Java bytecode - wikipedia Java bytecode Java bytecode is the instruction set of the Java virtual machine ( JVM ). Contents 1 Relation to Java 2 Instruction set architecture 2.1 Instruction set 3 Example 4 Generation 5 Execution 6 Support for dynamic languages 7 See also 8 References 9 External links Relation to Java A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, `` Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembly helps the C or C++ programmer. '' Instruction set architecture The JVM is both a stack machine and a register machine. Each frame for a method call has an `` operand stack '' and an array of `` local variables ''. The operand stack is used for operands to computations and for receiving the return value of a called method, while local variables serve the same purpose as registers and are also used to pass method arguments. The maximum size of the operand stack and local variable array, computed by the compiler, is part of the attributes of each method. Each can be independently sized from 0 to 65535 values, where each value is 32 bits. long and double types, which are 64 bits, take up two consecutive local variables ( which need not be 64 - bit aligned in the local variables array ) or one value in the operand stack ( but are counted as two units in the depth of the stack ). Instruction set Further information : Java bytecode instruction listings Each bytecode is composed of one byte that represents the opcode, along with zero or more bytes for operands. Of the 256 possible byte - long opcodes, as of 2015, 202 are in use ( ~ 79 % ), 51 are reserved for future use ( ~ 20 % ), and 3 instructions ( ~ 1 % ) are permanently reserved for JVM implementations to use. Two of these ( impdep1 and impdep2 ) are to provide traps for implementation - specific software and hardware, respectively. The third is used for debuggers to implement breakpoints. Instructions fall into a number of broad groups : Load and store ( e.g. aload_0, istore ) Arithmetic and logic ( e.g. ladd, fcmpl ) Type conversion ( e.g. i2b, d2i ) Object creation and manipulation ( new, putfield ) Operand stack management ( e.g. swap, dup2 ) Control transfer ( e.g. ifeq, goto ) Method invocation and return ( e.g. invokespecial, areturn ) There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc. Many instructions have prefixes and / or suffixes referring to the types of operands they operate on. These are as follows : Prefix / suffix Operand type i integer l long s short byte character f float d double reference For example, iadd will add two integers, while dadd will add two doubles. The const, load, and store instructions may also take a suffix of the form _n, where n is a number from 0 -- 3 for load and store. The maximum n for const differs by type. The const instructions push a value of the specified type onto the stack. For example, iconst_5 will push an integer ( 32 bit value ) with the value 5 onto the stack, while dconst_1 will push a double ( 64 bit floating point value ) with the value 1 onto the stack. There is also an aconst_null, which pushes a null reference. The n for the load and store instructions specifies the index in the local variable array to load from or store to. The aload_0 instruction pushes the object in local variable 0 onto the stack ( this is usually the this object ). istore_1 stores the integer on the top of the stack into local variable 1. For local variables beyond 3 the suffix is dropped and operands must be used. Example Consider the following Java code : outer : for ( int i = 2 ; i < 1000 ; i + + ) ( for ( int j = 2 ; j < i ; j + + ) ( if ( i % j = = 0 ) continue outer ; ) System. out. println ( i ) ; ) A Java compiler might translate the Java code above into byte code as follows, assuming the above was put in a method : 0 : iconst_2 1 : istore_1 2 : iload_1 3 : sipush 1000 6 : if_icmpge 44 9 : iconst_2 10 : istore_2 11 : iload_2 12 : iload_1 13 : if_icmpge 31 16 : iload_1 17 : iload_2 18 : irem 19 : ifne 25 22 : goto 38 25 : iinc 2, 1 28 : goto 11 31 : getstatic # 84 ; / / Field java / lang / System. out : Ljava / io / PrintStream ; 34 : iload_1 35 : invokevirtual # 85 ; / / Method java / io / PrintStream. println : ( I ) V 38 : iinc 1, 1 41 : goto 2 44 : return Generation Further information : List of JVM languages The most common language targeting Java virtual machine by producing Java bytecode is Java. Originally only one compiler existed, the javac compiler from Sun Microsystems, which compiles Java source code to Java bytecode ; but because all the specifications for Java bytecode are now available, other parties have supplied compilers that produce Java bytecode. Examples of other compilers include : Jikes, compiles from Java to Java bytecode ( developed by IBM, implemented in C++ ) Espresso, compiles from Java to Java bytecode ( Java 1.0 only ) GNU Compiler for Java ( GCJ ), compiles from Java to Java bytecode ; it can also compile to native machine code and was part of the GNU Compiler Collection ( GCC ) up until version 6. Some projects provide Java assemblers to enable writing Java bytecode by hand. Assembly code may be also generated by machine, for example by a compiler targeting a Java virtual machine. Notable Java assemblers include : Jasmin, takes text descriptions for Java classes, written in a simple assembly - like syntax using Java virtual machine instruction set and generates a Java class file Jamaica, a macro assembly language for the Java virtual machine. Java syntax is used for class or interface definition. Method bodies are specified using bytecode instructions. Krakatau Bytecode Tools, currently contains three tools : a decompiler and disassembler for Java classfiles and an assembler to create classfiles. Lilac, an assembler and disassembler for the Java virtual machine. Others have developed compilers, for different programming languages, to target the Java virtual machine, such as : ColdFusion JRuby and Jython, two scripting languages based on Ruby and Python Apache Groovy, a scripting language based on Java Scala, a type - safe general - purpose programming language supporting object - oriented and functional programming JGNAT and AppletMagic, compile from the language Ada to Java bytecode C to Java byte - code compilers Clojure, a functional, immutable, general - purpose programming language in the Lisp family with a strong emphasis on concurrency MIDletPascal JavaFX Script code is compiled to Java bytecode Kotlin Object Pascal source code is compiled to Java bytecode using the Free Pascal 3.0 + compiler. Execution There are several machines available today, both free and commercial products. Further information : Java virtual machine If executing Java bytecode in a Java virtual machine is undesirable, a developer can also compile Java source code or bytecode directly to native machine code with tools such as the GNU Compiler for Java ( GCJ ). Some processors can execute Java bytecode natively. Such processors are termed Java processors. Support for dynamic languages Further information : List of JVM languages The Java virtual machine provides some support for dynamically typed languages. Most of the extant JVM instruction set is statically typed - in the sense that method calls have their signatures type - checked at compile time, without a mechanism to defer this decision to run time, or to choose the method dispatch by an alternative approach. JSR 292 ( Supporting Dynamically Typed Languages on the Java TM Platform ) added a new invokedynamic instruction at the JVM level, to allow method invocation relying on dynamic type checking ( instead of the extant statically type - checked invokevirtual instruction ). The Da Vinci Machine is a prototype virtual machine implementation that hosts JVM extensions aimed at supporting dynamic languages. All JVMs supporting JSE 7 also include the invokedynamic opcode. See also Java ( programming language ) portal Java bytecode instruction listings Java class file List of JVM languages Java backporting tools C to Java Virtual Machine compilers JStik Common Intermediate Language ( CIL ), Microsoft 's rival to Java bytecode ObjectWeb ASM Byte Code Engineering Library References Jump up ^ Understanding bytecode makes you a better programmer ^ Jump up to : Lindholm, Tim ; Yellin, Frank ; Bracha, Gilad ; Buckley, Alex ( 2015 - 02 - 13 ). The Java Virtual Machine Specification ( Java SE 8 ed. ). Jump up ^ Jasmin home page Jump up ^ Jamaica : The Java virtual machine ( JVM ) macro assembler Jump up ^ Krakatau home page Jump up ^ Lilac home page Jump up ^ Free Pascal 3.0 release notes Jump up ^ Free Pascal JVM Target Jump up ^ Nutter, Charles ( 2007 - 01 - 03 ). `` InvokeDynamic : Actually Useful ? ''. Retrieved 2008 - 01 - 25. Jump up ^ see JSR 292 External links The Wikibook Java Programming has a page on the topic of : Java bytecode Oracle 's Java Virtual Machine Specification Programming Languages for the Java Virtual Machine Bytecode Visualizer -- bytecode viewer and debugger ( free Eclipse plugin ) AdaptJ StackTrace -- bytecode level debugging with a full control of the stack, the local variables, and the execution flow Java Class Unpacker -- plugin for Total Commander, it lets open class files as compressed archives and see fields and methods as files. The bytecode can be viewed as text using F3 Java Platforms Java language JVM Micro Edition Standard Edition Enterprise Edition Java Card Android SDK Oracle technologies Squawk Java Development Kit OpenJDK Java virtual machine JavaFX Maxine VM Platform technologies Applets Servlets MIDlets JSP JSF Web Start ( JNLP ) Major third - party technologies Blackdown Eclipse GNU Classpath GWT Harmony Hibernate IcedTea Jazelle Spring Struts TopLink WildFly History Java version history Java Community Process Sun Microsystems Free Java implementations Major JVM languages BeanShell Clojure Groovy Oxygene JRuby Jython Kotlin Processing Rhino Scala Tcl / Java Community Conferences JavaOne Devoxx Developers Apache Software Foundation Java Community Process Oracle Corporation Sun Microsystems, Sun Microsystems Laboratories People James Gosling Arthur van Hoff Urs Hölzle Patrick Naughton Category Portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Java_bytecode&oldid=856595043 '' Categories : Assembly languages Java platform Hidden categories : Articles containing potentially dated statements from 2015 All articles containing potentially dated statements Talk Contents About Wikipedia Bosanski Català Español Français 한국어 Bahasa Indonesia Magyar 日本 語 Polski Português Русский Српски / srpski Srpskohrvatski / српскохрватски Türkçe 中文 6 more Edit links This page was last edited on 26 August 2018, at 09 : 35 ( UTC ). 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Question: what is byte code in the context of java
Answer: the instruction set of the Java virtual machine (JVM)