Abstract:
The disclosed system represents an improvement over conventional systems for initializing static arrays by reducing the amount of code executed by the virtual machine to statically initialize an array. To realize this reduction, when consolidating class files, the preloader identifies all &lt;clinit&gt; methods and play executes these methods to determine the static initialization performed by them. The preloader then creates an expression indicating the static initialization performed by the &lt;clinit&gt; method and stores this expression in the .mclass file, replacing the &lt;clinit&gt; method. As such, the code of the &lt;clinit&gt; method, containing many instructions, is replaced by a single expression instructing the virtual machine to perform static initialization, thus saving a significant amount of memory. The virtual machine is modified to recognize this expression and perform the appropriate static initialization of an array.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to data processing systems and, more particularly, to a method and system for performing static initialization. 
     BACKGROUND OF THE INVENTION 
     Java™ describes both a programming language and a programming environment for generating and running platform-independent code. This platform-independent code runs on a Java™ virtual machine, which is an abstract computing machine that interprets the platform-independent code. The Java™ virtual machine is described in greater detail in Lindholm and Yellin, The Java Virtual Machine Specification, Addison-Wesley (1997), which is hereby incorporated by reference. The Java™ virtual machine does not specifically recognize the Java™ programming language or any other programming language; instead, the Java virtual machine only recognizes a particular file format, the class file format. A class file contains the Java virtual machine instructions (or byte codes) that constitute the platform-independent code. 
     As part of running a Java program, a developer performs a number of steps, as shown in FIG. 1. First, a developer compiles a computer program (step 102). Typically, the developer has developed a computer program containing source code in a high-level language, such as the Java programming language, and invokes the Java™ compiler to compile the code. The Java compiler is part of the Java™ software development kit available from Sun Microsystems of Mountain View, Calif,. The Java compiler outputs one or more class files containing byte codes suitable for execution on the Java virtual machine. Each class file contains one type of the Java programming language, either a class or an interface. The class file format is described in greater detail on pp. 83-137 of The Java Virtual Machine Specification. Although the class file format is a robust file format, it is unable to instruct the virtual machine to statically initialize an array efficiently, thus posing a problem, discussed in greater detail below. 
     After compiling the program, the developer consolidates the class files output in step 102 into a single file, known as a .mclass file, by using a preloader (step 104). The preloader also available from Sun Microsystems, Inc., concatenates the class files and performs preprocessing to facilitate the execution of the class files. After consolidating the class files, the developer loads the .mclass file into a virtual machine (step 106). In this step, the Java virtual machine stores the .mclass file in memory and interprets the byte codes contained in the .mclass file by reading the byte codes and then processing and executing them. Until interpretation of the byte codes is completed, the .mclass file is stored in memory. The byte codes recognized by the Java virtual machine are more clearly described on pp. 151-338 of The Java Virtual Machine Specification. 
     As stated above, the class file format cannot instruct the virtual machine to statically initialize arrays. To compensate for this problem, the Java™ compiler generates a special method, &lt;clinit&gt;, to perform class initialization, including initialization of static arrays. An example of the initialization of a static array follows: 
     Code Table #1 
     static int setup[ ]={1, 2, 3, 4}; 
     In this example, an array &#34;setup&#34; contains four integers statically initialized to the following values: 1, 2, 3, and 4. Given this static initialization, the Java™ compiler creates a &lt;clinit&gt; method that performs the static initialization as functionally described below in pseudo-code: 
     Code Table #2 
     temp=new int [4]; 
     temp=[0]=1; 
     temp=[1]=2; 
     temp=[2]=3; 
     temp=[3]=4; 
     this.setup=temp; 
     As the above code table shows, merely describing the &lt;clinit&gt; method functionally requires a number of statements. More importantly, however, the actual processing of the &lt;clinit&gt; method, performed by byte codes, requires many more statements. These byte codes manipulate a stack resulting in the requested static initialization. A stack is a portion of memory used by the methods in the Java programming environment. The steps performed by the &lt;clinit&gt; method for the example static initialization described above are expressed below in byte codes. 
     Code Table #3 
     Method void &lt;clinit&gt;() 
     0 iconst --  4 //push an integer value of 4 on the stack 
     1 newarray int //create a new array of integers and put it on the stack. 
     3 dup //duplicate top of stack 
     4 iconst --  0 //push an integer value of 0 on the stack 
     5 iconst --  1 //push an integer value of 1 on the stack 
     6 iastore //store a 1 at index 0 of array 
     7 dup //duplicate the top of the stack 
     8 iconst --  1 //push an integer value of 1 on the stack 
     9 iconst --  2 //push an integer value of 2 on the stack 
     10 iastore //store a 2 at index 1 of array 
     11 dup //duplicate top of stack 
     12 iconst --  2 //push an integer value of 2 on the stack 
     13 iconst --  3 //push an integer value of 3 on the stack 
     14 iastore //store a 3 at index 2 of array 
     15 dup //duplicate top of stack 
     16 iconst --  3 //push an integer of value 3 on stack 
     17 iconst --  4 //push an integer of value 4 on stack 
     18 iastore //store a 4 at index 3 of array 
     19 putstatic #3&lt;Field foobar.setup [I&gt; //modify set up array according to new array on stack 
     22 return 
     Although using the &lt;clinit&gt; method provides the Java™ compiler with a way to instruct the virtual machine to initialize a static array, the amount of code required to initialize the array is many times the size of the array, thus requiring a significant amount of memory. It is therefore desirable to improve static initialization. 
     SUMMARY OF THE INVENTION 
     The disclosed system represents an improvement over conventional systems for initializing static arrays by reducing the amount of code executed by the virtual machine to statically initialize an array. To realize this reduction, when consolidating class files, the preloader identifies all &lt;clinit&gt; methods and simulates executing (&#34;play executes&#34;) these methods to determine the static initialization performed by them. The preloader then creates an expression indicating the static initialization performed by the &lt;clinit&gt; method and stores this expression in the .mclass file, replacing the &lt;clinit&gt; method. As such, the code of the &lt;clinit&gt; method, containing many instructions, is replaced by a single expression instructing the virtual machine to perform static initialization, thus saving a significant amount of memory. The virtual machine is modified to recognize this expression and perform the appropriate static initialization of an array. 
     Methods consistent with the present invention receive code to be run on a processing component to perform an operation. The code is then play executed on the memory without running the code on the processing component to identify the operation if the code were run by the processing component. Thereafter, a directive is created for the processing component to perform the operation. 
     A data processing system consistent with the present invention contains a secondary storage device, a memory, and a processor. The secondary storage device contains a program with source code that statically initializes the data structure and class files, where one of the class files contains a &lt;clinit&gt; method that statically initializes the data structure. The memory contains a compiler for compiling the program and for generating the class files and a preloader for consolidating the class files, for simulating execution of the &lt;clinit&gt; method to determine the static initialization the &lt;clinit&gt; method performs, and for creating an instruction to perform the static initialization. The processor runs the compiler and the preloader. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a flowchart of the steps performed when developing a program in the Java™ programming environment. 
     FIG. 2 depicts a data processing system consistent with the present invention. 
     FIG. 3 depicts a flowchart of the steps performed by the preloader depicted in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Systems and methods consistent with the present invention provide an improved system for initializing static arrays in the Java™ programming environment by replacing the &lt;clinit&gt; method with one or more directives which, when read by the virtual machine, causes the virtual machine to perform the same static initialization performed by the &lt;clinit&gt; method, except using a significantly less amount of memory and significantly less time. As a result, such systems and methods can significantly reduce memory utilization when statically initializing an array. 
     Overview 
     Systems and methods consistent with the present invention eliminate the need for the &lt;clinit&gt; method by performing certain preprocessing in the preloader. Specifically, the preloader receives class files for consolidation and scans them looking for a &lt;clinit&gt; method. When the preloader finds the &lt;clinit method, it simulates execting (&#34;play executes&#34;) the &lt;clinit&gt; method against memory to determine the effects that the &lt;clinit&gt; method would have on the memory if interpreted by the Java virtual machine. That is, the preloader simulates execution of the &lt;clinit&gt; method to identify the static initialization that would result had the &lt;clinit&gt; method been executed by the Java™ virtual machine. After identifying this static initialization, the preloader generates one or more directives (or instructions) to cause the same static initialization as the &lt;clinit&gt; method and outputs these directives to the Java virtual machine, thus replacing the &lt;clinit&gt; method. These directives are then read at runtime by the Java virtual machine causing the Java virtual machine to perform the same static initialization performed by the &lt;clinit&gt; method. The directives require significantly less memory space than the &lt;clinit&gt; method. For example, the byte codes described above in code table #3 could be reduced to the following directives contained within the .mclass file indicating that an array of four integers has the initial values 1, 2, 3, and 4: 
     CONSTANT --  Array T --  INT 4 1 2 3 4 
     The virtual machine of an exemplary embodiment recognizes this expression and statically initializes the array to the appropriate values. As a result, the exemplary embodiment reduces memory consumption over conventional systems when initializing a static array. 
     Implementation Details 
     FIG. 2 depicts a data processing system 200 consistent with the present invention. The data processing system 200 comprises a computer system 202 connected to the Internet 204. Computer system 202 contains a memory 206, a secondary storage device 208, a central processing unit (CPU) 210, an input device 212, and a video display 214. The memory 206 further includes the Java™ compiler 218, the Java™ preloader 220, and the Java™ runtime system 221. The Java™ runtime system 221 includes the Java™ virtual machine 222. The secondary storage device 208 contains a program 224 with source code, various class files 226, and a .mclass file 228. The Java™ compiler 218 compiles the program 224 into one or more class files 226. The preloader 220 then receives the class files 226 and generates a .mclass file 228 representing the consolidation of all of the class files. After consolidation, the .mclass file 228 can be run on the virtual machine 222. 
     Processing consistent with the present invention is performed by the preloader 220 searching for a &lt;clinit&gt; method, and when it is found, the preloader (1) simulates execution of the &lt;clinit&gt; method to determine the effects it would have on memory if it was interpreted by the virtual machine 222, (2) creates static initialization directives to replicate these effects, and (3) outputs these directives in the .mclass file to replace the &lt;clinit&gt; method, thus saving significant amounts of memory. 
     In addition, processing consistent with the present invention is performed by the virtual machine 222 because it is modified to recognize the static initialization directives of the preloader. Although an exemplary embodiment of the present invention is described as being stored in memory 206, one skilled in the art will appreciate that it may also be stored on other computer-readable media, such as secondary storage devices like hard disks, floppy disks, or CD-Rom; a carrier wave received from the Internet 204; or other forms of RAM or ROM. Additionally, one skilled in the art will appreciate that computer 202 may contain additional or different components. 
     The Preloader 
     FIG. 3 depicts a flowchart of the steps performed by the preloader 220 consistent with the present invention to perform initialization of a static array. The first step performed by the preloader is to read a class file to obtain the &lt;clinit&gt; method (step 302). After obtaining a &lt;clinit&gt; method, the preloader allocates various variables for use during play execution (step 304). When play executing, discussed below, the preloader simulates execution of the byte codes contained in the &lt;clinit&gt; method by the virtual machine. These byte codes manipulate various data structures associated with the &lt;clinit&gt; method, such as the constant pool, the stack, or local variables (or registers). 
     The constant pool is a table of variable-length structures representing various string constants, class names, field names, and other constants referred to within the class file. The stack is a portion of memory for use in storing operands during the execution of the method. Thus, the size of the stack is the largest amount of space occupied by the operands at any point during execution of this method. The local variables are the variables that are used by this method. 
     When allocating variables, the preloader obtains a pointer to the constant pool of the &lt;clinit&gt; method, allocates a stack to the appropriate size, and allocates an array such that one entry of the array corresponds to each of the local variables. As described below, the play execution operates on these variables. 
     After allocating the variables, the preloader reads a byte code from the &lt;clinit&gt; method (step 306). Next, the preloader determines if it recognizes this byte code (step 308). In this step, the preloader recognizes a subset of all byte codes where this subset contains only those byte codes that are generally used to perform static initialization of an array. Following is a list of the byte codes recognized by the preloader of an exemplary embodiment: 
     
         ______________________________________Code Table #4______________________________________  aconst.sub.-- null                iastore  iconst.sub.-- m1                lastore  iconst.sub.-- 0                fastore  iconst.sub.-- 1                dastore  iconst.sub.-- 2                aastore  iconst.sub.-- 3                bastore  iconst.sub.-- 4                lastore  iconst.sub.-- 5                sastore  lconst.sub.-- 0                dup  lconst.sub.-- 1                newarray  fconst.sub.-- 0                anewarray  fconst.sub.-- 1                return  fconst.sub.-- 2                ldc  dconst.sub.-- 0                ldc.sub.-- w  dconst.sub.-- 1                ldc2.sub.-- w  bipush        putstatic  sipush______________________________________ 
    
     Any byte codes other than those listed above are not recognized. The appearance of other byte codes beyond those described above indicates that the &lt;clinit&gt; method performs functionality in addition to statically initializing an array. In this case, the &lt;clinit&gt; method cannot be optimized. If a byte code is not recognized, the preloader considers it unsuitable for optimization (or play execution) and processing continues to step 316. 
     If the preloader recognizes the byte code, however, the preloader performs the operation reflected by the byte code (step 310). In this step, the preloader play executes the byte code on the variables allocated in step 304, and as a result, a value may be popped from the stack, a local variable may be updated, or a value from the constant pool may be retrieved. Additionally, the preloader may encounter a &#34;put static&#34; byte code indicating that a particular static variable (e.g., array) is to be initialized in a particular manner. If the preloader receives such a byte code, it stores an indication of the requested initialization into a hash table for later use. An example of such an entry in the hash table follows: 
     
         Setup:=Array (1,2,3,4) 
    
     After performing the operation reflected by the byte code, the preloader determines if there are more byte codes in the &lt;clinit&gt; method (step 314). If so, processing returns to step 306. However, if there are no more byte codes, the preloader stores directions in the .mclass file to statically initialize the arrays (step 318). In this step, the preloader stores constant pool entries into the .mclass file like the following: 
     
         ______________________________________Tag             Type      Size     Values______________________________________CONSTANT.sub.-- Array           T.sub.-- INT                     4        1 2 3 4______________________________________ 
    
     This entry in the constant pool indicates that a particular array has four integers that have the initial values of 1, 2, 3, and 4. At run time, when the virtual machine initializes the class .mclass file, it will encounter a reference to this constant pool entry and create the appropriate array. As a result, the many instructions contained in the &lt;clinit&gt; method are reduced to this one expression, saving significant amounts of memory and time. 
     Example Implementation of the Preloader 
     The following pseudo-code describes sample processing of the preloader of an exemplary embodiment. The preloader receives as a parameter a method information data structure that defines the &lt;clinit&gt; method, described in the Java™ Virtual Machine Specification at pp. 104-106, and play executes the byte codes of this &lt;clinit&gt; method. It should be noted that the processing described is only exemplary; as such, only a few byte codes are described as being processed by the preloader. However, one skilled in the art will appreciate that all of the byte codes in code table #4 may be processed by the exemplary embodiment. 
     
         ______________________________________Code Table #5______________________________________void emulateByteCodes(Method.sub.-- info mb) int numberRegisters = mb.max.sub.-- locals();                //number of local variables int stackSize = mb.max.sub.-- stack();                //stack size byte byteCode [] = mb.code();                //get the byte code ConstantPool constantPool = mb.constantPool();                   // get constant pool Object stack[] = new Object[stackSize];                    //create stack for                     play execution Object registers[] = new Object[numberRegisters];                    //create local                     variables                     for play                      //execution /* Start with an empty stack. */ int stackTop = -1;  //just below valid element /* Map of static objects */ Hashtable changes = new Hashtable();try {  boolean success; execution.sub.-- loop:  for (int codeOffset = 0, nextCodeOffset;    ;codeOffset = nextCodeOffset) {   int opcode = byteCode[codeOffset] &amp; 0xFF;  //0..255   nextCodeOffset = codeOffset + 1; // the most usual value   switch(opcode) {    case opc.sub.-- iconst.sub.-- m1: // push -1 on the stackstack[++stackTop] = new Integer(-1);break;    case opc.sub.-- bipush:nextCodeOffset = codeOffset + 2;stack[++stackTop] = new Integer(bytecode[codeOffset + 1]);break;    case opc.sub.-- 1load.sub.-- 3:   //load the contents of register 3stack[++stackTop] = (Long)register [3];stack[++stackTop] = null;  //longs use two words on stackbreak;    case opc.sub.-- fsub: {   // subtract top of stack from item belowfloat b = stack[stackTop--].floatValue();float a = stack[stackTop].floatValue();stack[stackTop] = new Float(a - b);break;    }    case opc.sub.-- 1dc:nextCodeOffset = codeOffset + 2;stack[++stackTop] =  constantPool.getItem(byteCode (codeOffset + 1));break;    case sastore: {// store the contents into a &#34;short&#34; arrayshort value = (short) (stack[StackTop--].intValue());int index = stack[StackTop--].intvalue();short[] array = (short[])stack[StackTop--];array[index] = value;break;    }    case opc.sub.-- putstatic: {nextCodeOffset = codeOffset + 3;int index = ((byteCode[codeOffset + 1]) &amp; 0xFF) &lt;&lt; 8) + (byteCode[codeOffset + 2] &amp; 0xFF);Field f = constantPool.getItem(byteCode[codeOffset + 1]);if (f.getClass() ! = mb.getClass()) { // we can only modify static&#39;s in our own class throw new RuntimeException();}Type t = f.getType();if (t.isLong() ∥ t.isDouble()) ++stackTop;Object value = stack[++stackTop]changes.put(f, value); // put entry into hashtablebreak;    case opc.sub.-- return:success = true;break execution.sub.-- loop;    default: // some byte code we do not understandsuccess = false;break execution.sub.-- loop;    }   }  } catch (RuntimeException) {   // any runtime exception indicates failure.   success = false;  }  if (success) {   &lt;modify .class file as indicated by &#34;changes&#34; hashtable&gt;   &lt;Remove this &lt;clinit&gt; method from the class&gt;  } else {   &lt;ran into something we cannot understand&gt;   &lt;do not replace this method&gt; }______________________________________ 
    
     The Virtual Machine of the Exemplary Embodiment 
     As stated above, the Java virtual machine 222 is an otherwise standard Java virtual machine as defined in the Java Virtual Machine Specification, except that it is modified as will be described below. Conventional virtual machines recognize various constant pool entries, such as CONSTANT --  Integer, CONSTANT --  String, and CONSTANT --  Long. Constant pool entries of these types store various variable information, including the initial value. The virtual machine of an exemplary embodiment, however, additionally recognizes the CONSTANT --  Array entry in the constant pool. 
     The format of the CONSTANT --  Array constant pool entry in the class file format follows: 
     
         ______________________________________Code Table #6______________________________________CONSTANT.sub.-- Array.sub.-- info {ul tag;     /* The literal value CONSTANT.sub.-- Array */ul type;    /* see below */u4 length;  /* number of elements of the array */ux objects[length];       /* Actual values *//* The following field is included only if type ==T.sub.-- CLASS */u2 type2;  /* index of CONSTANT.sub.-- Class in constant pool */______________________________________ 
    
     The ul type field is one of the values listed in the following table: 
     
         ______________________________________  Array Type           Value______________________________________  T.sub.-- CLASS           2  T.sub.-- BOOLEAN           4  T.sub.-- CHAR           5  T.sub.-- FLOAT           6  T.sub.-- DOUBLE           7  T.sub.-- BYTE           8  T.sub.-- SHORT           9  T.sub.-- INT           10  T.sub.-- LONG           11______________________________________ 
    
     The field ux objects[length] is an array of values, providing the elements of the array. The number of elements in the array is given by the length field of the constant pool entry. The actual size of each of these values is shown below: 
     
         ______________________________________Type                  ux      Meaning______________________________________T.sub.-- BOOLEAN, T.sub.-- BYTE                 u1      1 byteT.sub.-- CHAR, T.sub.-- SHORT, T.sub.-- CLASS                 u2      2 bytesT.sub.-- INT, T.sub.-- FLOAT                 u4      4 bytesT.sub.-- LONG, T.sub.-- DOUBLE                 u8      8 bytes______________________________________ 
    
     For all of the above types except for T --  CLASS, the bytes shown are the actual value that are stored in that element of the array. For T --  CLASS, however, each u2 is itself an index to an entry into the constant pool. The constant pool entry referred to must itself be either a CONSTANT --  Array, CONSTANT --  Object, or the special constant pool entry 0, indicating a NULL value. 
     For example, to indicate the following array: 
     
         int[ ]={10, 20, 30, 40 }; 
    
     the constant pool entry would be as follows: 
     
         ______________________________________Tag            Type     Size    Initial Values______________________________________CONSTANT.sub.-- Array          T.sub.-- INT                   4       10 20 30 40______________________________________ 
    
     As another example, to indicate the following array: 
     
         new Foo[3 ]/* all initialized to NULL */ 
    
     the constant pool entry would be as follows: 
     
         ______________________________________Tag          Type       Size   Initial Values                                   Class______________________________________CONSTANT.sub.-- Array        T.sub.-- CLASS                   3      0 0 0    XX______________________________________ 
    
     where &#34;xx&#34; is an index into the constant pool indicating the class Foo in the constant pool. 
     Two-dimensional arrays like the following: 
     
         new byte[ ][ ]={{1,2,3,4 }, {5,6,7,8 }}; 
    
     are encoded by having two constant pool entries encode the sub-arrays and by having two additional entries indicate the association between the subarrays. This encoding corresponds to the Java™ notion of an array as a type of object and a multi-dimensional array as an array of arrays. The constant pool entries of the above two-dimensional array follows: 
     
         Entry1: CONSTANT.sub.-- Array T.sub.-- BYTE 4 1 2 3 4 
    
     
         Entry2: CONSTANT.sub.-- Array T.sub.-- BYTE 4 5 6 7 8 
    
     
         Entry3: CONSTANT.sub.-- Class with name &#34;[[B&#34; 
    
     and then 
     
         ______________________________________Tag             Type   Size   Initial Values                                  Class______________________________________Entry4: CONSTANT.sub.-- Array T.sub.-- Class                  2      Entry1 Entry2                                  Entry3______________________________________ 
    
     where each of Entry1, Entry2, and Entry3 are the two-byte encodings of the index of the corresponding constant-pool entry. 
     While the systems and methods of the present invention have been described with reference to a preferred embodiment, those skilled in the art will know of various changes in form and detail which may be made without departing from the spirit and scope of the present invention as defined in the appended claims.