forfrequency times every interval. For example, if a PeriodicTimer is given an instanceof RationalTime of (29,232) then the system will guarantee that the timer will fire
exactly 29 times every 232 milliseconds even if the system has to slightly adjust thetime between firings.

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Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. All time objects must maintain nanosecond precision and report their values interms of millisecond and nanosecond constituents.
2. Time objects must be constructed from other time objects, or from millisecond/nanosecond values.
3. Time objects must provide simple addition and subtraction operations, both forthe entire object and for constituent parts.
4. Time objects must implement the Comparable interface if it is available. The
compareTo() method must be implemented even if the interface is not available.
5. Any method of constructor that accepts a RationalTime of (x,y) must guaranteethat its activity occurs exactly x times in every y milliseconds even if the intervals
between occurrences of the activity have to be adjusted slightly. The RTSJ does
not impose any required distribution on the lengths of the intervals but strongly
suggests that implementations attempt to make them of approximately equal
lengths.

Rationale
Time is the essence of real-time systems, and a method of expressing absolute timewith sub-millisecond precision is an absolute minimum requirement. Expressing timein terms of nanoseconds has precedent and allows the implementation to provide
time-based services, such as timers, using whatever precision it is capable of while theapplication requirements are expressed to an arbitrary level of precision.
The expression of millisecond and nanosecond constituents is consistent withother Java interfaces.
The expression of relative times allows for time-based metaphors such as
deadline-based periodic scheduling where the cost of the task is expressed as arelative time and deadlines are usually represented as times relative to the beginningof the period.

HIGHRESOLUTIONTIME

7.1

HighResolutionTime

Syntax: public abstract class HighResolutionTime implements
java.lang.Comparable

Direct Known Subclasses: AbsoluteTime99, RelativeTime102
All Implemented Interfaces: java.lang.Comparable

Used to express time with nanosecond accuracy. This class is never used directly:
it is abstract and has no public constructors. Instead, use one of its subclasses
AbsoluteTime99 , RelativeTime102 , or RationalTime105 . When an API is definedthat has an HighResolutionTime as a parameter, it can take either an absolute,
relative, or rational time and will do something appropriate. All of the arithmetic
functions come in both allocating and non-allocating forms.
The standard Java java.util.Date class uses milliseconds as its basic unit inorder to provide sufficient range for a wide variety of applications. Real-time
programming generally requires nanosecond resolution, but even a 64 bit real-timeclock based in nanoseconds would be problematic in some situations, so a compound
format composed of 64 bits of millisecond timing, and 32 bits of nanoseconds withina millisecond, was chosen.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

7.1.1

Methods

public abstract AbsoluteTime99 absolute(Clock110 clock,
AbsoluteTime99 dest)

Convert this time to an absolute time, relative to some clock. Convenient
for situations where you really need an absolute time, but would like toallow relative times to be used too. Allocates a destination object ifnecessary. See the derived class comments for more specific information.
Parameters:
clock - The Clock110 reference for relative times.
dest - If null, a new object may or may not need to be allocated forthe result.
Returns: AbsoluteTime99 version of this object.
public int compareTo(HighResolutionTime97 time)

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CHAPTER 7 TIME

Compare this HighResolutionTime with the specified
HighResolutionTime. This method is provided in preference to individual
methods for each of the six boolean comparison operators (<, ==, >, >=, !=,<=). The suggested idiom for performing these comparisons is:(x.compareTo(y) <op> 0), where <op> is one of the six comparison
operators.
public int compareTo(java.lang.Object object)

For the Comparable interface.
public boolean equals(HighResolutionTime97 time)

Return true if the argument object has the same values as this.
Parameters:
time - Values are compared to this.
public boolean equals(java.lang.Object object)
Return true if the argument is a HighResolutionTime reference and has thesame values as this.
Overrides: java.lang.Object.equals(java.lang.Object) in classjava.lang.Object
Parameters:
object - Values are compared to this.
public final long getMilliseconds()

Return the milliseconds component of this.
Returns: The milliseconds component of the time past the epoch
represented by this.
public final int getNanoseconds()

Return nanoseconds component of this.
Returns: The nanoseconds component of the time past the epoch
represented by this.
public int hashCode()

Overrides: java.lang.Object.hashCode() in class java.lang.Object
public void set(HighResolutionTime97 time)

Changes the time represented by the argument to some time between the
invocation of the method and the return of the method.
Parameters:
time - The HighResolutionTime which will be set to represent thecurrent time.

ABSOLUTETIME

99

public void set(long millis)

Set the millisecond component of this to the given argument.
Parameters:
millis - This value will be the value of the millisecond componentof this at the completion of the call. If millis is negative themillisecond value of this is set to the negative value. Althoughlogically this may represent time before the epoch, invalid
results may occur if a HighResolutionTime representing time
before the epoch is given as a parameter to other methods.
public void set(long millis, intnanos)

Set the millisecond and nanosecond components of this to the given
arguments. If millis plus nanos result in a negative value the time
represented by this is time before the epoch. Although reasonable invalid
results may occur if a HighResolutionTime representing time before theepoch is given as a parameter to other methods.
Parameters:
millis - This value will be the value of the millisecond componentof this at the completion of the call.
nanos - This value will be the value of the nanosecond component of

this at the completion of the call.

7.2

AbsoluteTime

Syntax: public class AbsoluteTime extends HighResolutionTime97
All Implemented Interfaces: java.lang.Comparable

An object that represents a specific point in time given by milliseconds plus
nanoseconds past the epoch (January 1, 1970, 00:00:00 GMT). This representation
was designed to be compatible with the standard Java representation of an absolute
time in the java.util.Date class.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

7.2.1

Constructors

public AbsoluteTime()
Equivalent to new AbsoluteTime(0,0)
public AbsoluteTime(AbsoluteTime99 time)100

CHAPTER 7 TIME

Make a new AbsoluteTime99 object from the given AbsoluteTime99
object.
Parameters:
time - The AbsoluteTime99 object used as the source for the copy.
public AbsoluteTime(java.util.Date date)
Equivalent to new AbsoluteTime(date.getTime(),0).

Parameters:
date - The java.util.Date representation of time past the epoch.
public AbsoluteTime(long millis, int nanos)
Constructs an AbsoluteTime99 object, which means a time millis
milliseconds plus nanos nanoseconds past 00:00:00 GMT on January 1,
1970. If the addition of millis and nanos results in a negative value,

although reasonable in that it represents a time before the epoch, theninvalid output may occur when this is used as an argument to othermethods.
Parameters:
millis - The milliseconds component of the time past the epoch.
nanos - The nanosecond component of the time past the epoch.

7.2.2

Methods

public AbsoluteTime99 absolute(Clock110 clock,
AbsoluteTime99 destination)
Convert this time to an absolute time. For an AbsoluteTime99 , this is realeasy: it just returns itself. Presume that this time is already relative to thegiven clock.
Overrides: public abstract AbsoluteTime99 absolute(Clock110
clock, AbsoluteTime99 dest)97 in class
HighResolutionTime97

Parameters:
clock - Clock110 on which this is based.
destination - Converted to an absolute time.
public AbsoluteTime99 add(long millis, intnanos)

A new object is allocated for the result.
Parameters:
millis - Values are added to this.
nanos - Rest of value added to this.

ABSOLUTETIME

101

public AbsoluteTime99 add(long millis, intnanos,
AbsoluteTime99 destination)
If destination is non-null, the result is placed there and destination isreturned. Otherwise a new object is allocated for the result.
Parameters:
millis - Value is added to this.
nanos - Rest of value added to this.
destination - Result is placed here if non-null.

Returns: An AbsoluteTime99 . A result is always returned. A new object
is created if destination is null.
public final AbsoluteTime99 add(RelativeTime102 time)
Return this+b. A new object is allocated for the result.

Parameters:
time - Values are added to this.
public AbsoluteTime99 add(RelativeTime102 time,
AbsoluteTime99 destination)
Return this+time. If dest is non-null, the result is placed there and dest isreturned. Otherwise a new object is allocated for the result.
Parameters:
time - Values are added to this.
destination - Result is placed here if non-null.
Returns: An AbsoluteTime99 . A result is always returned. A new object
is created if dest is null.
public java.util.Date getDate()

Return the time past the epoch represented by this as a java.util.Date .
public void set(java.util.Date date)

Change the time represented by this.
Parameters:
date - java.util.Date which becomes the time represented by thisafter the completion of this method.
public final RelativeTime102 subtract(AbsoluteTime99 time)
Return this-time. A new object is allocated for the result.

Parameters:
time - Values are added to this.
public RelativeTime102 subtract(AbsoluteTime99 time,
RelativeTime102 destination)

102

CHAPTER 7 TIME

Return this-time. If destination is non-null, the result is placed there
and destination is returned. Otherwise a new object is allocated for the
resultParameters:
time - Values are subtracted from this.
destination - Result is placed here if non-null.
Returns: An AbsoluteTime99 . A result is always returned. A new object
is created if destination is null.
public final AbsoluteTime99 subtract(RelativeTime102 time)
Return this-time. A new object is allocated for the result.

Parameters:
time - Values are added to this.
public AbsoluteTime99 subtract(RelativeTime102 time,
AbsoluteTime99 destination)
Return this-time. If destination is non-null, the result is placed there
and destination is returned. Otherwise a new object is allocated for the

result.
Parameters:
time - Values are subtracted from this.
destination - Result is placed here if non-null.
Returns: An AbsoluteTime99 . A result is always returned. A new object
is created if destination is null.
public java.lang.String toString()

Return a printable version of this Time, in a format that matchesjava.util.Date.toString() with a postfix to detail the sub-second
value.
Overrides: java.lang.Object.toString() in class java.lang.Object

7.3

RelativeTime

Syntax: public class RelativeTime extends HighResolutionTime97
Direct Known Subclasses: RationalTime105
All Implemented Interfaces: java.lang.Comparable

An object that represents a time interval millis/1E3+nanos/1E9 seconds long. It
generally is used to represent a time relative to now.

RELATIVETIME

103

Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

7.3.1

Constructors

public RelativeTime()
Equivalent to new RelativeTime(0,0).
public RelativeTime(long millis, int nanos)
Construct a new RelativeTime object from the given millisecond andnanosecond components.
public RelativeTime(RelativeTime102 time)
Construct a new RelativeTime object from the given RelativeTime.

7.3.2

Methods

public AbsoluteTime99 absolute(Clock110 clock,
AbsoluteTime99 destination)
Convert this time to an absolute time. For a RelativeTime, this involves

adding the clock’s conception of now to this interval and constructing a
new AbsoluteTime99 based on the sum.
Overrides: public abstract AbsoluteTime99 absolute(Clock110
clock, AbsoluteTime99 dest)97 in class
HighResolutionTime97

Parameters:
clock - If null Clock.getRealtimeClock() is used.
destination - If null it is set to c.getTime() otherwisec.getTime(dest) is called.
public RelativeTime102 add(long millis, int nanos)

A new object is allocated for the result.
Parameters:
millis - Values are added to this.
nanos - Rest of value added to this.
public RelativeTime102 add(long millis, int nanos,
RelativeTime102 destination)
If destination is non-null, the result is placed there and destination isreturned. Otherwise a new object is allocated for the result.

104

CHAPTER 7 TIME

Parameters:
millis - Value is added to this.
nanos - Rest of value added to this.
destination - Result is placed here if non-null.

Returns: A RelativeTime. A result is always returned. A new object iscreated if destination is null.
public final RelativeTime102 add(RelativeTime102 time)
Return this+time. A new object is allocated for the result.

Parameters:
time - Values are added to this.
public RelativeTime102 add(RelativeTime102 time,
RelativeTime102 destination)
Return this+time. If destination is non-null, the result is placed there
and destination is returned. Otherwise a new object is allocated for the

result.
Parameters:
time - Values are added to this.
destination - Result is placed here if non-null.
Returns: A RelativeTime. A Result is always returned. A new object iscreated if destination is null.
public void addInterarrivalTo(AbsoluteTime99 destination)
Add this time to an AbsoluteTime99 . It is almost the same as
destination.add(this,dest) except that it accounts for (i.e., divides by)

the frequency.
public RelativeTime102 getInterarrivalTime(RelativeTime102
destination)

Return the interarrival time that is the result of dividing this interval by its
frequency. For a RelativeTime, and RationalTime105 s with a frequencyof 1, it just returns this. The interarrival time is necessarily anapproximation.
public final RelativeTime102 subtract(RelativeTime102 time)
Return this-time. A new object is allocated for the result.

Parameters:
time - Values are added to this.
public RelativeTime102 subtract(RelativeTime102 time,
RelativeTime102 destination)

RATIONALTIME

Return this-time. If destination is non-null, the result is placed there
and destination is returned. Otherwise a new object is allocated for the
result.
Parameters:
time - Values are subtracted from this.
destination - Result is placed here if non-null.
Returns: A RelativeTime. A result is always returned. A new object iscreated if destination is null.
public java.lang.String toString()

Return a printable version of this time.
Overrides: java.lang.Object.toString() in class java.lang.Object

7.4

RationalTime

Syntax: public class RationalTime extends RelativeTime102
All Implemented Interfaces: java.lang.Comparable

An object that represents a time interval millis/1E3+nanos/1E9 seconds long thatis divided into subintervals by some frequency. This is generally used in periodic
events, threads, and feasibility analysis to specify periods where there is a basic
period that must be adhered to strictly (the interval), but within that interval the
periodic events are supposed to happen frequency times, as uniformly spaced as
possible, but clock and scheduling jitter is moderately acceptable.
Caution: This class is explicitly unsafe in multithreaded situations when it isbeing changed. No synchronization is done. It is assumed that users of this class who
are mutating instances will be doing their own synchronization at a higher level.

7.4.1

Constructors

public RationalTime(int frequency)
Equivalent to new RationalTime(frequency,1000,0) which represents acycles-per-second value.
public RationalTime(int frequency, longmillis, intnanos)Create a RationalTime that indicates frequency occurrences of something(e.g. firings of a PeriodicTimer114 ) in an interval of time millis/

1E3+nanos/1E9 seconds long.
Parameters:

105

106

CHAPTER 7 TIME
frequency - The number of occurrences indicated for the given

time.
millis - The millisecond component of the time interval.
nanos - The nanosecond component of the time interval.

Throws: IllegalArgumentException - Thrown if the frequence, millis,
or nanos value is less than zero, or if the computed time interval
is less than or equal to zero.
public RationalTime(int frequency, RelativeTime102 interval)
Create a RationalTime that indicates frequency occurrences of something(e.g. firings of a PeriodicTimer114 ) in an interval of time.

Parameters:
frequency - The number of occurrences indicated for the given

interval.
interval - The interval expressed as a RelativeTime102 .

Throws: IllegalArgumentException - Thrown if the frequence is lessthan zero.

7.4.2

Methods

public AbsoluteTime99 absolute(Clock110 clock,
AbsoluteTime99 destination)

Convert this time to an absolute time, relative to some clock. Convenient
for situations where you really need an absolute time, but would like toallow rational times to be used too. Allocates a destination object ifnecessary. See the derived class comments for more specific information.
Overrides: public AbsoluteTime99 absolute(Clock110 clock,
AbsoluteTime99 destination)103 in class RelativeTime102
Parameters:
clock - The Clock110 reference for relative times.
destination - If null, a new object may or may not need to beallocated for the result.
Returns: An AbsoluteTime99 version of this object.
public void addInterarrivalTo(AbsoluteTime99 destination)
Add this time to an AbsoluteTime99 . It is almost the same as
destination.add(this,destination) except that it accounts for (i.e.,

divides by) the frequency.
Overrides: public void addInterarrivalTo(AbsoluteTime99
destination)104 in class RelativeTime102

RATIONALTIME

107

public int getFrequency()

Return the frequency component of this.
public RelativeTime102 getInterarrivalTime(RelativeTime102 dest)

Return the interarrival time that is the result of dividing this interval by its
frequency. For RationalTime instances with a frequency of 1, it justreturns this. The interarrival time is necessarily an approximation (partly
because of numerical imprecision and partly because of clock/scheduling
jitter).
Overrides: public RelativeTime102
getInterarrivalTime(RelativeTime102 destination)104 inclass RelativeTime102
public void set(long millis, intnanos)

Change the indicated interval of this to the sum of the values of thearguments.
Overrides: public void set(long millis, intnanos) 99 in class
HighResolutionTime97

Throws: IllegalArgumentException - Thrown if the millis, or nanos
value is less than zero, or if the computed time interval is lessthan or equal to zero.
public void setFrequency(int frequency)

Change the frequency of this to the given value.
Throws: ArithmeticException - Thrown if the frequency is less thanzero.

HighResolutionTime Example
HighResolutionTime defines the base class for AbsoluteTime and RelativeTime.
You cannot create HighResolutionTime objects directly, you must use one of the

subclasses:
AbsoluteTime at;

All high resolution times are a normal java time: a long(64 bit) time in milliseconds;
plus an offset in nanoseconds. All constructors take the same (milliseconds,
nanoseconds) parameters, along with some variants for convenience:at = new AbsoluteTime(System.currentTimeMillis(), 0);
System.out.print(“at=” + at + “\n”);

Relative times refer to an interval and can be added to another time:
RelativeTime step = new RelativeTime(0, 500); //500nanosecond s
System.out.print(“sum=” + at.add(step) + “\n”);

108

CHAPTER 7 TIME

Offset computations can be performed more simply with built in methods:
System.out.print(“sum2=” + at.addNanoseconds(500) + “\n”);

All of the math methods return their results as a HighResolutionTime. They all
normally allocate a new object for their return value, but they all also have alternative
forms that allow the destination to be specified:AbsoluteTime dest = new AbsoluteTime(0, 0);at.add(step, dest);
System.out.print(“sum3=” + dest + “\n”);

The destination can be the same as the object to which the method is applied. This
allows for in-place modification of the time:
at.addNanoseconds(500, at);
System.out.print(“sum4=” + at + “\n”);

Output from running the example
at=Thu Mar 23 14:01:04 PST 2000+517ms
sum=Thu Mar 23 14:01:04 PST 2000+517000500ns
sum2=Thu Mar 23 14:01:04 PST 2000+517001000ns
sum3=Thu Mar 23 14:01:04 PST 2000+517000500ns
sum4=Thu Mar 23 14:01:04 PST 2000+517001000ns

CHAP TER

8Timers
Chapter 8 Timers

This section contains classes that:• Allow creation of a timer whose expiration is either periodic or set to occur at aparticular time as kept by a system-dependent time base (clock).
• Trigger some behavior to occur on expiration of a timer, using the asynchronous
event mechanisms provided by the specification.
The classes provided by this section are Clock, Timer, PeriodicTimer, and
OneShotTimer.

An instance of the Clock class is provided by the implementation. There isnormally one clock provided, the system real-time clock. This object provides themechanism for triggering behavior on expiration of a timer. It also reports theresolution of timers provided by the implementation.
An instance of PeriodicTimer fires an AsyncEvent at constant intervals.
An instance of OneShotTimer describes an event that is to be triggered exactly
once at either an absolute time, or at a time relative to the creation of the timer. It may
be used as the source for timeouts.
Instances of Timer are not used. The Timer class provides the interface andunderlying implementation for both one-shot and periodic timers.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
109

110

CHAPTER 8 TIMERS

methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. The Clock class shall be capable of reporting the achievable resolution of timers
based on that clock.
2. The OneShotTimer class shall ensure that a one-shot timer is triggered exactly
once, regardless of whether or not the timer is enabled after expiration of theindicated time.
3. The PeriodicTimer class shall allow the period of the timer to be expressed interms of a RelativeTime or a RationalTime. In the latter case, theimplementation shall provide a best effort to perform any correction necessary tomaintain the frequency at which the event occurs.
4. If a periodic timer is enabled after expiration of the start time, the first event shalloccur immediately and thus mark the start of the first period.

Rationale
The importance of the use of one-shot timers for timeout behavior and the vagaries inthe execution of code prior to enabling the timer for short timeouts dictate that thetriggering of the timer should be guaranteed. The problem is exacerbated for periodic
timers where the importance of the periodic triggering outweighs the precision of thestart time. In such cases, it is also convenient to allow, for example, a relative time ofzero to be used as the start time for relative timers.
In many situations, it is important that a periodic task be represented as afrequency and that the period remain synchronized. In these cases, a relatively simple
correction can be enforced by the implementation at the expense of some additional
overhead for the timer.

8.1

Clock

Syntax: public abstract class ClockA clock advances from the past, through the present, into the future. It has aconcept of now that can be queried through Clock.getTime(), and it can have events
queued on it which will be fired when their appointed time is reached. There are many
possible subclasses of clocks: real-time clocks, user time clocks, simulation timeclocks. The idea of using multiple clocks may at first seem unusual but we allow it as
a possible resource allocation strategy.
Consider a real-time system where the natural events of the system have different
tolerances for jitter (jitter refers to the distribution of the differences between when

CLOCK111

the events are actually raised or noticed by the software and when they should havereally occurred according to time in the real-world). Assume the system functions
properly if event A is noticed or raised within plus or minus 100 seconds of the actual
time it should occur but event B must be noticed or raised within 100 microseconds of
its actual time. Further assume, without loss of generality, that events A and B are
periodic. An application could then create two instances of PeriodicTimer114 basedon two clocks. The timer for event B should be based on a Clock which checks its
queue at least every 100 microseconds but the timer for event A could be based on a
Clock that checked its queue only every 100 seconds. This use of two clocks reduces
the queue size of the accurate clock and thus queue management overhead is reduced.

8.1.1

Constructors

public Clock()

8.1.2

Methods

public static Clock110 getRealtimeClock()

There is always one clock object available: a real-time clock that advancesin sync with the external world. This is the default Clock.
Returns: An instance of the default Clock.
public abstract RelativeTime102 getResolution()

Return the resolution of the clock, that is, the interval between ticks. Notethat neither a clock or any software using this clock can know about events
that occur between ticks. In some sense all events happen in the past and
we only care if the past gets too long.
Returns: A RelativeTime102 object representing the resolution of this.
public AbsoluteTime99 getTime()

Return the current time in a freshly allocated object.
Returns: An AbsoluteTime99 that represents the current time of this.
public abstract void getTime(AbsoluteTime99 time)

Return the current time in an existing object. The time represented by thegiven AbsoluteTime99 is changed some time between the invocation of
the method and the return of the method.
Parameters:
time - The AbsoluteTime99 object which will have its timechanged. If null then nothing happens.

112

CHAPTER 8 TIMERS
public abstract void setResolution(RelativeTime102 resolution)

Sets the resolution of this. However, some hardware clocks do not allowchanges in resolution. If this method is called on one those the method willcomplete but the resolution will not change.
Parameters:
resolution - The new resolution of this.

8.2

Timer

Syntax: public abstract class Timer extends AsyncEvent125
Direct Known Subclasses: OneShotTimer113, PeriodicTimer114

A Timer is a timed event that measures time relative to a given Clock110 . This
class defines basic functionality available to all timers. Applications will generally
use either PeriodicTimer114 to create an event that is fired repeatedly at regularintervals, or OneShotTimer113 for an event that just fires once at a specific time. A
timer is always based on a Clock110 , which provides the basic facilities of somethingthat ticks along following some time line (real-time, cpu-time, user-time, simulationtime, etc.). All timers are created disabled and do nothing until start() is called.

8.2.1

Constructors

protected Timer(HighResolutionTime97 t, Clock110 c,
AsyncEventHandler127 handler)
Create a Timer.

Parameters:
t - The time to fire the event. Will be converted to absolute time.
c - The Clock110 on which to base this time. If null, the system realtime clock is used.
handler - The default handler to use for this event. If null, no
handler is associated with it and nothing will happen when thisevent fires until a handler is provided.

8.2.2

Methods

public ReleaseParameters43 createReleaseParameters()

Overrides: public ReleaseParameters43
createReleaseParameters()126 in class AsyncEvent125

ONESHOTTIMER
public void disable()

Disable this timer, preventing it from firing. It may subsequently be reenabled. If the timer is disabled when its fire time occurs, then it will not
fire. However, a disabled timer continues to count while it is disabled, andif it is subsequently re-enabled before its fire time occurs and is enabled
when its fire time occurs it will fire then. If it is enabled after its fire time
has occurred then it will fire immediately.
public void enable()

Re-enable this timer after it has been disabled.
public Clock110 getClock()

Used to determine the clock with which this is associated and is thus used
to decrement the implicit counter for this.
Returns: A Clock110 object which is the clock associated with this.
public AbsoluteTime99 getFireTime()

Get the time at which this event will fire. The value returned is notdependent on whether or not this is enabled or disabled.
Returns: An AbsoluteTime99 object representing the absolute time atwhich this will fire.
public void reschedule(HighResolutionTime97 time)

Change the scheduled time for this event. Can take either absolute or
relative times.
Parameters:
time - The new time at which this will fire. If null the previous firetime is still the time at which this will fire.
public void start()
A Timer starts measuring time from when it is started.

8.3

OneShotTimer

Syntax: public class OneShotTimer extends Timer112
A timed AsyncEvent125 that is driven by a clock. It will fire off once, when theclock time reaches the timeout time. If clock time has already passed the timeout time,
it will fire immediately.

8.3.1

Constructors

public OneShotTimer(HighResolutionTime97 time,
AsyncEventHandler127 handler)

113

114

CHAPTER 8 TIMERS

Create an instance of AsyncEvent125 that will execute its fire method at
the expiration of the given time.
Parameters:
time - Will fire at time.absolute(). Null equals now.
handler - The AsyncEventHandler127 that will be scheduled when
AsyncEvent.fire() is executed.
public OneShotTimer(HighResolutionTime97 start, Clock110 clock,AsyncEventHandler127 handler)
Create an instance of AsyncEvent125 , based on the given clock, that will

execute its fire method at the expiration of the given time.
Parameters:
start - Will fire at start.absolute(). Null equals now.
clock - The timer will increment based on this clock.
handler - The AsyncEventHandler127 that will be scheduled when
AsyncEvent.fire() is executed.

8.4

PeriodicTimer

Syntax: public class PeriodicTimer extends Timer112
An AsyncEvent125 whose fire method is executed periodically according to thegiven parameters. If a clock is given, calculation of the period uses the increments ofthe clock. If an interval is given or set the system guarantees that the fire method will
execute interval time units after the last execution or its given start time asappropriate. If one of the HighResolutionTime97 argument types is
RationalTime105 then the system guarantees that the fire method will be executed
exactly frequency times every unit time (see RationalTime105 constructors) by
adjusting the interval between executions of fire().
This is similar to a thread with PeriodicParameters45 except that it is lighter
weight.
If a PeriodicTimer is disabled, it still counts, and if enabled at some later time, itwill fire at its next scheduled fire time.

8.4.1

Constructors

public PeriodicTimer(HighResolutionTime97 start,
RelativeTime102 interval,
AsyncEventHandler127 handler)
Create an instance of AsyncEvent125 that executes its fire() method

periodically.

PERIODICTIMER

115

Parameters:
start - When the first interval begins. Null equals now.
interval - The time between successive executions of the fire()

method.
handler - The instance of AsyncEventHandler127 that will bescheduled each time the fire method is executed.
public PeriodicTimer(HighResolutionTime97 start,
RelativeTime102 interval, Clock110 clock,AsyncEventHandler127 handler)
Create an instance of AsyncEvent125 that executes its fire() method

periodically.
Parameters:
start - When the first interval begins. Null equals now.
interval - The time between successive executions of the fire()

method.
clock - The Clock110 whose increments are used to calculate theinterval.
handler - The instance of AsyncEventHandler127 that will bescheduled each time the fire method is executed.

8.4.2

Methods

public ReleaseParameters43 createReleaseParameters()
Create a ReleaseParameters43 object with the next fire time as the start

time and the interval of this as the period.
Overrides: public ReleaseParameters43
createReleaseParameters()112 in class Timer112
public void fire()

The only real difference between a periodic timer and a one-shot timer isthat a periodic timer contiues to fire once each period.
Overrides: public synchronized void fire()126 in class
AsyncEvent125
public AbsoluteTime99 getFireTime()

Get the next time at which this will fire. The value returned is notdependent on whether or not this is enabled or disabled.
Overrides: public AbsoluteTime99 getFireTime()113 in class Timer112
Returns: An AbsoluteTime99 object representing the absolute time atwhich this will fire.

116

CHAPTER 8 TIMERS
public RelativeTime102 getInterval()
Return the interval of this Timer112 .

Returns: A RelativeTime102 object which is the current interval of this.
public void setInterval(RelativeTime102 interval)
Reset the interval of this Timer112 .

Timer Example
Here’s a definition of the Scheduling parameters for a high priority task that we’llcreate later:
SchedulingParameters highPriority =new PriorityParameters(PriorityScheduler.getMaxPriority());

This method is a generic testbed for showing what timers do:
private static void TestTimer(String title, Timer t){
System.out.print(“\n” + title + “ test:\n”);

Figure out the start time:
final long T0 = t.getFireTime().getMilliseconds();

Ask the timer to create the appropriate release parameters:
ReleaseParameters rp = t.createReleaseParameters();

Fill in a guess at the handlers runtime:rp.setCost(new RelativeTime(10, 0)); / / agues satruntim e i n
System.out.print(“ Release parameters=” + rp + “\n”);

Add a handler that prints a message when the timer fires off:t.addHandler(new AsyncEventHandler(highPriority, rp, null) {public void handleAsyncEvent() {System.out.print(“ Timer went off at ”
+ (System.currentTimeMillis() - T0) + “\n”);}});

Give the timer a kick:t.start();PERIODICTIMER

And wait a while to watch things happen:try {Thread.sleep(1000);} catch(InterruptedException ie) {}System.out.print(“ After sleeping, t=”+ (System.currentTimeMillis() - T0) + “\n”);

Run the test bed with a one shot timer:
TestTimer(“One Shot”,
new OneShotTimer(new RelativeTime(100, 0), null));Then with a periodic timer:
TestTimer(“Periodic”,
new PeriodicTimer(new RelativeTime(100, 0),
new RelativeTime(100, 0), null));117

CHAP TER

9Asynchrony
Chapter 9 Asynchronization

This section contains classes that:• Provide mechanisms that bind the execution of program logic to the occurrenceof internal and external events.
• Provide mechanisms that allow the asynchronous transfer of control.
• Provide mechanisms that allow the asynchronous termination of threads.
This specification provides several facilities for arranging asynchronous control of
execution, some of which apply to threads in general while others apply only to realtime threads. These facilities fall into two main categories: asynchronous event
handling and asynchronous transfer of control (ATC), which includes threadtermination.
Asynchronous event handling is captured by the non-abstract class AsyncEventand the abstract classes AsyncEventHandler and BoundAsyncEventHandler. An
instance of the AsyncEvent class is an object corresponding to the possibility of anasynchronous event occurrence. An event occurrence may be initiated by either
application logic or by the occurrence of a happening external to the JVM (such as asoftware signal or a hardware interrupt handler). An event occurrence is expressed inprogram logic by the invocation of the fire() method of an instance of the
AsyncEvent class. The initiation of an event occurrence due to a happening is
implementation dependent.
An instance of the class AsyncEventHandler is an object embodying code that isscheduled in response to the occurrence of an event. The run() method of an instanceof AsyncEventHandler acts like a thread, and indeed one of its constructors takes
references to instances of SchedulingParameters, ReleaseParameters, and
119

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CHAPTER 9 ASYNCHRONIZATION
MemoryParameters. However, there is not necessarily a separate thread for eachrun() method. The class BoundAsyncEventHandler extends AsyncEventHandler,and should be used if it is necessary to ensure that a handler has a dedicated thread.
An event count is maintained so that a handler can cope with event bursts —situations where an event is fired more frequently than its handler can respond.
The interrupt() method in java.lang.Thread provides rudimentaryasynchronous communication by setting a pollable/resettable flag in the target thread,and by throwing a synchronous exception when the target thread is blocked at an
invocation of wait(), sleep(), or join(). This specification extends the effect of
Thread.interrupt() and adds an overloaded version in RealtimeThread, offering amore comprehensive and non-polling asynchronous execution control facility. It isbased on throwing and propagating exceptions that, though asynchronous, aredeferred where necessary in order to avoid data structure corruption. The mainelements of ATC are embodied in the class AsynchronouslyInterruptedException(AIE), its subclass Timed, the interface Interruptible, and in the semantics of theinterrupt methods in Thread and RealtimeThread.
A method indicates its willingness to be asynchronously interrupted by including
AIE on its throws clause. If a thread is asynchronously interrupted while executing amethod that identifies AIE on its throws clause, then an instance of AIE will bethrown as soon as the thread is outside of a section in whichATC is deferred. Several
idioms are available for handling an AIE, giving the programmer the choice of using
catch clauses and a low-level mechanism with specific control over propagation, or ahigher-level facility that allows specifying the interruptible code, the handler, and theresult retrieval as separate methods.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable to AsyncEvent
objects. Semantics that apply to particular classes, constructors, methods, and fields
will be found in the class description and the constructor, method, and field detail
sections.
1. When an instance of AsyncEvent occurs (by either program logic or ahappening), all run() methods of instances of the AsyncEventHandler class thathave been added to the instance of AsyncEvent by the execution of
addHandler() are scheduled for execution. This action may or may not be
idempotent. Every occurrence of an event increments a counter in each associatedhandler. Handlers may elect to execute logic for each occurrence of the event ornot.

PERIODICTIMER

2. Instances of AsyncEvent and AsyncEventHandler may be created and used byany program logic.
3. More than one instance of AsyncEventHandler may be added to an instance ofAsyncEvent.
4. An instance of AsyncEventHandler may be added to more than one instance ofAsyncEvent.
This list establishes the semantics and requirements that are applicable to
AsynchronouslyInterruptedException. Semantics that apply to particular classes,
constructors, methods, and fields will be found in the class description and theconstructor, method, and field detail sections.
1. Instances of the class AsynchronouslyInterruptedException can be generated
by execution of program logic and by internal virtual machine mechanisms thatare asynchronous to the execution of program logic which is the target of the
exception.
2. Program logic that exists in methods that throw
AsynchronouslyInterruptedException is subject to receiving an instance ofAsynchronouslyInterruptedException at any time during execution.
3. The RTSJ specifically requires that blocking methods in java.io.* must be
prevented from blocking indefinitely when invoked from a method with AIE in
its throws clause. The implementation, when either AIE.fire() or
RealtimeThread.interrupt() is called when control is in a java.io.* method
invoked from an interruptible method, may either unblock the blocked call, raise
an IOException on behalf of the call, or allow the call to complete normally if theimplementation determines that the call would eventually unblock.
4. Program logic executing within a synchronized block within a method with
AsynchronouslyInterruptedException in its throws clause is not subject toreceiving an instance of AIE. The interrupted state of the execution context is set
to pending and the program logic will receive the instance when control passes
out of the synchronized block if other semantics in this list so indicate.
5. Constructors are allowed to include AsynchronouslyInterruptedException in
their throws clause and will thus be interruptible.

Definitions
The RTSJ’s approach to ATC is designed to follow these principles. It is based on
exceptions and is an extension of the current Java language rules forjava.lang.Thread.interrupt(). The following terms and abbreviations will beused:
ATC - Asynchronous Transfer of Control

121

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CHAPTER 9 ASYNCHRONIZATION

AIE - (Asynchronously Interrupted Exception) The classjavax.realtime.AsynchronouslyInterruptedException, a subclass ofjava.lang.InterruptedException.

AI - (Asynchronously Interruptible) A method is said to be asynchronouslyinterruptible if it includes AIE in its throws clause.
ATC-deferred section - a synchronized method, a synchronized statement, or any
method or constructor without AIE in its throws clause.

Summary of Operation
In summary, ATC works as follows:
If t is an instance of RealtimeThread or NoHeapRealtimeThread andt.interrupt() or AIE.fire() is executed by any thread in the system then:
1. If control is in an ATC-deferred section, then the AIE is put into a pending state.
2. If control is in an AI method and not in a try block or a synchronized block, thenthe method from which the AI method had been invoked immediately receives
the fired AIE without further execution of the logic in the AI method and the
AIE’s state is set to pending.
3. As with normal Java exception semantics, if control is within a try blockcontained within an AI method control transfers to the first statement of theappropriate catch clause. If no appropriate catch clause exists, then the calling
method receives the fired AIE and the AIE’s state is set to pending.
4. If control is in either wait(), sleep(), or join(), the thread is awakened and thefired AIE (which is a subclass of I nterruptedException) is thrown. Then ATC
follows option 1, 2, or 3 as appropriate.
5. If control is in a non-AI method, control continues normally until the first attempt
to return to an AI method or invoke an AI method. Then ATC follows option 1, 2,
or 3 as appropriate.
6. If control is transferred from a non-AI method to an AI method through the actionof propagating an exception and if an AIE is pending then when the transition to
the AI-method occurs the thrown exception is discarded and replaced by the AIE.
If an AIE is in a pending state then this AIE is thrown only when:
1. Control enters an AI-method.
2. Control returns to an AI-method.
3. Control leaves a synchronized block within an AI-method.

PERIODICTIMER

When happened() is called on an AIE or that AIE is superseded by another the first
AIE’s state is made non-pending.
An AIE may be raised while another AIE is pending or in action. Because AI
code blocks are nested by method invocation (a stack-based nesting) there is a naturalprecedence among active instances of AIE. Let AIE_0 be the AIE raised whent.interrupt() is invoked and AIE_i (i = 1,...,n, for n unique instances of AIE) be the
AIE raised when AIE_i.fire() is invoked. Assume stacks grow down and therefore
the phrase “a frame lower on the stack than this frame” refers to a method at a deeper
nesting level.
1. If the current AIE is an AIE_0 and the new AIE is an AIE_x associated with any
frame on the stack then the new AIE (AIE_x) is discarded.
2. If the current AIE is an AIE_x and the new AIE is an AIE_0, then the current AIE(AIE_x) is replaced by the new AIE (AIE_0).
3. If the current AIE is an AIE_x and the new AIE is an AIE_y from a frame lower
on the stack, then the new AIE discarded.
4. If the current AIE is an AIE_x and the new AIE is an AIE_y from a frame higher
on the stack, the current AIE is replaced by the new AIE.

Non-Blocking I/O
The RTSJ will provide mechanisms and programming disciplines to allow
applications to bound waiting on I/O calls. There are two cases: (1) the device on
which I/O is being performed (and thus its associated stream) is no longer needed and(2) timed, non-blocking I/O (where the device and associated streams remain viable).
For case 1 the RTSJ requires that when stream.close() is called on a stream, allblocked I/O calls will throw appropriate instances of IOException. Note that thisrequirement adds additional semantics to stream.close() which require blocked calls
to throw an appropriate exception in addition to just checking for closed streams at thecommencement of the I/O call. For case 2 the RTSJ recommends a programming
discipline in which one thread uses the blocking calls from java.io.* and provides
timed, non-blocking methods used by other threads. (See the examples in the section
on asynchrony).

Rationale
The design of the asynchronous event handling was intended to provide the necessaryfunctionality while allowing efficient implementations and catering to a variety ofreal-time applications. In particular, in some real-time systems there may be a large
number of potential events and event handlers (numbering in the thousands or perhaps
even the tens of thousands), although at any given time only a small number will be

123

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used. Thus it would not be appropriate to dedicate a thread to each event handler. The
RTSJ addresses this issue by allowing the programmer to specify an event handler
either as not bound to a specific thread (the class AsyncEventHandler) or
alternatively as bound to a thread (BoundAsyncEventHandler).
Events are dataless: the fire method does not pass any data to the handler. Thiswas intentional in the interest of simplicity and efficiency. An application that needs
to associate data with an AsyncEvent can do so explicitly by setting up a buffer; it willthen need to deal with buffer overflow issues as required by the application.
The ability for one thread to trigger an ATC in another thread is necessary inmany kinds of real-time applications but must be designed carefully in order tominimize the risks of problems such as data structure corruption and deadlock. Thereis, invariably, a tension between the desire to cause an ATC to be immediate, and thedesire to ensure that certain sections of code are executed to completion.
One basic decision was to allow ATC in a method only if the method explicitly
permits this. The default of no ATC is reasonable, since legacy code might be written
expecting no ATC, and asynchronously aborting the execution of such a method could
lead to unpredictable results. Since the natural way to model ATC is with an exception(AsynchronouslyInterruptedException, or AIE), the way that a method indicates
its susceptibility toATC is by including AIE on its throws clause. Causing thisexception to be thrown in a thread t as an effect of calling t.interrupt() was anatural extension of the semantics of interrupt as currently defined byjava.lang.Thread.
One ATC-deferred section is synchronized code. This is a context that needs tobe executed completely in order to ensure a program operates correctly. If
synchronized code is aborted, a shared object could be left in an inconsistent state.
Constructors and finally clauses are subject to interruption. If a constructor isaborted, an object might be only partially initialized. If a finally clause is aborted,needed cleanup code might not be performed. It is the programmer’s responsibility toensure that executing these constructs does not induce unwanted ATC latency. Notethat by making synchronized code ATC-deferred, this specification avoids the
problems that caused Thread.stop() to be deprecated and that have made the use of
Thread.destroy() prone to deadlock.
A potential problem with using the exception mechanism to model ATC is that amethod with a “catch-all” handler (for example a catch clause identifying Exception
or even Throwable as the exception class) can inadvertently intercept an exception
intended for a caller. This problem is avoided by having special semantics forcatching an instance of AIE. Even though a catch clause may catch an AIE, theexception will be propagated unless the handler invokes the happened method from

ASYNCEVENT

125

AIE. Thus, if a thread is asynchronously interrupted while in a try block that has a
handler such as
catch (Throwable e){ return; }then the AIE instance will still be propagated to the caller.
This specification does not provide a special mechanism for terminating a thread;ATC can be used to achieve this effect. This means that, by default, a thread cannot beterminated; it needs to invoke methods that have AIE in their throws clauses.
Allowing termination as the default would have been questionable, bringing the same
insecurities that are found in Thread.stop() and Thread.destroy().

9.1

AsyncEvent

Syntax: public class AsyncEvent
Direct Known Subclasses: Timer112

An asynchronous event represents something that can happen, like a light turning
red. It can have a set of handlers associated with it, and when the event occurs, thehandler is scheduled by the scheduler to which it holds a reference (seeAsyncEventHandler127 and Scheduler36 ).
A major motivator for this style of building events is that we expect to have lotsof events and lots of event handlers. An event handler is logically very similar to athread, but it is intended to have a much lower cost (in both time and space) —assuming that a relatively small number of events are fired and in the process of being
handled at once. AsyncEvent.fire() differs from a method call because the handler(a) has scheduling parameters and (b) is executed asynchronously.

9.1.1

Constructors

public AsyncEvent()

9.1.2

Methods

public synchronized void addHandler(AsyncEventHandler127 handler)

Add a handler to the set of handlers associated with this event. An
AsyncEvent may have more than one associated handler.
Parameters:
handler - The new handler to add to the list of handlers alreadyassociated with this. If handler is null then nothing happens.

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Since this affects the constraints expressed in the release parametersof the existing schedulable objects, this may change thefeasibility of the current schedule.
public void bindTo(java.lang.String happening)

Binds this to an external event (a happening). The meaningful values ofhappening are implementation dependent. This AsyncEvent is considered
to have occurred whenever the external event occurs.
Parameters:
happening - An implementation dependent value that binds thisAsyncEvent to some external event.
public ReleaseParameters43 createReleaseParameters()
Create a ReleaseParameters43 block appropriate to the timing

characteristics of this event. The default is the most pessimistic:
AperiodicParameters47 . This is typically called by code that is setting up

a handler for this event that will fill in the parts of the release parametersthat it knows the values for, like cost.
public synchronized void fire()
Fire (schedule the run() methods of) the handlers associated with thisevent.
public boolean handledBy(AsyncEventHandler127 target)

Returns true if and only if this event is handled by this handler.
Parameters:
target - The handler to be tested to determine if it is associated withthis. Returns false if target is null.
public synchronized void removeHandler(AsyncEventHandler127
handler)

Remove a handler from the set associated with this event.
Parameters:
handler - The handler to be disassociated from this. If null nothing

happens. If not already associated with this then nothing
happens.
public synchronized void setHandler(AsyncEventHandler127 handler)

Associate a new handler with this event, removing all existing handlers.
Since this affects the constraints expressed in the release parameters of theexisting schedulable objects, this may change the feasibility of the current
schedule.

ASYNCEVENTHANDLER

Parameters:
handler - The new and only handler to be associated with this. If
handler is null then no handler will be associated with this (i.e.,remove all handlers).

9.2

AsyncEventHandler

Syntax: public abstract class AsyncEventHandler implements Schedulable35
Direct Known Subclasses: BoundAsyncEventHandler132
All Implemented Interfaces: java.lang.Runnable, Schedulable35

An asynchronous event handler encapsulates code that gets run at some timeafter an AsyncEvent125 occurs.
It is essentially a java.lang.Runnable with a set of parameter objects, making
it very much like a RealtimeThread22 . The expectation is that there may bethousands of events, with corresponding handlers, averaging about one handler per
event. The number of unblocked (i.e., scheduled) handlers is expected to be relatively
small.
It is guaranteed that multiple firings of an event handler will be serialized. It isalso guaranteed that (unless the handler explicitly chooses otherwise) for each firingof the handler, there will be one execution of the handleAsyncEvent() method.
There is no restriction on what handlers may do. They may run for a long or shorttime, and they may block. (Note: blocked handlers may hold system resources.)
Normally, handlers are bound to an execution context dynamically, when their
AsyncEvent125 occurs. This can introduce a (small) time penalty. For criticalhandlers that can not afford the expense, and where this penalty is a problem, use a
BoundAsyncEventHandler132 .

9.2.1

Constructors

public AsyncEventHandler()
Create a handler whose SchedulingParameters40 are inherited from thecurrent thread and does not have either ReleaseParameters43 orMemoryParameters79 .
public AsyncEventHandler(boolean nonheap)

Create a handler whose parameters are inherited from the current thread, if
it is a RealtimeThread22 , or null otherwise.

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Parameters:
nonheap - A flag meaning, when true, that this will havecharacteristics identical to a NoHeapRealtimeThread26 . A false

value means this will have characteristics identical to a
RealtimeThread22 . If true and the current thread is not a
NoHeapRealtimeThread26 or a RealtimeThread22 executing
within a ScopedMemory62 or ImmortalMemory62 scope then an
IllegalArgumentException is thrown.
public AsyncEventHandler(SchedulingParameters40 scheduling,
ReleaseParameters43 release,
MemoryParameters79 memory, MemoryArea60 area,
ProcessingGroupParameters50 group)

Create a handler with the specified parameters.
Parameters:
release - A ReleaseParameters43 object which will be associatedwith the constructed instance of this. If null this will have no
ReleaseParameters43 .
scheduling - A SchedulingParameters40 object which will be
associated with the constructed instance of this. If null this will
be assigned the reference to the SchedulingParameters40 of
the current thread.
memory - A MemoryParameters79 object which will be associatedwith the constructed instance of this. If null this will have noMemoryParameters79 .
area - The MemoryArea60 for this. If null the memory area will bethat of the current thread.
group - A ProcessingGroupParameters50 object to which this will
be associated. If null this will not be associated with any
processing group.
public AsyncEventHandler(SchedulingParameters40 scheduling,
ReleaseParameters43 release,
MemoryParameters79 memory, MemoryArea60 area,
ProcessingGroupParameters50 group,boolean nonheap)

Create a handler with the specified parameters.
Parameters:
scheduling - A SchedulingParameters40 object which will be

associated with the constructed instance of this. If null this will
be assigned the reference to the SchedulingParameters40 of
the current thread.

ASYNCEVENTHANDLER
release - A ReleaseParameters43 object which will be associatedwith the constructed instance of this. If null this will have no
ReleaseParameters43 .
memory - A MemoryParameters79 object which will be associatedwith the constructed instance of this. If null this will have noMemoryParameters79 .
area - The MemoryArea60 for this. Must be a reference to a
ScopedMemory62 or ImmortalMemory62 object if noheap is true.
group - A ProcessingGroupParameters50 object to which this will
be associated. If null this will not be associated with any
processing group.
nonheap - A flag meaning, when true, that this will havecharacteristics identical to a NoHeapRealtimeThread26 .

9.2.2

Methods

public void addToFeasibility()

Inform the scheduler and cooperating facilities that this thread’s feasibility
parameters should be considered in feasibility analysis until further
notified.
protected final synchronized int getAndClearPendingFireCount()

Atomically set to zero the number of pending executions of this handler
and returns the value from before it was cleared. This is used in handlersthat can handle multiple firings and that want to collapse them together.
The general form for using this is:public void handleAsyncEvent() {int fireCount = getAndClearPendingFireCount();<handle the events>}

Returns: The pending fire count.
protected synchronized int getAndDecrementPendingFireCount()

Atomically decrements the number of pending executions of this handler(if it was non-zero) and returns the value from before the decrement. Thiscan be used in the handleAsyncEvent() method in this form to handle
multiple firings:

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public void handleAsyncEvent() {<setup>do {<handle the event>} while(getAndDecrementPendingFireCount()>0);}

This construction is necessary only in the case where one wishes to avoid
the setup costs since the framework guarantees that handleAsyncEvent()
will be invoked the appropriate number of times.
Returns: The pending fire count.
protected synchronized int getAndIncrementPendingFireCount()

Atomically increments the number of pending executions of this handler
and returns the value from before the increment. The handleAsyncEvent()
method does not need to do this, since the surrounding framework
guarantees that the handler will be re-executed the appropriate number oftimes. It is only of value when there is common setup code that isexpensive.
Returns: The pending fire count.
public MemoryArea60 getMemoryArea()

Get the current memory area.
Returns: The current memory area in which allocations occur.
public MemoryParameters79 getMemoryParameters()

Get the memory parameters associated with this handler.
Returns: The MemoryParameters79 object associated with this.
public ProcessingGroupParameters50 getProcessingGroupParameters()
Returns a reference to the ProcessingGroupParameters50 object.
public ReleaseParameters43 getReleaseParameters()

Get the release parameters associated with this handler.
Returns: The ReleaseParameters43 object associated with this.
public Scheduler36 getScheduler()
Return the Scheduler36 for this handler.

Returns: The instance of the scheduler managing this.
public SchedulingParameters40 getSchedulingParameters()

Returns a reference to the scheduling parameters object.
Returns: The SchedulingParameters40 object associated with this.

ASYNCEVENTHANDLER
public abstract void handleAsyncEvent()

Override this method to define the action to be taken by this handler. Thismethod will be invoked repeatedly while fireCount is greater than zero.
public void removeFromFeasibility()

Inform the scheduler and cooperating facilities that this thread’s feasibility
parameters should not be considered in feasibility analysis until further
notified.
public final void run()

Used by the asynchronous event mechanism, see AsyncEvent125 . Thismethod invokes handleAsyncEvent() repeatedly while the fire count isgreater than zero. Applications cannot override this method and should
thus override handleAsyncEvent() in subclasses with the logic of thehandler.
public void setMemoryParameters(MemoryParameters79 memory)

Set the memory parameters associated with this handler. When it is next
fired, the executing thread will use these parameters to control memory
allocation. Does not affect the current invocation of the run() of thishandler.
Parameters:
memory - A MemoryParameters79 object which will become theMemoryParameters79 associated with this after the method call.
public void
setProcessingGroupParameters(ProcessingGroupParameters50 parameters)
Sets the reference to the ProcessingGroupParameters50 object.
public void setReleaseParameters(ReleaseParameters43 parameters)

Set the release parameters associated with this handler. When it is next
fired, the executing thread will use these parameters to control scheduling.
If the scheduling parameters of a handler is set to null, the handler will be
executed immediately when it is fired, in the thread of the firer. Does notaffect the current invocation of the run() of this handler.
Since this affects the constraints expressed in the release parameters of theexisting schedulable objects, this may change the feasibility of the current
schedule.
Parameters:
parameters - A ReleaseParameters43 object which will become
the ReleaseParameters43 associated with this after the method

call.

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public void setScheduler(Scheduler36 scheduler)

Set the scheduler for this handler. A reference to the scheduler which will
manage the execution of this thread.
Parameters:
scheduler - An instance of Scheduler36 (or subclasses) which will
manage the execution of this thread. If scheduler is nullnothing happens.
Throws: IllegalThreadStateException
public void setSchedulingParameters(SchedulingParameters40
parameters)

Set the scheduling parameters associated with this handler. When it is next
fired, the executing thread will use these parameters to control scheduling.
Does not affect the current invocation of the run() of this handler.
Parameters:
parameters - A SchedulingParameters40 object which will
become the SchedulingParameters40 object associated withthis after the method call.

9.3

BoundAsyncEventHandler

Syntax: public abstract class BoundAsyncEventHandler extends
AsyncEventHandler127

All Implemented Interfaces: java.lang.Runnable, Schedulable35

A bound asynchronous event handler is an asynchronous event handler that ispermanently bound to a thread. Bound asynchronous event handlers are meant for use
in situations where the added timeliness is worth the overhead of binding the handler
to a thread.

9.3.1

Constructors

public BoundAsyncEventHandler()

Create a handler whose parameters are inherited from the current thread, if
it is a RealtimeThread22 , or null otherwise.
public BoundAsyncEventHandler(SchedulingParameters40 scheduling,
ReleaseParameters43 release,
MemoryParameters79 memory, MemoryArea60 area,
ProcessingGroupParameters50 group,boolean nonheap)

INTERRUPTIBLE

133

Create a handler with the specified ReleaseParameters43 andMemoryParameters79 .
Parameters:
scheduling - A SchedulingParameters40 object which will be

associated with the constructed instance of this. If null this will
be assigned the reference to the SchedulingParameters40 of
the current thread.
release - The ReleaseParameters43 object for this. A value ofnull will construct this without a ReleaseParameters43 object.
memory - The MemoryParameters79 object for this. A value of null
will construct this without a MemoryParameters79 object.
area - The MemoryArea60 for this. Must be a reference to a
ScopedMemory62 or ImmortalMemory62 object if noheap is true.
nonheap - A flag meaning, when true, that this will havecharacteristics identical to a NoHeapRealtimeThread26 .
group - A ProcessingGroupParameters50 object to which this will
be associated. If null this will not be associated with any
processing group.

9.4

Interruptible

Syntax: public interface Interruptible
Interruptible is an interface implemented by classes that will be used asarguments on the doInterruptible() ofAsynchronouslyInterruptedException134 and its subclasses. doInterruptible()
invokes the implementation of the method in this interface. Thus the system canensure correctness before invoking run() and correctly cleaned up after run()
returns.

9.4.1

Methods

public void interruptAction(AsynchronouslyInterruptedException134
exception)
This method is called by the system if the run() method is excepted. Using
this the program logic can determine if the run() method completed

normally or had its control asynchronously transferred to its caller.
Parameters:
exception - Used to invoke methods on
AsynchronouslyInterruptedException134 from within theinterruptAction() method.

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CHAPTER 9 ASYNCHRONIZATION
public void run(AsynchronouslyInterruptedException134 exception)

The main piece of code that is executed when an implemention is given to
doInterruptible(). When you create a class that implements thisinterface (usually through an anonymous inner class) you must remember
to include the throws clause to make the method interruptible. If thethrows clause is omitted the run() method will not be interruptible.
Parameters:
exception - Used to invoke methods on
AsynchronouslyInterruptedException134 from within therun() method.

Throws: AsynchronouslyInterruptedException134

9.5

AsynchronouslyInterruptedException

Syntax: public class AsynchronouslyInterruptedException extends
java.lang.InterruptedException

Direct Known Subclasses: Timed137
All Implemented Interfaces: java.io.Serializable

An special exception that is thrown in response to an attempt to asynchronouslytransfer the locus of control of a RealtimeThread22 .
When a method is declared with AsynchronouslyInterruptedException in its
throws clause the platform is expected to asynchronously throw this exception if
RealtimeThread.interrupt() is called while the method is executing, or if such aninterrupt is pending any time control returns to the method. The interrupt is not
thrown while any methods it invokes are executing, unless they are, in turn, declared
to throw the exception. This is intended to allow long-running computations to beterminated without the overhead or latency of polling withjava.lang.Thread.interrupted() .
The throws AsynchronouslyInterruptedException clause is a marker on astack frame which allows a method to be statically marked as asynchronouslyinterruptible. Only methods that are marked this way can be interrupted.
When Thread.interrupt(), public synchronized void interrupt()24 , or
this.fire() is called, the AsynchronouslyInterruptedException is comparedagainst any currently pending AsynchronouslyInterruptedException on the thread.
If there is none, or if the depth of the AsynchronouslyInterruptedException is lessthan the currently pending AsynchronouslyInterruptedException — i.e., it isASYNCHRONOUSLYINTERRUPTEDEXCEPTION

targeted at a less deeply nested method call — it becomes the currently pending
interrupt. Otherwise, it is discarded.
If the current method is interruptible, the exception is thrown on the thread.
Otherwise, it just remains pending until control returns to an interruptible method, atwhich point the AsynchronouslyInterruptedException is thrown. When aninterrupt is caught, the caller should invoke the happened() method on the
AsynchronouslyInterruptedException in which it is interested to see if it matches
the pending AsynchronouslyInterruptedException. If so, the pending
AsynchronouslyInterruptedException is cleared from the thread. Otherwise, itwill continue to propagate outward.
Thread.interrupt() and RealtimeThread.interrupt() generate a system
available generic AsynchronouslyInterruptedException which will always
propagate outward through interruptible methods until the generic
AsynchronouslyInterruptedException is identified and stopped. Other sources(e.g., this.fire() and Timed137 ) will generate a specific instance ofAsynchronouslyInterruptedException which applications can identify and thus
limit propogation.

9.5.1

Constructors

public AsynchronouslyInterruptedException()
Create an instance of AsynchronouslyInterruptedException.

9.5.2

Methods

public synchronized boolean disable()

Defer the throwing of this exception. If interrupt() is called when thisexception is disabled, the exception is put in pending state. The exception
will be thrown if this exception is subsequently enabled. This is valid only
within a call to doInterruptible(). Otherwise it returns false and doesnothing.
Returns: True if this is disabled otherwise returns false.
public boolean doInterruptible(Interruptible133 logic)
Execute the run() method of the given Interruptible133 . This method
may be on the stack in exactly one RealtimeThread22 . An attempt to

invoke this method in a thread while it is on the stack of another or thesame thread will cause an immediate return with a value of false.

135

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Parameters:
code - An instance of an Interruptible133 whose run() method
will be called.
Returns: True if the method call completed normally. Returns false ifanother call to doInterruptible has not completed.
public synchronized boolean enable()

Enable the throwing of this exception. This is valid only within a call to
doInterruptible(). Otherwise it returns false and does nothing.
Returns: True if this is enabled otherwise returns false.
public synchronized boolean fire()

Make this exception the current exception if doInterruptible() has been
invoked and not completed.
Returns: True if this was fired. If there is no current invocation of
doInterruptible(), then false is returned with no other effect.
False is also returned if there is already a current
doInterruptible() or if disable() has been called.
public static AsynchronouslyInterruptedException134 getGeneric()
Return the system generic AsynchronouslyInterruptedException,
which is generated when RealtimeThread.interrupt() is invoked.
public boolean happened(boolean propagate)

Used with an instance of this exception to see if the current exception is
this exception.
Parameters:
propagate - Propagate the exception if true and this exception is not

the current one. If false, then the state of this is set tononpending (i.e., it will stop propagating).
Returns: True if this is the current exception. Returns false if this is not thecurrent exception.
public boolean isEnabled()

Query the enabled status of this exception.
Returns: True if this is enabled otherwise returns false.
public void propagate()

Cause the current exception to continue up the stack.

TIMED

9.6

137

Timed

Syntax: public class Timed extends AsynchronouslyInterruptedException134
All Implemented Interfaces: java.io.Serializable

Create a scope in a RealtimeThread22 for which interrupt() will be called at
the expiration of a timer. This timer will begin measuring time at some point between
the time doInterruptible() is invoked and the time the run() method of theInterruptible object is invoked. Each call of doInterruptible() on an instance ofTimed will restart the timer for the amount of time given in the constructor or the mostrecent invocation of resetTime(). All memory use of Timed occurs during
construction or the first invocation of doInterruptible(). Subsequent invokes of
doInterruptible() do not allocate memory.
Usage: new Timed(T).doInterruptible(interruptible);

9.6.1

Constructors

public Timed(HighResolutionTime97 time)
Create an instance of Timed with a timer set to timeout. If the time is in thepast the AsynchronouslyInterruptedException134 mechanism isimmediately activated.
Parameters:
time - The interval of time between the invocation of
doInterruptible() and when interrupt() is called on
currentRealtimeThread(). If null thejava.lang.IllegalArgumentException is thrown.
Throws: IllegalArgumentException

9.6.2

Methods

public boolean doInterruptible(Interruptible133 logic)

Execute a timeout method. Starts the timer and executes the run() methodof the given Interruptible133 object.
Overrides: public boolean doInterruptible(Interruptible133
logic)135 in class AsynchronouslyInterruptedException134
Parameters:
logic - Implements an Interruptible133 run() method. If nullnothing happens.

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public void resetTime(HighResolutionTime97 time)

To reschedule the timeout for the next invocation of doInterruptible().
Parameters:
time - This can be an absolute time or a relative time. If null the
timeout is not changed.

AsyncEvent Example
An easy way to construct event handlers is with anonymous inner classes:
AsyncEventHandler h = new AsyncEventHandler() {public void handleAsyncEvent() {System.out.print(“The first handler ran!\n”);}};

They get associated with events by adding them to the event’s handler list. There is aslight naming issue that sometimes causes confusion: in the java.awt package (andcommon gui api usage), an ‘event’ refers to something that has happened. In therealtime package, (and common real-time system usage) an event refers to somethingthat may happen in the future. To have our handler h associated with the inputReady
event:
inputReady.addHandler(h);Sometime in the future, the event gets fired:
System.out.print(“Test 1\n”);
inputReady.fire();Thread.yield();
System.out.print(“Fired the event\n”);

Event handlers are like threads in that they have release, scheduling, and memory
parameters. This complicates the preceeding example: by default, handlers are createdwith the same priority as the creating thread. When inputReady is fired, h becomesrunnable, but the current thread is already running. So h just sits in the run queue
waiting for the current process to do something that gives up the processor.

TIMEDFor example, we can create a low and high priority handler like this:SchedulingParameters low = new PriorityParameters(
PriorityScheduler.getMinPriority(null));
inputReady.setHandler(new AsyncEventHandler(low,null,null) {public void handleAsyncEvent() {System.out.print(“The low priority handler ran!\n”);}});
SchedulingParameters high = new PriorityParameters(
PriorityScheduler.getMaxPriority(null));
inputReady.addHandler(new AsyncEventHandler(high, null, null) {public void handleAsyncEvent() {System.out.print(“The high priority handler ran!\n”);}});

If we fire the event off, the low priority handler doesn’t run until there’s some idle
time on the processor:
System.out.print(“\nTest 2\n”);
inputReady.fire();System.out.print(“After the fire\n”);
Thread.sleep(100);
System.out.print(“After the sleep\n”);
ReleaseParameters are somewhat problematic with respect to AsyncEvents. They
encapsulate the information needed for feasibility analysis, which consists of a
combination of information about when things happen and about the computation thatis triggered. In the case of instances of AsyncEvent, the knowledge about those twocollections of information is seperated: the event knows about when things happen,
while the handler knows about the computation that is triggered. When setting up
ReleaseParameters for an AsyncEvent, the following pattern should be followed:
ReleaseParameters rp = inputReady.createReleaseParameters();rp.setCost(new RelativeTime(1,0));AsyncEventHandler h2 = new AsyncEventHandler(high, rp, null){public void handleAsyncEvent() { System.out.print(“Whatever...\n”); }};

The call, inputReady.createReleaseParameters() creates a ReleaseParameters
object (actually some subclass of ReleaseParameters) and populates it withinformation about when the event will fire. For example, if inputReady were a
PeriodicTimer event, createReleaseParameters() would create a
PeriodicParameters object and fill in the periodicity fields.

139

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CHAPTER 9 ASYNCHRONIZATION

Output from running the example
Test 1
The first handler ran!
Fired the event
Test 2The high priority handler ran!
After the fire
The low priority handler ran!
After the sleep

AIE Example
An AsynchronouslyInterruptedException allows code to be written so that it canbe aborted in a controlled fashion in response to an action by another thread, or by an
external event. A block of interruptible code is associated with the exception that canbe used to terminate its execution. If the asynchronous exception is fired at any point
during the execution of the interruptible code, control is transferred to the end of theexecutable section of code. If the interruptible code calls some other code that isn’t
interruptible and the exception is fired, the exception remains pending until
superseded by a more pertinent exception, or until control returns to the interruptible
section. In the latter case, the interruptible section is then terminated. To make a
block of code interruptible by a particular asynchronous exception, it must be
encapsulated in a class that implements the Interruptible interface. An instance ofthe class is passed to the doInterruptible() method on the
AsynchronouslyInterruptedException that can interrupt the code block. This
causes the run method of the interruptible object to be executed. Execution can beinterrupted at any point during the run method. Note: Only one thread can be
executing interruptible code within an asynchronous exception at a given time. To
interrupt more than one thread it is necessary to multiplex a source, such as an
AsyncEvent, to multiple asynchronous exceptions. An anonymous inner class can beused to code inline interruptible code, as in the following:
MyInterrupt aie = new MyInterrupt();
aie.doInterruptible(new Interruptible() {public void runNonInterruptible() {//do something that can’t be interrupted}
public void run(AsynchronouslyInterruptedException e)
throws AsynchronouslyInterruptedException {TIMED

141

This method can be interrupted at any point in time do something “interrupt”-safe
Call to a non-interruptible method. If the asynchronous exception is fired duringexecution of this method, it will be deferred until return from the method.
runNonInterruptible();We can also disable the asynchronous exception for a period of time. If it is fired, it
will be deferred until it’s enabled again:e.disable();And enable it again later:e.enable();Upon return from run, aie can no longer effect execution of the thread.
public void interruptAction(AsynchronouslyInterruptedException e)

If we want to know whether the method was actually interrupted, we can make use of
the interruptAction() entry point of the Interruptible object. This is only called
if the run method was interrupted.
aie.doInterruptible(new Interruptible() {public void run(AsynchronouslyInterruptedException e)
throws AsynchronouslyInterruptedException {//do something interrupt-safe}
public void interruptAction(AsynchronouslyInterruptedException e){try {MyInterrupt myAie = (MyInterrupt)e;
myAie.wasInterrupted = true;} catch (ClassCastException ce) {}}//do something about it - abort or retry}

AIE Example 2
In order to asynchronously interrupt code running in another thread, it is necessary toobtain a reference to the AsynchronouslyInterruptedException that the thread is
expecting. This will usually be stored in a field on the thread, or may be kept in a
globally accessible object. Once the reference is obtained, the other thread can beinterrupted by calling the fire method on the asynchronous exception.
getInterrupt().fire();142

CHAPTER 9 ASYNCHRONIZATION

An asynchronous exception may be bound to an event, in which case, firing the event
will result in the asynchronous exception being fired automatically. The real-time
extensions package does this to implement timed expressions, where expiration of thetimer automatically interrupts the expression:(new Timed(new RelativeTime(50,0))).doInterruptible(
new Interruptible() {public void run(AsynchronouslyInterruptedException e) {The run method will have 50 ms to execute. At the end of this time an asynchronous
exception will be fired, interrupting the run method.
public void interruptAction(AsynchronouslyInterruptedException e)

AIE Example 3
An asynchronous exception may be bound to an AsyncEvent. This allows a single
asynchronous event to be used to interrupt multiple threads. It also allows
implementation dependent external events (happenings) to be used to fireasynchronous exceptions that interrupt threads.
class Interrupt extends AsynchronouslyInterruptedException {private class EventHandler extends AsyncEventHandler {AsyncEvent event;
AsynchronouslyInterruptedException aie;
public EventHandler(AsyncEvent event,
AsynchronouslyInterruptedException aie) {super(new SchedulingParameters(RealtimeThread.MAX_PRIORITY),null,null);
this.event = event;this.aie = aie;try {event.addHandler(this);} catch (AdmissionControlException e) {}}public void handleAsyncEvent() {aie.fire();}}EventHandler handler;
public Interrupt(AsyncEvent event) {super();Create the EventHandler for firing the asynchronous exception.
handler = new EventHandler(event, this);TIMED

In order to asynchronously interrupt code running in another thread(s), it is necessary
to obtain a reference to the AsyncEvent that has been bound to the asynchronous
exceptions that those threads are expecting. Once the reference is obtained, the otherthreads can be interrupted by calling the fire method on the asynchronous event.
getInterrupt().fire();For the special case of POSIX systems, this can also be initiated in response to asignal:POSIXSignalHandler.addHandler(POSIXSignalHandler.SIGINT,
new AsyncEventHandler() {public void handleAsyncEvent() {AIEExample3.getInterrupt().fire();
}
});
}
}

AIE Example 4Interruptible blocks of code can be nested. In this case the asynchronous exception of
the less deeply nested interruptible block takes precedence over the moreasynchronous exception of the more deeply nested block. If an asynchronous
exception is “in flight” for the most deeply nested interruptible block when the otherasynchronous exception is fired, the new exception supersedes the first, causing theinterrupt to transfer control to the end of the outer block. An anonymous inner classcan be used to code inline interruptible code, as in the following:
AsynchronouslyInterruptedException hiPriority =new AsynchronouslyInterruptedException();hiPriority.doInterruptible(new Interruptible() {public void run(AsynchronouslyInterruptedException e)
throws AsynchronouslyInterruptedException {AsynchronouslyInterruptedException loPriority =new AsynchronouslyInterruptedException();This method can be interrupted at any point in time by the hiPriority exception
loPriority.doInterruptible(new Interruptible() {public void run(AsynchronouslyInterruptedException e)
throws AsynchronouslyInterruptedException {This method can be interrupted at any point in time by either the hiPriority or the
loPriority exception. In the case of the hiPriority exception, control is transferredto the end of the outer run method.
public void interruptAction(AsynchronouslyInterruptedException e)143CHAP TER

10

System and Options
Chapter 10 System and Options

This section contains classes that:• Provide a common idiom for binding POSIX signals to instances of AsyncEventwhen POSIX signals are available on the underlying platform.
• Provide a class that contains operations and semantics that affect the entire
system.
• Provide the security semantics required by the additional features in the entiretyof this specification, which are additional to those required by implementations of
the Java Language Specification.
The RealtimeSecurity class provides security primarily for physical memory access.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. The POSIX signal handler class is required to be available when implementationsof this specification execute on an underlying platform that provides POSIX
signals or any subset of signals named with the POSIX names.
2. The RealtimeSecurity class is required.

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Rationale
This specification accommodates the variation in underlying system variation in a
number of ways. One of the most important is the concept of optionally requiredclasses (e.g., the POSIX signal handler class). This class provides a commonality thatcan be relied upon by program logic that intends to execute on implementations thatthemselves execute on POSIX compliant systems.
The RealtimeSystem class functions in similar capacity to java.lang.System.
Similarly, the RealtimeSecurity class functions similarly tojava.lang.SecurityManager.

10.1 POSIXSignalHandler
Syntax: public final class POSIXSignalHandler
Use instances of AsyncEvent125 to handle POSIX signals. Usage:
POSIXSignalHandler.addHandler(SIGINT, intHandler);This class is required to be implemented only if the underlying operating system
supports POSIX signals.

10.1.1 Fields
public static final int SIGABRTUsed by abort, replace SIGIOT in the future.
public static final int SIGALRM

Alarm clock.
public static final int SIGBUS

Bus error.
public static final int SIGCANCEL

Thread cancellation signal used by libthread.
public static final int SIGCHLD

Child status change alias (POSIX).
public static final int SIGCLD

Child status change.
public static final int SIGCONT

Stopped process has been continued.
public static final int SIGEMTEMT instruction.

POSIXSIGNALHANDLER
public static final int SIGFPE

Floating point exception.
public static final int SIGFREEZESpecial signal used by CPR.
public static final int SIGHUP

Hangup.
public static final int SIGILL

Illegal instruction (not reset when caught).
public static final int SIGINT

Interrupt (rubout).
public static final int SIGIO

Socket I/O possible (SIGPOLL alias).
public static final int SIGIOTIOT instruction.
public static final int SIGKILL

Kill (cannot be caught or ignored).
public static final int SIGLOST

Resource lost (e.g., record-lock lost).
public static final int SIGLWP

Special signal used by thread library.
public static final int SIGPIPE

Write on a pipe with no one to read it.
public static final int SIGPOLL

Pollable event occured.
public static final int SIGPROF

Profiling timer expired.
public static final int SIGPWR

Power-fail restart.
public static final int SIGQUIT

Quit (ASCII FS).
public static final int SIGSEGV

Segmentation violation.
public static final int SIGSTOP

Stop (cannot be caught or ignored).
public static final int SIGSYS

Bad argument to system call.

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CHAPTER 10 SYSTEM AND OPTIONS
public static final int SIGTERM

Software termination signal from kill.
public static final int SIGTHAWSpecial signal used by CPR.
public static final int SIGTRAP

Trace trap (not reset when caught).
public static final int SIGTSTP

User stop requested from tty.
public static final int SIGTTIN

Background tty read attempted.
public static final int SIGTTOU

Background tty write attempted.
public static final int SIGURG

Urgent socket condition.
public static final int SIGUSR1User defined signal = 1.
public static final int SIGUSR2User defined signal = 2.
public static final int SIGVTALRM

Virtual timer expired.
public static final int SIGWAITING

Process’s lwps are blocked.
public static final int SIGWINCH

Window size change.
public static final int SIGXCPU

Exceeded cpu limit.
public static final int SIGXFSZ

Exceeded file size limit.

10.1.2 Methods
public static synchronized void addHandler(int signal,
AsyncEventHandler127 handler)
Add the given AsyncEventHandler127 to the list of handlers of the
AsyncEvent125 of the given signal.

Parameters:
signal - One of the POSIX signals from this (e.g., this.SIGLOST).

REALTIMESECURITY

149

handler - An AsyncEventHandler127 which will be scheduled

when the given signal occurs.
public static synchronized void removeHandler(int signal,
AsyncEventHandler127 handler)
Remove the given AsyncEventHandler127 to the list of handlers of the
AsyncEvent125 of the given signal.

Parameters:
signal - One of the POSIX signals from this (e.g., this.SIGLOST).
handler - An AsyncEventHandler127 which will be scheduled

when the given signal occurs.
public static synchronized void setHandler(int signal,
AsyncEventHandler127 handler)
Set the given AsyncEventHandler127 as the handler of the AsyncEvent125

of the given signal.
Parameters:
signal - One of the POSIX signals from this (e.g., this.SIGLOST).
handler - An AsyncEventHandler127 which will be scheduled

when the given signal occurs. If h is null then no handler will be
associated with this (i.e., remove all handlers).

10.2 RealtimeSecurity
Syntax: public class RealtimeSecurity
Security policy object for real-time specific issues. Primarily used to control
access to physical memory.

10.2.1 Constructors
public RealtimeSecurity()

10.2.2 Methods
public void checkAccessPhysical()Check whether the application is allowed to access physical memory.
Throws: SecurityException - the application doesn’t have permission.
public void checkAccessPhysicalRange(long base, longsize)

Check whether the application is allowed to access physical memory
within the specified range.

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CHAPTER 10 SYSTEM AND OPTIONS

Throws: SecurityException - the application doesn’t have permission.
public void checkSetFactory()

Check whether the application is allowed to set factory objects.
Throws: SecurityException - the application doesn’t have permission.
public void checkSetScheduler()

Check whether the application is allowed to set the scheduler.
Throws: SecurityException - the application doesn’t have permission.

10.3 RealtimeSystem
Syntax: public class RealtimeSystem
RealtimeSystem provides a means for tuning the behavior of the implementationby specifying parameters such as the maximum number of locks that can be in use
concurrently, and the monitor control policy. In addition, RealtimeSystem provides amechanism for obtaining access to the security manager, garbage collector andscheduler, to make queries from them or to set parameters.

10.3.1 Fields
public static final byte BIG_ENDIAN
public static final byte BYTE_ORDER
public static final byte LITTLE_ENDIAN

10.3.2 Methods
public static GarbageCollector81 currentGC()

Return a reference to the currently active garbage collector for the heap.
Returns: A GarbageCollector81 object which is the current collector
collecting objects on the traditional Java heap.
public int getConcurrentLocksUsed()Get the maximum number of locks that have been used concurrently. Thisvalue can be used for tuning the concurrent locks parameter, which is used
as a hint by systems that use a monitor cache.
Returns: An int whose value is the number of locks in use at the time of
the invocation of the method.
public int getMaximumConcurrentLocks()

REALTIMESYSTEM

151

Get the maximum number of locks that can be used concurrently without
incurring an execution time increase as set by the
setMaximumConcurrentLocks() methods.
Returns: An int whose value is the maximum number of locks that can bein simultaneous use.
public static RealtimeSecurity149 getSecurityManager()Get a reference to the security manager used to control access to real-timesystem features such as access to physical memory.
Returns: A RealtimeSecurity149 object representing the default realtime security manager.
public void setMaximumConcurrentLocks(int number)

Set the anticipated maximum number of locks that may be held or waited
on concurrently. Provide a hint to systems that use a monitor cache as tohow much space to dedicate to the cache.
Parameters:
number - An integer whose value becomes the number of locks thatcan be in simultaneous use without incurring an execution timeincrease. If number is less than or equal to zero nothing happens.
public void setMaximumConcurrentLocks(int number, boolean hard)

Set the anticipated maximum number of locks that may be held or waited
on concurrently. Provide a limit for the size of the monitor cache on
systems that provide one if hard is true.
Parameters:
number - The maximum number of locks that can be in simultaneous
use without incurring an execution time increase. If number isless than or equal to zero nothing happens.
hard - If true, number sets a limit. If a lock is attempted which would
cause the number of locks to exceed number then aResourceLimitError156 is thrown.
public static void setSecurityManager(RealtimeSecurity149 manager)

Set a new real-time security manager.
Parameters:
manager - A RealtimeSecurity149 object which will become the

new security manager.
Throws: SecurityException - Thrown if security manager has alreadybeen set.

CHAP TER

11

Exceptions
Chapter 11 Exceptions

This section contains classes that:• Add additional exception classes required by the entirety of the other sections of
this specification.
• Provide for the ability to asynchronously transfer the control of program logic.

Semantics and Requirements
This list establishes the semantics and requirements that are applicable across theclasses of this section. Semantics that apply to particular classes, constructors,
methods, and fields will be found in the class description and the constructor, method,and field detail sections.
1. All classes in this section are required.
2. All exceptions, except AsynchronouslyInterruptedException, are required tohave semantics exactly as those of their eventual superclass in the java.*
hierarchy.
3. Instances of the class AsynchronouslyInterruptedException can be generated
by execution of program logic and by internal virtual machine mechanisms thatare asynchronous to the execution of program logic which is the target of the
exception.
4. Program logic that exists in methods that throw
AsynchronouslyInterruptedException is subject to receiving an instance ofAsynchronouslyInterruptedException at any time during execution.

153

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CHAPTER 11 EXCEPTIONS

Rationale
The need for additional exceptions given the new semantics added by the othersections of this specification is obvious. That the specification attaches new,nontraditional, exception semantics to AsynchronouslyInterruptedException is,perhaps, not so obvious. However, after careful thought, and given our self-imposed
directive that only well-defined code blocks would be subject to having their control
asynchronously transferred, the chosen mechanism is logical.

11.1 IllegalAssignmentError
Syntax: public class IllegalAssignmentError extends java.lang.Error
All Implemented Interfaces: java.io.Serializable

The exception thrown on an attempt to make an illegal assignment. For example,
this will be thrown if logic attempts to assign a reference to an object inScopedMemory62 to a field in an object in ImmortalMemory62 .

11.1.1 Constructors
public IllegalAssignmentError()
A constructor for IllegalAssignmentError.
public IllegalAssignmentError(java.lang.String description)
A descriptive constructor for IllegalAssignmentError.

Parameters:
description - Description of the error.

11.2 MemoryAccessError
Syntax: public class MemoryAccessError extends java.lang.Error
All Implemented Interfaces: java.io.Serializable

The exception thrown on an attempt to refer to an object in an inaccessible
MemoryArea60 . For example this will be thrown if logic in a
NoHeapRealtimeThread26 attempts to refer to an object in the traditional Java heap.

MEMORYSCOPEEXCEPTION

11.2.1 Constructors
public MemoryAccessError()
A constructor for MemoryAccessError.
public MemoryAccessError(java.lang.String description)
A descriptive constructor for MemoryAccessError.

Parameters:
description - Description of the error.

11.3 MemoryScopeException
Syntax: public class MemoryScopeException extends java.lang.Exception
All Implemented Interfaces: java.io.Serializable

Thrown if construction of any of the wait-free queues is attempted with the endsof the queues in incompatible memory areas.

11.3.1 Constructors
public MemoryScopeException()
A constructor for MemoryScopeException.
public MemoryScopeException(java.lang.String description)
A descriptive constructor for MemoryScopeException.

Parameters:
description - A description of the exception.

11.4 OffsetOutOfBoundsException
Syntax: public class OffsetOutOfBoundsException extends
java.lang.Exception

All Implemented Interfaces: java.io.Serializable

Thrown if the constructor of a ImmortalPhysicalMemory69 ,ScopedPhysicalMemory71 , RawMemoryFloatAccess76 , or RawMemoryAccess72 isgiven an invalid address.

155

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CHAPTER 11 EXCEPTIONS

11.4.1 Constructors
public OffsetOutOfBoundsException()
A constructor for OffsetOutOfBoundsException.
public OffsetOutOfBoundsException(java.lang.String description)
A descriptive constructor for OffsetOutOfBoundsException.

Parameters:
description - A description of the exception.

11.5 ResourceLimitError
Syntax: public abstract class ResourceLimitError extends java.lang.Error
All Implemented Interfaces: java.io.Serializable

Thrown if an attempt is made to exceed a system resource limit, such as the
maximum number of locks.

11.5.1 Constructors
public ResourceLimitError()
A constructor for ResourceLimitError.
public ResourceLimitError(java.lang.String description)
A descriptive constructor for ResourceLimitError.

Parameters:
description - The description of the exception.

11.6 SizeOutOfBoundsException
Syntax: public class SizeOutOfBoundsException extends java.lang.Exception
All Implemented Interfaces: java.io.Serializable

Thrown if the constructor of a ImmortalPhysicalMemory69 ,ScopedPhysicalMemory71 , RawMemoryFloatAccess76 , or RawMemoryAccess72 isgiven an invalid size or if an accessor method on one of the above classes would cause
access to an invalid address.

THROWBOUNDARYERROR

11.6.1 Constructors
public SizeOutOfBoundsException()
A constructor for SizeOutOfBoundsException.
public SizeOutOfBoundsException(java.lang.String description)
A descriptive constructor for a SizeOutOfBoundsException.

Parameters:
description - The description of the exception.

11.7 ThrowBoundaryError
Syntax: public class ThrowBoundaryError extends java.lang.Error
All Implemented Interfaces: java.io.Serializable

The error thrown by public void enter(java.lang.Runnable logic)64
when a java.lang.Throwable allocated from memory that is not usable in thesurrounding scope tries to propagate out of the scope of the public voidenter(java.lang.Runnable logic)64 .

11.7.1 Constructors
public ThrowBoundaryError()
A constructor for ThrowBoundaryError.
public ThrowBoundaryError(java.lang.String description)
A descriptive constructor for ThrowBoundaryError.

Parameters:
description - Description of the error.

11.8 UnsupportedPhysicalMemoryException
Syntax: public class UnsupportedPhysicalMemoryException extends
java.lang.Exception

All Implemented Interfaces: java.io.Serializable

Thrown when the underlying hardware does not support the type of physicalmemory given to the physical memory create() method. See: RawMemoryAccess72
RawMemoryFloatAccess76 ImmortalPhysicalMemory69 ScopedPhysicalMemory71

157

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CHAPTER 11 EXCEPTIONS

11.8.1 Constructors
public UnsupportedPhysicalMemoryException()
A constructor for UnsupportedPhysicalMemoryException.
public UnsupportedPhysicalMemoryException(java.lang.String
description)
A descriptive constructor for a UnsupportedPhysicalMemoryException

Parameters:
description - The description of the exception.

159

160

LEGENDThe following is a very condensed summary of all of the classes defined in thisspecification, listed alphabetically. It is done in the style introduced by Patrick Chanin his excellent Java Developers Almanac.

1. The name of the class.
2. The name of the package containing the class
3. The chain of superclasses. Each class is a subclass of the one above it.
4. The names of the interfaces implemented by each class.
5. A static method.
6. A constructor. Other icons that may occur in this table are:❍ abstract● final■ static final© protected✍ field
7. The return type of a method or the declared type of an instance variable.
8. The name of the class member. If it is a method, the parameter list and optional
throws clause follows. Members are arranged alphabetically.

12

CHAP TER

Almanac

AbsoluteTime

javax.realtime

Object➥HighResolutionTime➥AbsoluteTime

Comparable

AbsoluteTime absolute(Clock clock, AbsoluteTime destination)❉

AbsoluteTime()❉

AbsoluteTime(AbsoluteTime time)❉

AbsoluteTime(java.util.Date date)❉AbsoluteTime(long millis, int nanos)
AbsoluteTime add(long millis, int nanos)●

AbsoluteTime add(long millis, int nanos,
AbsoluteTime destination)
AbsoluteTime add(RelativeTime time)
AbsoluteTime add(RelativeTime time, AbsoluteTime destination)
java.util.Date getDate()
void set(java.util.Date date)●

RelativeTime subtract(AbsoluteTime time)●

RelativeTime subtract(AbsoluteTime time,
RelativeTime destination)
AbsoluteTime subtract(RelativeTime time)
AbsoluteTime subtract(RelativeTime time,
AbsoluteTime destination)
String toString()

161

162

AperiodicParameters

javax.realtime

Object➥ReleaseParameters➥AperiodicParameters❉

AperiodicParameters(RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)

AsyncEvent

javax.realtime

Object➥AsyncEvent
void addHandler(AsyncEventHandler handler)❉

AsyncEvent()
ReleaseParameters createReleaseParameters()
void fire()boolean handledBy(AsyncEventHandler target)
void removeHandler(AsyncEventHandler handler)
void setHandler(AsyncEventHandler handler)

AsyncEventHandler

javax.realtime

Object➥AsyncEventHandler

Schedulable

void addToFeasibility()❉

AsyncEventHandler()❉

AsyncEventHandler(boolean nonheap)❉●♦

AsyncEventHandler(SchedulingParameters scheduling,
ReleaseParameters release,
MemoryParameters memory, MemoryArea area,
ProcessingGroupParameters group)
AsyncEventHandler(SchedulingParameters scheduling,
ReleaseParameters release,
MemoryParameters memory, MemoryArea area,
ProcessingGroupParameters group,boolean nonheap)
int getAndClearPendingFireCount()♦

int getAndDecrementPendingFireCount()❉♦

int getAndIncrementPendingFireCount()
MemoryArea getMemoryArea()
MemoryParameters getMemoryParameters()
ProcessingGroupParameters getProcessingGroupParameters()
ReleaseParameters getReleaseParameters()
Scheduler getScheduler()

ALMANAC

163
SchedulingParameters getSchedulingParameters()❍

void handleAsyncEvent()
void removeFromFeasibility()●

void run()void setMemoryParameters(MemoryParameters memory)
void setProcessingGroupParameters(ProcessingGroupParame
ters parameters)
void setReleaseParameters(ReleaseParameters parameters)
void setScheduler(Scheduler scheduler)
throws IllegalThreadStateException
void setSchedulingParameters(SchedulingParameters parameters)

AsynchronouslyInterruptedException

javax.realtime

Object➥Throwable
java.io.Serializable➥Exception➥InterruptedException➥AsynchronouslyInterruptedException❉

AsynchronouslyInterruptedException()boolean disable()boolean doInterruptible(Interruptible logic)
boolean enable()boolean fire()❏

AsynchronouslyInter- getGeneric()
ruptedException
boolean happened(boolean propagate)boolean isEnabled()
void propagate()

BoundAsyncEventHandler
Object➥AsyncEventHandler➥BoundAsyncEventHandler

javax.realtime

Schedulable❉

BoundAsyncEventHandler()❉

BoundAsyncEventHandler(SchedulingParameters schedu
ling, ReleaseParameters release,
MemoryParameters memory, MemoryArea area,
ProcessingGroupParameters group,boolean nonheap)

164

Clock

javax.realtime

Object➥Clock❉Clock()❏❍

Clock getRealtimeClock()
RelativeTime getResolution()
AbsoluteTime getTime()❍void getTime(AbsoluteTime time)❍

void setResolution(RelativeTime resolution)

GarbageCollector

javax.realtime

Object➥GarbageCollector❉❍

GarbageCollector()
RelativeTime getPreemptionLatency()

HeapMemory

javax.realtime

Object➥MemoryArea➥HeapMemory❏

HeapMemory instance()

HighResolutionTime
Object➥HighResolutionTime❍

javax.realtime

Comparable

AbsoluteTime absolute(Clock clock, AbsoluteTime dest)
int compareTo(HighResolutionTime time)
int compareTo(Object object)boolean equals(HighResolutionTime time)boolean equals(Object object)●long getMilliseconds()●

int getNanoseconds()
int hashCode()
void set(HighResolutionTime time)
void set(long millis)
void set(long millis, int nanos)

ALMANAC

165

IllegalAssignmentError
Object➥Throwable➥Error➥IllegalAssignmentError

javax.realtime

java.io.Serializable❉

IllegalAssignmentError()❉

IllegalAssignmentError(String description)

ImmortalMemory

javax.realtime

Object➥MemoryArea➥ImmortalMemory❏

ImmortalMemory instance()

ImmortalPhysicalMemory

javax.realtime

Object➥MemoryArea➥ImmortalPhysicalMemory❏ ImmortalPhysicalMemory❏ ImmortalPhysicalMemory

❉♦
❉♦
❏

create(Object type, long size)
throws SecurityException, SizeOutOfBoundsException, UnsupportedPhysicalMemoryException
create(Object type, long base, long size)
throws SecurityException, SizeOutOfBoundsException, OffsetOutOfBoundsException, UnsupportedPhysicalMemoryException
ImmortalPhysicalMemory(ImmortalPhysicalMemory memory, long base, long size)
ImmortalPhysicalMemory(long base, longsize)

void setFactory(PhysicalMemoryFactory factory)

ImportanceParameters

javax.realtime

Object➥SchedulingParameters➥PriorityParameters➥ImportanceParameters
int getImportance()❉

ImportanceParameters(int priority, int importance)void setImportance(int importance)
String toString()

166Interruptible

javax.realtime

Interruptible
void interruptAction(AsynchronouslyInterruptedException
exception)
void run(AsynchronouslyInterruptedException exception)
throws AsynchronouslyInterruptedException

LTMemory

javax.realtime

Object➥MemoryArea➥ScopedMemory➥LTMemory❉

LTMemory(long initialSizeInBytes,
long maxSizeInBytes)

MemoryAccessError

javax.realtime

Object➥Throwable➥Error➥MemoryAccessError

java.io.Serializable❉

MemoryAccessError()❉

MemoryAccessError(String description)

MemoryArea

javax.realtime

Object➥MemoryArea
void enter(Runnable logic)❏❉♦

MemoryArea getMemoryArea(Object object)
MemoryArea(long sizeInBytes)
long memoryConsumed()
long memoryRemaining()
Object newArray(Class type, int number)
throws InstantiationException, IllegalAccessException
Object newInstance(Class type)throws InstantiationException, IllegalAccessException
long size()ALMANAC

167

MemoryParameters

javax.realtime

Object➥MemoryParameters
long getAllocationRate()long getMaxImmortal()long getMaxMemoryArea()❉

MemoryParameters(long maxMemoryArea,long maxImmortal)
throws IllegalArgumentException
MemoryParameters(long maxMemoryArea,long maxImmortal, long allocationRate)
throws IllegalArgumentException
long NO_MAX❉✍■

void setAllocationRate(long rate)boolean setMaxImmortal(long maximum)
boolean setMaxMemoryArea(long maximum)

MemoryScopeException

javax.realtime

Object➥Throwable➥Exception➥MemoryScopeException

java.io.Serializable❉

MemoryScopeException()❉

MemoryScopeException(String description)

MonitorControl

javax.realtime

Object➥MonitorControl❉

MonitorControl()❏void setMonitorControl(MonitorControl policy)❏

void setMonitorControl(Object monitor,
MonitorControl policy)

168

NoHeapRealtimeThread

javax.realtime

Object➥Thread➥RealtimeThread➥NoHeapRealtimeThread
❉
❉

❉

✍■

Runnable
Schedulable

NoHeapRealtimeThread(SchedulingParameters scheduling, MemoryArea area)
throws IllegalArgumentException
NoHeapRealtimeThread(SchedulingParameters scheduling, ReleaseParameters release,
MemoryArea area)
throws IllegalArgumentException
NoHeapRealtimeThread(SchedulingParameters scheduling, ReleaseParameters release,
MemoryParameters memory, MemoryArea area,
ProcessingGroupParameters group,
Runnable logic)
throws IllegalArgumentException
int NORM_PRIORITY

OffsetOutOfBoundsException

javax.realtime

Object➥Throwable
java.io.Serializable➥Exception➥OffsetOutOfBoundsException❉

OffsetOutOfBoundsException()❉

OffsetOutOfBoundsException(String description)

OneShotTimerjavax.realtime

Object➥AsyncEvent➥Timer➥OneShotTimer❉❉

OneShotTimer(HighResolutionTime time,
AsyncEventHandler handler)
OneShotTimer(HighResolutionTime start,
Clock clock, AsyncEventHandler handler)

ALMANAC

169

PeriodicParameters

javax.realtime

Object➥ReleaseParameters➥PeriodicParameters
RelativeTime getPeriod()
HighResolutionTime getStart()❉

PeriodicParameters(HighResolutionTime start,
RelativeTime period, RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
void setPeriod(RelativeTime period)
void setStart(HighResolutionTime start)

PeriodicTimerjavax.realtime

Object➥AsyncEvent➥Timer➥PeriodicTimer
ReleaseParameters createReleaseParameters()
void fire()
AbsoluteTime getFireTime()
RelativeTime getInterval()❉❉

PeriodicTimer(HighResolutionTime start,
RelativeTime interval,
AsyncEventHandler handler)
PeriodicTimer(HighResolutionTime start,
RelativeTime interval, Clock clock,AsyncEventHandler handler)
void setInterval(RelativeTime interval)

PhysicalMemoryFactory

javax.realtime

Object➥PhysicalMemoryFactory✍■

String ALIGNED✍■

String BYTESWAP♦

Object create(Object memoryType, Class physMemType,long base, long size)
String DMA

✍■
♦
✍■

long getTypedMemoryBase(Object memoryType, long size)
String SHARED

170

POSIXSignalHandler

javax.realtime

Object➥POSIXSignalHandler❏

void addHandler(int signal, AsyncEventHandler handler)❏❏

void removeHandler(int signal,
AsyncEventHandler handler)
void setHandler(int signal, AsyncEventHandler handler)✍■

int SIGABRT✍■int SIGALRM✍■

int SIGBUS✍■

int SIGCANCEL✍■

int SIGCHLD✍■

int SIGCLD✍■

int SIGCONT✍■

int SIGEMT✍■

int SIGFPE✍■

int SIGFREEZE✍■

int SIGHUP✍■

int SIGILL✍■

int SIGINT✍■

int SIGIO✍■

int SIGIOT✍■

int SIGKILL✍■

int SIGLOST✍■

int SIGLWP✍■

int SIGPIPE✍■

int SIGPOLL✍■

int SIGPROF✍■

int SIGPWR✍■

int SIGQUIT✍■

int SIGSEGV✍■

int SIGSTOP✍■

int SIGSYS✍■

int SIGTERM✍■

int SIGTHAW✍■

int SIGTRAP✍■

int SIGTSTP✍■

int SIGTTIN✍■

int SIGTTOU✍■

int SIGURG✍■

int SIGUSR1✍■

int SIGUSR2

ALMANAC

171✍■

int SIGVTALRM✍■

int SIGWAITING✍■

int SIGWINCH✍■

int SIGXCPU✍■

int SIGXFSZ

PriorityCeilingEmulation

javax.realtime

Object➥MonitorControl➥PriorityCeilingEmulation
int getDefaultCeiling()❉

PriorityCeilingEmulation(int ceiling)

PriorityInheritancejavax.realtime

Object➥MonitorControl➥PriorityInheritance❏

PriorityInheritance instance()❉

PriorityInheritance()

PriorityParameters

javax.realtime

Object➥SchedulingParameters➥PriorityParameters
int getPriority()❉

PriorityParameters(int priority)
void setPriority(int priority)
throws IllegalArgumentException
String toString()

PriorityScheduler

javax.realtime

Object➥Scheduler➥PriorityScheduler♦

void addToFeasibility(Schedulable s)boolean changeIfFeasible(Schedulable schedulable,
ReleaseParameters release,
MemoryParameters memory)
void fireSchedulable(Schedulable schedulable)
int getMaxPriority()❏

int getMaxPriority(Thread thread)172
int getMinPriority()❏

int getMinPriority(Thread thread)int getNormPriority()❏

int getNormPriority(Thread thread)
String getPolicyName()❏

PriorityScheduler instance()boolean isFeasible()❉

PriorityScheduler()♦void removeFromFeasibility(Schedulable s)

ProcessingGroupParameters

javax.realtime

Object➥ProcessingGroupParameters
RelativeTime getCost()
AsyncEventHandler getCostOverrunHandler()
RelativeTime getDeadline()
AsyncEventHandler getDeadlineMissHandler()
RelativeTime getPeriod()
HighResolutionTime getStart()❉

ProcessingGroupParameters(HighResolutionTime start,
RelativeTime period, RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
void setCost(RelativeTime cost)
void setCostOverrunHandler(AsyncEventHandler handler)
void setDeadline(RelativeTime deadline)
void setDeadlineMissHandler(AsyncEventHandler handler)
void setPeriod(RelativeTime period)
void setStart(HighResolutionTime start)

RationalTime

javax.realtime

Object➥HighResolutionTime➥RelativeTime➥RationalTime

Comparable

AbsoluteTime absolute(Clock clock, AbsoluteTime destination)
void addInterarrivalTo(AbsoluteTime destination)
int getFrequency()
RelativeTime getInterarrivalTime(RelativeTime dest)❉

RationalTime(int frequency)❉

RationalTime(int frequency, longmillis,
int nanos) throws IllegalArgumentException

ALMANAC

173❉

RationalTime(int frequency, RelativeTimeinterval)
void set(long millis, int nanos)
throws IllegalArgumentException
void setFrequency(int frequency)

RawMemoryAccessjavax.realtime

Object➥RawMemoryAccess❏❏

RawMemoryAccess create(Object type, long size)
throws SecurityException, OffsetOutOfBoundsException, SizeOutOfBoundsException, UnsupportedPhysicalMemoryException
RawMemoryAccess create(Object type, long base, long size)
throws SecurityException, OffsetOutOfBoundsException, SizeOutOfBoundsException, UnsupportedPhysicalMemoryException
byte getByte(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getBytes(long offset, byte[] bytes, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
int getInt(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getInts(long offset, int[] ints, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
long getLong(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getLongs(long offset, long[] longs, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
long getMappedAddress()
short getShort(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getShorts(long offset, short[] shorts, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
long map()
long map(long base)long map(long base, long size)❉♦❉♦

RawMemoryAccess(long base, long size)
RawMemoryAccess(RawMemoryAccess memory, long base,long size)
void setByte(long offset, byte value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException

174
void setBytes(long offset, byte[] bytes, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setInt(long offset, int value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setInts(long offset, int[] ints, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setLong(long offset, long value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setLongs(long offset, long[] longs, int low,
int n) throws OffsetOutOfBoundsException,
SizeOutOfBoundsException
void setShort(long offset, short value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setShorts(long offset, short[] shorts, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void unmap()

RawMemoryFloatAccessjavax.realtime

Object➥RawMemoryAccess➥RawMemoryFloatAccess
❏

❏

❉♦
❉♦

RawMemoryFloatAccess createFloatAccess(Object type, long size)
throws SecurityException, OffsetOutOfBoundsException, SizeOutOfBoundsException, UnsupportedPhysicalMemoryException
RawMemoryFloatAccess createFloatAccess(Object type, long base,long size) throws SecurityException, OffsetOutOfBoundsException, SizeOutOfBoundsException, UnsupportedPhysicalMemoryException
byte getDouble(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getDoubles(long offset, double[] doubless,int low, int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
byte getFloat(long offset)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void getFloats(long offset, float[] floats, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
RawMemoryFloatAccess(long base, long size)
RawMemoryFloatAccess(RawMemoryAccess memory,long base, longsize)
void setDouble(long offset, double value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException

ALMANAC

175
void setDoubles(long offset, double[] doubles, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setFloat(long offset, float value)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException
void setFloats(long offset, float[] floats, int low,
int number)
throws OffsetOutOfBoundsException, SizeOutOfBoundsException

RealtimeSecurity

javax.realtime

Object➥RealtimeSecurity

void checkAccessPhysical() throws SecurityException
void checkAccessPhysicalRange(long base, long size)
throws SecurityException
void checkSetFactory() throws SecurityException
void checkSetScheduler() throws SecurityException❉

RealtimeSecurity()

RealtimeSystemjavax.realtime

Object➥RealtimeSystem✍■byte BIG_ENDIAN✍■

byte BYTE_ORDER❏

GarbageCollector currentGC()
int getConcurrentLocksUsed()
int getMaximumConcurrentLocks()❏

RealtimeSecurity getSecurityManager()✍■byte LITTLE_ENDIAN
void setMaximumConcurrentLocks(int number)
void setMaximumConcurrentLocks(int number,boolean hard)
void setSecurityManager(RealtimeSecurity manager)
throws SecurityException❏

RealtimeThread

javax.realtime

Object➥Thread➥RealtimeThread

Runnable
Schedulable

void addToFeasibility()❏

RealtimeThread currentRealtimeThread()void deschedulePeriodic()

176
MemoryArea getMemoryArea()
MemoryParameters getMemoryParameters()
ProcessingGroupParam- getProcessingGroupParameters()
eters
ReleaseParameters getReleaseParameters()
Scheduler getScheduler()
SchedulingParameters getSchedulingParameters()
void interrupt()❉

RealtimeThread()❉

RealtimeThread(SchedulingParameters scheduling)❉

RealtimeThread(SchedulingParameters scheduling,
ReleaseParameters release)
RealtimeThread(SchedulingParameters scheduling,
ReleaseParameters release,
MemoryParameters memory, MemoryArea area,
ProcessingGroupParameters group,
Runnable logic)
void removeFromFeasibility()❉

void schedulePeriodic()void setMemoryParameters(MemoryParameters parameters)
void setProcessingGroupParameters(ProcessingGroupParameters parameters)
void setReleaseParameters(ReleaseParameters parameters)❏❏

void setScheduler(Scheduler scheduler)
throws IllegalThreadStateException
void setSchedulingParameters(SchedulingParameters scheduling)
void sleep(Clock clock, HighResolutionTime time)
throws InterruptedException
void sleep(HighResolutionTime time)
throws InterruptedException
boolean waitForNextPeriod()
throws IllegalThreadStateException

RelativeTime

javax.realtime

Object➥HighResolutionTime➥RelativeTime

Comparable

AbsoluteTime absolute(Clock clock, AbsoluteTime destination)
RelativeTime add(long millis, int nanos)●

RelativeTime add(long millis, int nanos,
RelativeTime destination)
RelativeTime add(RelativeTime time)
RelativeTime add(RelativeTime time, RelativeTime destination)
void addInterarrivalTo(AbsoluteTime destination)
RelativeTime getInterarrivalTime(RelativeTime destination)❉

RelativeTime()❉

RelativeTime(long millis, int nanos)❉

RelativeTime(RelativeTime time)ALMANAC

177●

RelativeTime subtract(RelativeTime time)
RelativeTime subtract(RelativeTime time,
RelativeTime destination)
String toString()

ReleaseParameters

javax.realtime

Object➥ReleaseParameters
RelativeTime getCost()
AsyncEventHandler getCostOverrunHandler()
RelativeTime getDeadline()
AsyncEventHandler getDeadlineMissHandler()❉♦

ReleaseParameters(RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
void setCost(RelativeTime cost)
void setCostOverrunHandler(AsyncEventHandler handler)
void setDeadline(RelativeTime deadline)
void setDeadlineMissHandler(AsyncEventHandler handler)

ResourceLimitError

javax.realtime

Object➥Throwable➥Error➥ResourceLimitError

java.io.Serializable❉

ResourceLimitError()❉

ResourceLimitError(String description)

Schedulable

javax.realtime

Schedulable

Runnable
void addToFeasibility()
MemoryParameters getMemoryParameters()

ReleaseParameters getReleaseParameters()
Scheduler getScheduler()
SchedulingParameters getSchedulingParameters()
void removeFromFeasibility()
void setMemoryParameters(MemoryParameters memory)
void setReleaseParameters(ReleaseParameters release)void setScheduler(Scheduler scheduler)void setSchedulingParameters(SchedulingParameters scheduling)178

Scheduler

javax.realtime

Object➥Scheduler❍♦

void addToFeasibility(Schedulable schedulable)❏boolean changeIfFeasible(Schedulable schedulable,
ReleaseParameters release,
MemoryParameters memory)
Scheduler getDefaultScheduler()❍

String getPolicyName()❍boolean isFeasible()❍♦

void removeFromFeasibility(Schedulable schedulable)❉❏

Scheduler()void setDefaultScheduler(Scheduler scheduler)

SchedulingParameters

javax.realtime

Object➥SchedulingParameters❉

SchedulingParameters()

ScopedMemory

javax.realtime

Object➥MemoryArea➥ScopedMemory
void enter(Runnable logic)
int getMaximumSize()
MemoryArea getOuterScope()
Object getPortal()❉

ScopedMemory(long size)
void setPortal(Object object)

ALMANAC

179

ScopedPhysicalMemory

javax.realtime

Object➥MemoryArea➥ScopedMemory➥ScopedPhysicalMemory
❏

❉♦
❉♦
❏

ScopedPhysicalMemory create(Object type, long base, long size)
throws SecurityException, SizeOutOfBoundsException, OffsetOutOfBoundsException, UnsupportedPhysicalMemoryException
ScopedPhysicalMemory(long base, long size)
ScopedPhysicalMemory(ScopedPhysicalMemory memory,long base, long size)
void setFactory(PhysicalMemoryFactory factory)

SizeOutOfBoundsException
Object➥Throwable➥Exception➥SizeOutOfBoundsException

javax.realtime

java.io.Serializable❉

SizeOutOfBoundsException()❉

SizeOutOfBoundsException(String description)

SporadicParameters

javax.realtime

Object➥ReleaseParameters➥AperiodicParameters➥SporadicParameters
RelativeTime getMinimumInterarrival()
void setMinimumInterarrival(RelativeTime minimum)❉

SporadicParameters(RelativeTime minInterarrival,
RelativeTime cost, RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)

ThrowBoundaryError

javax.realtime

Object➥Throwable➥Error➥ThrowBoundaryError

java.io.Serializable❉

ThrowBoundaryError()❉ThrowBoundaryError(String description)

180

Timed

javax.realtime

Object➥Throwable
java.io.Serializable➥Exception➥InterruptedException➥AsynchronouslyInterruptedException➥Timedboolean doInterruptible(Interruptible logic)
void resetTime(HighResolutionTime time)❉

Timed(HighResolutionTime time)
throws IllegalArgumentExceptionTimerjavax.realtime

Object➥AsyncEvent➥TimerReleaseParameters createReleaseParameters()
void disable()
void enable()
Clock getClock()
AbsoluteTime getFireTime()void reschedule(HighResolutionTime time)
void start()❉♦

Timer(HighResolutionTime t, Clockc,
AsyncEventHandler handler)

UnsupportedPhysicalMemoryException

javax.realtime

Object➥Throwable
java.io.Serializable➥Exception➥UnsupportedPhysicalMemoryException❉

UnsupportedPhysicalMemoryException()❉

UnsupportedPhysicalMemoryException(String description)

ALMANAC

181

VTMemory

javax.realtime

Object➥MemoryArea➥ScopedMemory➥VTMemory❉

VTMemory(int initial, int maximum)

WaitFreeDequeue

javax.realtime

Object➥WaitFreeDequeue
Object blockingRead()boolean blockingWrite(Object object)
throws MemoryScopeException
boolean force(Object object)
Object nonBlockingRead()❉boolean nonBlockingWrite(Object object)
throws MemoryScopeException
WaitFreeDequeue(Thread writer, Thread reader,
int maximum, MemoryArea area)
throws IllegalArgumentException, IllegalAccessException, ClassNotFoundException,
InstantiationException

WaitFreeReadQueuejavax.realtime

Object➥WaitFreeReadQueuevoid clear()boolean isEmpty()
boolean isFull()Object read()
int size()
void waitForData()❉❉

WaitFreeReadQueue(Thread writer, Thread reader,
int maximum, MemoryArea memory)
throws IllegalArgumentException, InstantiationException, ClassNotFoundException, IllegalAccessException
WaitFreeReadQueue(Thread writer, Thread reader,
int maximum, MemoryArea memory,
boolean notify)
throws IllegalArgumentException, InstantiationException, ClassNotFoundException, IllegalAccessException
boolean write(Object object) throws MemoryScopeException

182

WaitFreeWriteQueuejavax.realtime

Object➥WaitFreeWriteQueuevoid bind(Thread writer, Thread reader,
MemoryArea memory)
throws IllegalArgumentException, IllegalAccessException, InstantiationException
void clear()boolean force(Object object)boolean isEmpty()
boolean isFull()Object read()
int size()❉

WaitFreeWriteQueue(Thread writer, Thread reader,
int maximum, MemoryArea memory)
throws IllegalArgumentException, IllegalAccessException, ClassNotFoundException,
InstantiationException
boolean write(Object object) throws MemoryScopeException

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187

Colophon
This specification document was generated from a set of Java and HTML source files.
They were compiled using javadoc and the doclet-from-hell: mifdoclet. The recent
development of mifdoclet was driven largely by the Real Time for Java Expert
Group. We wanted to be able to produce a specification document that had beenchecked, as much as possible, by whatever compilation tools we could find. The
specification source compiles as a Java program, and even contains a scaffold
implementation which was used to compile and run the examples.
The mifdoclet generates its output in MIF format, which was processed through
Adobe FrameMaker, http://www.adobe.com/products/framemaker, a truely wonderful
publishing package without which this book would have been much more difficult.
The source files used to produce this specification will eventually be available at
http://www.rtj.org.

189IndexAabsolute 97, 100, 103, 106
AbsoluteTime 99, 100
add 100, 101, 103, 104
addHandler 125, 148
addInterarrivalTo 104, 106
addToFeasibility 24, 35, 36, 38, 129
ALIGNED 68
AperiodicParameters 47, 48
AsyncEvent 125
AsyncEventHandler 127, 128
AsynchronouslyInterruptedException
134, 135

B
BIG_ENDIAN 150
bind 93bindTo 126
blockingRead 89
blockingWrite 89
BoundAsyncEventHandler 132
BYTE_ORDER 150
BYTESWAP 68

CchangeIfFeasible 37, 38
checkAccessPhysical 149
checkAccessPhysicalRange 149
checkSetFactory 150
checkSetScheduler 150
clear 91, 93
Clock 110, 111
compareTo 97, 98

create 69, 70, 71, 72, 73
createFloatAccess 77
createReleaseParameters 112, 115,126
currentGC 150
currentRealtimeThread 24

DdeschedulePeriodic 24
disable 113, 135
DMA 68doInterruptible 135, 137

Eenable 113, 136
enter 60, 64
equals 98

Ffire 115, 126, 136
fireSchedulable 39
force 89, 93

G
GarbageCollector 81, 82
getAllocationRate 80
getAndClearPendingFireCount 129
getAndDecrementPendingFireCount
129
getAndIncrementPendingFireCount130
getByte 73
191

192

INDEX

getBytes 73
getClock 113
getConcurrentLocksUsed 150
getCost 44, 51
getCostOverrunHandler 44, 51
getDate 101
getDeadline 44, 52
getDeadlineMissHandler 44, 52
getDefaultCeiling 88
getDefaultScheduler 37
getDouble 77
getDoubles 78
getFireTime 113, 115
getFloat 78
getFloats 78
getFrequency 107
getGeneric 136
getImportance 42
getInt 73
getInterarrivalTime 104, 107
getInterval 116
getInts 74
getLong 74
getLongs 74
getMappedAddress 74
getMaxImmortal 80
getMaximumConcurrentLocks 150
getMaximumReclamationRate 82
getMaximumSize 64
getMaxMemoryArea 80
getMaxPriority 39
getMemoryArea 24, 60, 130
getMemoryParameters 24, 35, 130
getMilliseconds 98
getMinimumInterarrival 50
getMinPriority 39
getNanoseconds 98
getNormPriority 39
getOuterScope 64
getPeriod 47, 52
getPolicyName 37, 40

getPortal 64
getPreemptionLatency 82, 83
getPriority 41
getProcessingGroupParameters 24, 130
getReadBarrierOverhead 83
getRealtimeClock 111
getReleaseParameters 24, 35, 130
getResolution 111
getScheduler 24, 35, 130
getSchedulingParameters 24, 35, 130
getSecurityManager 151
getShort 74
getShorts 74
getStart 47, 52
getTime 111
getTypedMemoryBase 69
getWriteBarrierOverhead 83

HhandleAsyncEvent 131
handledBy 126
happened 136
hashCode 98
HeapMemory 61
HighResolutionTime 97

I
IllegalAssignmentError 154
ImmortalMemory 62
ImmortalPhysicalMemory 69
ImportanceParameters 42
IncrementalCollectorExample 82
instance 40, 62, 88
interrupt 24
interruptAction 133
Interruptible 133
isEmpty 91, 93
isEnabled 136
isFeasible 37, 40
isFull 91, 93

193

L

R

LITTLE_ENDIAN 150
LTMemory 65, 66

RationalTime 105, 106
RawMemoryAccess 72
RawMemoryFloatAccess 76, 77
read 91, 93
RealtimeSecurity 149
RealtimeSystem 150
RealtimeThread 22, 23
RelativeTime 102, 103
ReleaseParameters 43
removeFromFeasibility 25, 35, 37, 40,131removeHandler 126, 149
reschedule 113
resetTime 138
ResourceLimitError 156
run 131, 134M
map 74, 75
MarkAndSweepCollectorExample 83
MemoryAccessError 154, 155
MemoryArea 60
memoryConsumed 60
MemoryParameters 79, 80
memoryRemaining 60
MemoryScopeException 155
MonitorControl 86, 87

N
newArray 61
newInstance 61
NO_MAX 79
NoHeapRealtimeThread 26, 27, 28
nonBlockingRead 89
nonBlockingWrite 90

O
OffsetOutOfBoundsException 155, 156
OneShotTimer 113, 114

P
PeriodicParameters 45, 46
PeriodicTimer 114, 115
PhysicalMemoryFactory 68
POSIXSignalHandler 146
PriorityCeilingEmulation 87
PriorityInheritance 88
PriorityParameters 41
PriorityScheduler 38
ProcessingGroupParameters 50, 51
propagate 136

S
Schedulable 35
schedulePeriodic 25
Scheduler 36
SchedulingParameters 40, 41
ScopedMemory 62, 63
ScopedPhysicalMemory 71
set 98, 99, 101, 107
setAllocationRate 80
setByte 75
setBytes 75
setCost 44, 52
setCostOverrunHandler 45, 52
setDeadline 45, 52
setDeadlineMissHandler 45, 53
setDefaultScheduler 37
setDouble 78
setDoubles 78
setFactory 70, 71
setFloat 78
setFloats 78

194

INDEX

setFrequency 107
setHandler 126, 149
setImportance 42
setInt 75
setInterval 116
setInts 75
setLong 75
setLongs 76
setMaxImmortal 81
setMaximumConcurrentLocks 151
setMaxMemoryArea 81
setMemoryParameters 25, 35, 131
setMinimumInterarrival 50
setMonitorControl 87
setPeriod 47, 53
setPortal 64
setPriority 41
setProcessingGroupParameters 25, 131
setReclamationRate 83
setReleaseParameters 25, 36, 131
setResolution 112
setScheduler 25, 36, 132
setSchedulingParameters 25, 36, 132
setSecurityManager 151
setShort 76
setShorts 76
setStart 47, 53
SHARED 68
SIGABRT 146
SIGALRM 146
SIGBUS 146
SIGCANCEL 146
SIGCHLD 146
SIGCLD 146
SIGCONT 146
SIGEMT 146
SIGFPE 147
SIGFREEZE 147
SIGHUP 147
SIGILL 147
SIGINT 147

SIGIO 147
SIGIOT 147
SIGKILL 147
SIGLOST 147
SIGLWP 147
SIGPIPE 147
SIGPOLL 147
SIGPROF 147
SIGPWR 147
SIGQUIT 147
SIGSEGV 147
SIGSTOP 147
SIGSYS 147
SIGTERM 148
SIGTHAW 148
SIGTRAP 148
SIGTSTP 148
SIGTTIN 148
SIGTTOU 148
SIGURG 148
SIGUSR1 148
SIGUSR2 148
SIGVTALRM 148
SIGWAITING 148
SIGWINCH 148
SIGXCPU 148
SIGXFSZ 148
size 61, 91, 93
SizeOutOfBoundsException 156, 157
sleep 25, 26
SporadicParameters 49
start 113
subtract 101, 102, 104T
ThrowBoundaryError 157
Timed 137
Timer 112
toString 41, 42, 102, 105

195

U
unmap 76
UnsupportedPhysicalMemoryException
157, 158

V
VTMemory 65

W
waitForData 91
waitForNextPeriod 26
WaitFreeDequeue 88
WaitFreeReadQueue 90, 91
WaitFreeWriteQueue 92
write 92, 94