Method and apparatus for pre-processing and packaging class files

Abstract

A method and apparatus for pre-processing and packaging class files. Embodiments remove duplicate information elements from a set of class files to reduce the size of individual class files and to prevent redundant resolution of the information elements. Memory allocation requirements are determined in advance for the set of classes as a whole to reduce the complexity of memory allocation when the set of classes are loaded. The class files are stored in a single package for efficient storage, transfer and processing as a unit. In an embodiment, a pre-processor examines each class file in a set of class files to locate duplicate information in the form of redundant constants contained in a constant pool. The duplicate constant is placed in a separate shared table, and all occurrences of the constant are removed from the respective constant pools of the individual class files. During pre-processing, memory allocation requirements are determined for each class file, and used to determine a total allocation requirement for the set of class files. The shared table, the memory allocation requirements and the reduced class files are packaged as a unit in a multi-class file.

Claims

We claim:

1. A method of pre-processing class files comprising:

determining plurality of duplicated elements in a plurality of class files;

forming a shared table comprising said plurality of duplicated elements;

removing said duplicated elements from said plurality of class files to obtain a plurality of reduced class files; and

forming a multi-class file comprising said plurality of reduced class files and said shared table.

2. The method of claim 1, further comprising:

computing an individual memory allocation requirement for each of said plurality of reduced class files;

computing a total memory allocation requirement for said plurality of class files from said individual memory allocation requirement of each of said plurality of reduced class files; and

storing said total memory allocation requirement in said multi-class file.

3. The method of claim 2, further comprising:

reading said total memory allocation requirement from said multi-class file;

allocating a portion of memory based on said total memory allocation requirement; and

loading said reduced class files and said shared table into said portion of memory.

4. The method of claim 3, further comprising:

accessing said shared table in said portion of memory to obtain one or more elements not found in one or more of said reduced class files.

5. The method of claim 1, wherein said step of determining a plurality of duplicated elements comprises:

determining one or more constants shared between two or more class files.

6. The method of claim 5, wherein said step of forming a shared table comprises:

forming a shared constant table comprising said one or more constants shared between said two or more class files.

7. A computer program product comprising:

a computer usable medium having computer readable program code embodied therein for pre-processing class files, said computer program product comprising:

computer readable program code configured to cause a computer to determine a plurality of duplicated elements in a plurality of class files;

computer readable program code configured to cause a computer to form a shared table comprising said plurality of duplicated elements;

computer readable program code configured to cause a computer to remove said duplicated elements from said plurality of class files to obtain a plurality of reduced class files; and

computer readable program code configured to cause a computer to form a multi-class file comprising said plurality of reduced class files and said shared table.

8. The computer program product of claim 7, further comprising:

computer readable program code configured to cause a computer to compute an individual memory allocation requirement of each of said plurality of reduced class files;

computer readable program code configured to cause a computer to compute a total memory allocation requirement of said plurality of class files from said individual memory allocation requirement of each of said plurality of reduced class files; and

computer readable program code configured to cause a computer to store said total memory allocation requirement in said multi-class file.

9. The computer program product of claim 8, further comprising:

computer readable program code configured to cause a computer to read said total memory allocation requirement from said multi-class file;

computer readable program code configured to cause a computer to allocate a portion of memory based on said total memory allocation requirement; and

computer readable program code configured to cause a computer to load said reduced class files and said shared table into said portion of memory.

10. The computer program product of claim 9, further comprising:

computer readable program code configured to cause a computer to access said shared table in said portion of memory to obtain one or more elements not found in one or more of said reduced class files.

11. The computer program product of claim 7, wherein said computer readable program code configured to cause a computer to determine said plurality of duplicated elements comprises:

computer readable program code configured to cause a computer to determine one or more constants shared between two or more class files.

12. The computer program product of claim 11, wherein said computer readable program code configured to cause a computer to form said shared table comprises:

computer readable program code configured to cause a computer to form a shared constant table comprising said one or more constants shared between said two or more class files.

13. An apparatus comprising:

a processor;

a memory coupled to said processor;

a plurality of class files stored in said memory;

a process executing on said processor, said process configured to form a multi-class file comprising:

a plurality of reduced class files obtained from said plurality of class files by removing one or more elements that are duplicated between two or more of said plurality of class files; and

a shared table comprising said duplicated elements.

14. The apparatus of claim 13, wherein said multi-class file further comprises a memory requirement, said memory requirement being computed by said process.

15. The apparatus of claim 13, wherein said duplicated elements comprise elements of constant pools of respective class files, said shared table comprising a shared constant pool.

16. The apparatus of claim 13, further comprising:

a virtual machine having a class loader and a runtime data area, said class loader configured to obtain and load said multi-class file into said runtime data area.

17. The apparatus of claim 16, wherein said class loader is configured to allocate a portion of said runtime data area based on said memory requirement in said multi-class file.

18. The apparatus of claim 17, wherein said class loader is configured to load said plurality of reduced class files and said shared table into said portion of said runtime data area.

19. The apparatus of claim 16, wherein said virtual machine is configured to access said shared table when a desired element associated with a first class file is not present in a corresponding one of said plurality of reduced class files.

20. A memory configured to store data for access by a virtual machine executing in a computer system, comprising:

a data structure stored in said memory, said data structure comprising:

a plurality of reduced class files associated with a plurality of corresponding classes, said plurality of reduced class files configured to be loaded by the virtual machine for execution of said plurality of classes;

a shared table comprising one or more elements that are duplicated between two or more of said plurality of classes, said shared table configured to be loaded into the virtual machine to be accessed for said duplicated elements; and

a memory requirement value configured to be read by a class loader of the virtual machine to allocate a portion of a runtime data area for loading said plurality of reduced class files and said shared table.

21. The memory of claim 20, wherein said duplicated elements are removed from said plurality of reduced class files.

22. The memory of claim 20, wherein said duplicated elements comprise constants and said shared table comprises a shared constant pool.

23. The memory of claim 20, wherein said memory requirement value is computed from individual memory requirements of said plurality of reduced class files and a memory requirement of said shared table.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of computer software, and, more specifically, to object-oriented computer applications.

Portions of the disclosure of this patent document contain material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office file or records, but otherwise reserves all copyright rights whatsoever.

2. Background Art

With advancements in network technology, the use of networks for facilitating the distribution of media information, such as text, graphics, and audio, has grown dramatically, particularly in the case of the Internet and the World Wide Web. One area of focus for current developmental efforts is in the field of web applications and network interactivity. In addition to passive media content, such as HTML definitions, computer users or "clients" coupled to the network are able to access or download application content, in the form of applets, for example, from "servers" on the network.

To accommodate the variety of hardware systems used by clients, applications or applets are distributed in a platform-independent format such as the Java.RTM. class file format. Object-oriented applications are formed from multiple class files that are accessed from servers and downloaded individually as needed. Class files contain bytecode instructions. A "virtual machine" process that executes on a specific hardware platform loads the individual class files and executes the bytecodes contained within.

A problem with the class file format and the class loading process is that class files often contain duplicated data. The storage, transfer and processing of the individual class files is thus inefficient due to the redundancy of the information. Also, an application may contain many class files, all of which are loaded and processed in separate transactions. This slows down the application and degrades memory allocator performance. Further, a client is required to maintain a physical connection to the server for the duration of the application in order to access class files on demand.

These problems can be understood from a review of general object-oriented programming and an example of a current network application environment.

Object-Oriented Programming

Object-oriented programming is a method of creating computer programs by combining certain fundamental building blocks, and creating relationships among and between the building blocks. The building blocks in object-oriented programming systems are called "objects." An object is a programming unit that groups together a data structure (one or more instance variables) and the operations (methods) that can use or affect that data. Thus, an object consists of data and one or more operations or procedures that can be performed on that data. The joining of data and operations into a unitary building block is called "encapsulation."

An object can be instructed to perform one of its methods when it receives a "message." A message is a command or instruction sent to the object to execute a certain method. A message consists of a method selection (e.g., method name) and a plurality of arguments. A message tells the receiving object what operations to perform.

One advantage of object-oriented programming is the way in which methods are invoked. When a message is sent to an object, it is not necessary for the message to instruct the object how to perform a certain method. It is only necessary to request that the object execute the method. This greatly simplifies program development.

Object-oriented programming languages are predominantly based on a "class" scheme. The class-based object-oriented programming scheme is generally described in Lieberman, "Using Prototypical Objects to Implement Shared Behavior in Object-Oriented Systems," OOPSLA 86 Proceedings, September 1986, pp. 214-223.

A class defines a type of object that typically includes both variables and methods for the class. An object class is used to create a particular instance of an object. An instance of an object class includes the variables and methods defined for the class. Multiple instances of the same class can be created from an object class. Each instance that is created from the object class is said to be of the same type or class.

To illustrate, an employee object class can include "name" and "salary" instance variables and a "set.sub.-- salary" method. Instances of the employee object class can be created, or instantiated for each employee in an organization. Each object instance is said to be of type "employee." Each employee object instance includes "name" and "salary" instance variables and the "set.sub.-- salary" method. The values associated with the "name" and "salary" variables in each employee object instance contain the name and salary of an employee in the organization. A message can be sent to an employee's employee object instance to invoke the "set.sub.-- salary" method to modify the employee's salary (i.e., the value associated with the "salary" variable in the employee's employee object).

A hierarchy of classes can be defined such that an object class definition has one or more subclasses. A subclass inherits its parent's (and grandparent's etc.) definition. Each subclass in the hierarchy may add to or modify the behavior specified by its parent class. Some object-oriented programming languages support multiple inheritance where a subclass may inherit a class definition from more than one parent class. Other programming languages support only single inheritance, where a subclass is limited to inheriting the class definition of only one parent class. The Java programming language also provides a mechanism known as an "interface" which comprises a set of constant and abstract method declarations. An object class can implement the abstract methods defined in an interface. Both single and multiple inheritance are available to an interface. That is, an interface can inherit an interface definition from more than one parent interface.

An object is a generic term that is used in the object-oriented programming environment to refer to a module that contains related code and variables. A software application can be written using an object-oriented programming language whereby the program's functionality is implemented using objects.

A Java program is composed of a number of classes and interfaces. Unlike many programming languages, in which a program is compiled into machine-dependent, executable program code, Java classes are compiled into machine independent bytecode class files. Each class contains code and data in a platform-independent format called the class file format. The computer system acting as the execution vehicle contains a program called a virtual machine, which is responsible for executing the code in Java classes. The virtual machine provides a level of abstraction between the machine independence of the bytecode classes and the machine-dependent instruction set of the underlying computer hardware. A "class loader" within the virtual machine is responsible for loading the bytecode class files as needed, and either an interpreter executes the bytecodes directly, or a "just-in-time" (JIT) compiler transforms the bytecodes into machine code, so that they can be executed by the processor. FIG. 1 is a block diagram illustrating a sample Java network environment comprising a client platform 102 coupled over a network 101 to a server 100 for the purpose of accessing Java class files for execution of a Java application or applet.

Sample Java Network Application Environment

In FIG. 1, server 100 comprises Java development environment 104 for use in creating the Java class files for a given application. The Java development environment 104 provides a mechanism, such as an editor and an applet viewer, for generating class files and previewing applets. A set of Java core classes 103 comprise a library of Java classes that can be referenced by source files containing other/new Java classes. From Java development environment 104, one or more Java source files 105 are generated. Java source files 105 contain the programmer readable class definitions, including data structures, method implementations and references to other classes. Java source files 105 are provided to Java compiler 106, which compiles Java source files 105 into compiled ".class" files 107 that contain bytecodes executable by a Java virtual machine. Bytecode class files 107 are stored (e.g., in temporary or permanent storage) on server 100, and are available for download over network 101.

Client platform 102 contains a Java virtual machine (JVM) 111 which, through the use of available native operating system (O/S) calls 112, is able to execute bytecode class files and execute native O/S calls when necessary during execution.

Java class files are often identified in applet tags within an HTML (hypertext markup language) document. A web server application 108 is executed on server 100 to respond to HTTP (hypertext transport protocol) requests containing URLs (universal resource locators) to HTML documents, also referred to as "web pages." When a browser application executing on client platform 102 requests an HTML document, such as by forwarding URL 109 to web server 108, the browser automatically initiates the download of the class files 107 identified in the applet tag of the HTML document. Class files 107 are typically downloaded from the server and loaded into virtual machine 111 individually as needed.

It is typical for the classes of a Java program to be loaded as late during the program's execution as possible; they are loaded on demand from the network (stored on a server), or from a local file system, when first referenced during the Java program's execution. The virtual machine locates and loads each class file, parses the class file format, allocates memory for the class's various components, and links the class with other already loaded classes. This process makes the code in the class readily executable by the virtual machine.

The individualized class loading process, as it is typically executed, has disadvantages with respect to use of storage resources on storage devices, allocation of memory, and execution speed and continuity. Those disadvantages are magnified by the fact that a typical Java application can contain hundreds or thousands of small class files. Each class file is self-contained. This often leads to information redundancy between class files, for example, with two or more class files sharing common constants. As a result, multiple classes inefficiently utilize large amounts of storage space on permanent storage devices to separately store duplicate information. Similarly, loading each class file separately causes unnecessary duplication of information in application memory as well. Further, because common constants are resolved separately per class during the execution of Java code, the constant resolution process is unnecessarily repeated.

Because classes are loaded one by one, each small class requires a separate set of dynamic memory allocations. This creates memory fragmentation, which wastes memory, and degrades allocator performance. Also, separate loading "transactions" are required for each class. The virtual machine searches for a class file either on a network device, or on a local file system, and sets up a connection to load the class and parse it. This is a relatively slow process, and has to be repeated for each class. The execution of a Java program is prone to indeterminate pauses in response/execution caused by each class loading procedure, especially, when loading classes over a network. These pauses create a problem for systems in which interactive or real-time performance is important.

A further disadvantage of the individual class loading process is that the computer executing the Java program must remain physically connected to the source of Java classes during the duration of the program's execution. This is a problem especially for mobile or embedded computers without local disk storage or dedicated network access. If the physical connection is disrupted during execution of a Java application, class files will be inaccessible and the application will fail when a new class is needed. Also, it is often the case that physical connections to networks such as the Internet have a cost associated with the duration of such a connection. Therefore, in addition to the inconvenience associated with maintaining a connection throughout application execution, there is added cost to the user as a result of the physical connection.

A Java archive (JAR) format has been developed to group class files together in a single transportable package known as a JAR file. JAR files encapsulate Java classes in archived, compressed format. A JAR file can be identified in an HTML document within an applet tag. When a browser application reads the HTML document and finds the applet tag, the JAR file is downloaded to the client computer and decompressed. Thus, a group of class files may be downloaded from a server to a client in one download transaction. After downloading and decompressing, the archived class files are available on the client system for individual loading as needed in accordance with standard class loading procedures. The archived class files remain subject to storage inefficiencies due to duplicated data between files, as well as memory fragmentation due to the performance of separate memory allocations for each class file.

SUMMARY OF THE INVENTION

A method and apparatus for pre-processing and packaging class files is described. Embodiments of the invention remove duplicate information elements from a set of class files to reduce the size of individual class files and to prevent redundant resolution of the information elements. Memory allocation requirements are determined in advance for the set of classes as a whole to reduce the complexity of memory allocation when the set of classes are loaded. The class files are stored in a single package for efficient storage, transfer and processing as a unit.

In an embodiment of the invention, a pre-processor examines each class file in a set of class files to locate duplicate information in the form of redundant constants contained in a constant pool. The duplicate constant is placed in a separate shared table, and all occurrences of the constant are removed from the respective constant pools of the individual class files. During pre-processing, memory allocation requirements are determined for each class file, and used to determine a total allocation requirement for the set of class files. The shared table, the memory allocation requirements and the reduced class files are packaged as a unit in a multi-class file.

When a virtual machine wishes to load the classes in the multi-class file, the location of the multi-class file is determined and the multi-class file is downloaded from a server, if needed. The memory allocation information in the multi-class file is used by the virtual machine to allocate memory from the virtual machine's heap for the set of classes. The individual classes, with respective reduced constant pools, are loaded, along with the shared table, into the virtual machine. Constant resolution is carried out on demand on the respective reduced constant pools and the shared table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a Java network application environment.

FIG. 2 is a block diagram of an embodiment of a computer system capable of providing a suitable execution environment for an embodiment of the invention.

FIG. 3 is a block diagram of an embodiment of a class file format.

FIG. 4 is a flow diagram of a class file pre-processing method in accordance with an embodiment of the invention.

FIG. 5 is a block diagram of an multi-class file format in accordance with an embodiment of the invention.

FIG. 6 is a block diagram of the runtime data areas of a virtual machine in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a method and apparatus for pre-processing and packaging class files. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the invention.

Embodiment of Computer Execution Environment (Hardware)

An embodiment of the invention can be implemented as computer software in the form of computer readable program code executed on a general purpose computer such as computer 200 illustrated in FIG. 2, or in the form of bytecode class files executable by a virtual machine running on such a computer. A keyboard 210 and mouse 211 are coupled to a bi-directional system bus 218. The keyboard and mouse are for introducing user input to the computer system and communicating that user input to central processing unit (CPU) 213. Other suitable input devices may be used in addition to, or in place of, the mouse 211 and keyboard 210. I/O (input/output) unit 219 coupled to bi-directional system bus 218 represents such I/O elements as a printer, A/V (audio/video) I/O, etc.

Computer 200 includes a video memory 214, main memory 215 and mass storage 212, all coupled to bidirectional system bus 218 along with keyboard 210, mouse 211 and CPU 213. The mass storage 212 may include both fixed and removable media, such as magnetic, optical or magnetic optical storage systems or any other available mass storage technology. Bus 218 may contain, for example, thirty-two address lines for addressing video memory 214 or main memory 215. The system bus 218 also includes, for example, a 32-bit data bus for transferring data between and among the components, such as CPU 213, main memory 215, video memory 214 and mass storage 212. Alternatively, multiplex data/address lines may be used instead of separate data and address lines.

In one embodiment of the invention, the CPU 213 is a microprocessor manufactured by Motorola.RTM., such as the 680X0 processor or a microprocessor manufactured by Intel.RTM., such as the 80X86, or Pentium.RTM. processor, or a SPARC.RTM. microprocessor from Sun Microsystems.RTM.. However, any other suitable microprocessor or microcomputer may be utilized. Main memory 215 is comprised of dynamic random access memory (DRAM). Video memory 214 is a dual-ported video random access memory. One port of the video memory 214 is coupled to video amplifier 216. The video amplifier 216 is used to drive the cathode ray tube (CRT) raster monitor 217. Video amplifier 216 is well known in the art and may be implemented by any suitable apparatus. This circuitry converts pixel data stored in video memory 214 to a raster signal suitable for use by monitor 217. Monitor 217 is a type of monitor suitable for displaying graphic images.

Computer 200 may also include a communication interface 220 coupled to bus 218. Communication interface 220 provides a two-way data communication coupling via a network link 221 to a local network 222. For example, if communication interface 220 is an integrated services digital network (ISDN) card or a modem, communication interface 220 provides a data communication connection to the corresponding type of telephone line, which comprises part of network link 221. If communication interface 220 is a local area network (LAN) card, communication interface 220 provides a data communication connection via network link 221 to a compatible LAN. Wireless links are also possible. In any such implementation, communication interface 220 sends and receives electrical, electromagnetic or optical signals which carry digital data streams representing various types of information.

Network link 221 typically provides data communication through one or more networks to other data devices. For example, network link 221 may provide a connection through local network 222 to host computer 223 or to data equipment operated by an Internet Service Provider (ISP) 224. ISP 224 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the "Internet" 225. Local network 222 and Internet 225 both use electrical, electromagnetic or optical signals which carry digital data streams. The signals through the various networks and the signals on network link 221 and through communication interface 220, which carry the digital data to and from computer 200, are exemplary forms of carrier waves transporting the information.

Computer 200 can send messages and receive data, including program code, through the network(s), network link 221, and communication interface 220. In the Internet example, server 226 might transmit a requested code for an application program through Internet 225, ISP 224, local network 222 and communication interface 220. In accord with the invention, one such downloaded application is the apparatus for pre-processing and packaging class files described herein.

The received code may be executed by CPU 213 as it is received, and/or stored in mass storage 212, or other non-volatile storage for later execution. In this manner, computer 200 may obtain application code in the form of a carrier wave.

The computer systems described above are for purposes of example only. An embodiment of the invention may be implemented in any type of computer system or programming or processing environment.

Class File Structure

Embodiments of the invention can be better understood with reference to aspects of the class file format. Description is provided below of the Java class file format. Also, enclosed as Section A of this specification are Chapter 4, "The class File Format," and Chapter 5, "Constant Pool Resolution," of The Java Virtual Machine Specification, by Tim Lindholm and Frank Yellin, published by Addison-Wesley in September 1996, .COPYRGT.Sun Microsystems, Inc.

The Java class file consists of a stream of 8-bit bytes, with 16-bit, 32-bit and 64-bit structures constructed from consecutive 8-bit bytes. A single class or interface file structure is contained in the class file. This class file structure appears as follows:

    ______________________________________
    ClassFile  {
    u4 magic;
    u2 minor.sub.-- version;
    u2 major.sub.-- version;
    u2 constant.sub.-- pool.sub.-- count;
    cp.sub.-- info constant.sub.-- pool[constant.sub.-- pool.sub.-- count-1];
    u2 access.sub.-- flags;
    u2 this.sub.-- class;
    u2 super.sub.-- class;
    u2 interfaces.sub.-- count;
    u2 interfaces[interfaces.sub.-- count];
    u2 fields.sub.-- count;
    field.sub.-- info fields[fields.sub.-- count];
    u2 methods.sub.-- count;
    method.sub.-- info methods[methods.sub.-- count];
    u2 attributes.sub.-- count;
    attribute.sub.-- info attributes[attributes.sub.-- count];
    ______________________________________

    ______________________________________
             cp.sub.-- info  {
               u1 tag;
               u1 info[ ];
             }
    ______________________________________

    ______________________________________
    MclassFile  {
    u2 shared.sub.-- pool.sub.-- count;
    cp.sub.-- info shared.sub.-- pool[shared.sub.-- pool.sub.-- count-1];
    u2 mem.sub.-- alloc.sub.-- req;
    u2 classfile.sub.-- count;
    ClassFile classfiles[classfile.sub.-- count];
    ______________________________________

    ______________________________________
    ClassFile  {
    u4 magic;
    u2 minor.sub.-- version;
    u2 major.sub.-- version;
    u2 constant.sub.-- pool.sub.-- count;
    cp.sub.-- info constant.sub.-- pool[constant.sub.-- pool.sub.-- count-1];
    u2 access.sub.-- flags;
    u2 this.sub.-- class;
    u2 super.sub.-- class;
    u2 interfaces.sub.-- count;
    u2 interfaces[interfaces.sub.-- count]
    u2 fields.sub.-- count;
    field.sub.-- info fields[fields.sub.-- count]
    u2 methods.sub.-- count;
    method.sub.-- info methods[methods.sub.-- count];
    u2 attributes.sub.-- count;
    attribute.sub.-- info attributes [attributes.sub.-- count];
    ______________________________________

    __________________________________________________________________________
    Flag Name
             Value
                 Meaning               Used By
    __________________________________________________________________________
    ACC.sub.-- PUBLIC
             0x0001
                 Is public; may be accessed from outside its
                                       Class, interface
    ACC.sub.-- FINAL
             0x0010
                 Is final; no subclasses allowed.
                                       Class
    ACC.sub.-- SUPER
             0x0020
                 Treat superclass methods specially in invokespecial.
                                       Class, interface
    ACC.sub.-- INTERFACE
             0x0200
                 Is an interface.      Interface
    ACC.sub.-- ABSTRACT
             0x0400
                 Is abstract; may not be instantiated.
                                       Class, interface
    __________________________________________________________________________

    ______________________________________
    B           byte     signed byte
    C           char     character
    D           double   double-precision IEEE 754 float
    F           float    single-precision IEEE 754 float
    I           int      integer
    J           long     long integer
    L<classname>;
                . . .    an instance of the class
    S           short    signed short
    Z           boolean  true or false
    [           . . .    one array dimension
    ______________________________________

    ______________________________________
             cp.sub.-- info {
               u1 tag;
               u1 info [ ];
             }
    ______________________________________

    ______________________________________
    Constant Type        Value
    ______________________________________
    CONSTANT.sub.-- Class
                         7
    CONSTANT.sub.-- Fieldref
                         9
    CONSTANT.sub.-- Methodref
                         10
    CONSTANT.sub.-- InterfaceMethodref
                         11
    CONSTANT.sub.-- String
                         8
    CONSTANT.sub.-- Integer
                         3
    CONSTANT.sub.-- Float
                         4
    CONSTANT.sub.-- Long 5
    CONSTANT.sub.-- Double
                         6
    CONSTANT.sub.-- NameAndType
                         12
    CONSTANT.sub.-- Utf8 1
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- Class.sub.-- info {
                u1 tag;
                u2 name.sub.-- index;
             }
    ______________________________________

    ______________________________________
    CONSTANT.sub.-- Fieldref.sub.-- info {
      u1 tag;
      u2 class.sub.-- index;
      u2 name.sub.-- and.sub.-- type.sub.-- index;
    CONSTANT.sub.-- Methodref.sub.-- info {
      u1 tag;
      u2 class.sub.-- index;
      u2 name.sub.-- and.sub.-- type.sub.-- index;
    }
    CONSTANT.sub.-- InterfaceMethodref.sub.-- info {
      u1 tag;
      u2 class.sub.-- index;
      u2 name.sub.-- and.sub.-- type.sub.-- index;
    }
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- String.sub.-- info {
                u1 tag;
                u2 string.sub.-- index;
             }
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- Integer.sub.-- info {
               u1 tag;
               u4 bytes;
             }
             CONSTANT.sub.-- Float.sub.-- info {
               u1 tag;
               u4 bytes;
             }
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- Long.sub.-- info {
               u1 tag;
               u4 high.sub.-- bytes;
               u4 low.sub.-- bytes;
             }
             CONSTANT.sub.-- Double.sub.-- info {
               u1 tag;
               u4 high.sub.-- bytes;
               u4 low.sub.-- bytes;
             }
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- NameAndType.sub.-- info {
               u1 tag;
               u2 name.sub.-- index;
               u2 descriptor.sub.-- index;
             }
    ______________________________________

    ______________________________________
             CONSTANT.sub.-- Utf8.sub.-- info {
               u1 tag;
               u2 length;
               u1 bytes[length];
             }
    ______________________________________

    ______________________________________
             field.sub.-- info {
                u2 access.sub.-- flags
                u2 name.sub.-- index;
                u2 descriptor.sub.-- index;
                u2 attributes.sub.-- count;
                attribute.sub.-- info attributes[attributes.sub.-- count];
             }
    ______________________________________

    __________________________________________________________________________
    Flag Name Value
                  Meaning               Used By
    __________________________________________________________________________
    ACC.sub.-- PUBLIC
              0x0001
                  Is public; may be accessed from outside its
                                        Any field
    ACC.sub.-- PRIVATE
              0x0002
                  Is private; usable only within the defining
                                        Class field
    ACC.sub.-- PROTECTED
              0x0004
                  Is protected; may be accessed within subclasses.
                                        Class field
    ACC.sub.-- STATIC
              0x0008
                  Is static.            Any field
    ACC.sub.-- FINAL
              0x0010
                  Is final; no further overriding or assignment
                                        Any field
                  initialization.
    ACC.sub.-- VOLATILE
              0x0040
                  Is volatile; cannot be cached.
                                        Class field
    ACC.sub.-- TRANSIENT
              0x0080
                  Is transient; not written or read by a persistent
                                        Class field
                  object manager.
    __________________________________________________________________________

    ______________________________________
    method.sub.-- info {
    u2 access.sub.-- flags;
    u2 name.sub.-- index;
    u2 descriptor.sub.-- index;
    u2 attributes.sub.-- count;
    attribute.sub.-- info attributes [attributes.sub.-- count];
    ______________________________________

    __________________________________________________________________________
    Flag Name   Value
                    Meaning           Used By
    __________________________________________________________________________
    ACC.sub.-- PUBLIC
                0x0001
                    Is public; may be accessed from outside
                                      Any method
                    package.
    ACC.sub.-- PRIVATE
                0x0002
                    Is private; usable only within the defining
                                      Class/instance method
                    class.
    ACC.sub.-- PROTECTED
                0x0004
                    Is protected; may be accessed within
                                      Class/instance method
                    subclasses.
    ACC.sub.-- STATIC
                0x0008
                    Is static.        Class/instance method
    ACC.sub.-- FINAL
                0x0010
                    Is final; no overriding is allowed.
                                      Class/instance method
    ACC.sub.-- SYNCHRONIZED
                0x0020
                    Is synchronized; wrap use in monitor lock.
                                      Class/instance method
    ACC.sub.-- NATIVE
                0x0100
                    Is native; implemented in a language other
                                      Class/instance method
                    than Java.
    ACC.sub.-- ABSTRACT
                0x0400
                    Is abstract, no implementation is provided.
                                      Any method
    __________________________________________________________________________

    ______________________________________
    attribute.sub.-- info {
            u2 attribute.sub.-- name.sub.-- index;
            u4 attribute.sub.-- length;
            u1 info[attribute.sub.-- length];
    ______________________________________

    ______________________________________
    SourceFile.sub.-- attribute {
            u2 attribute.sub.-- name.sub.-- index;
            u4 attribute.sub.-- length;
            u2 sourcefile.sub.-- index;
           }
    ______________________________________

    ______________________________________
    ConstantValue.sub.-- attribute {
            u2 attribute.sub.-- name.sub.-- index;
            u4 attribute.sub.-- length;
            u2 constantvalue.sub.-- index;
    ______________________________________

    ______________________________________
    Field Type          Entry Type
    ______________________________________
    long                CONSTANT.sub.-- Long
    float               CONSTANT.sub.-- Float
    double              CONSTANT.sub.-- Double
    int, short, char, byte, boolean
                        CONSTANT.sub.-- Integer
    java.lang.String    CONSTANT.sub.-- String
    ______________________________________

    ______________________________________
    Code.sub.-- attribute {
    u2 attribute.sub.-- name.sub.-- index;
    u4 attribute.sub.-- length;
    u2 max.sub.-- stack
    u2 max.sub.-- locals;
    u4 code.sub.-- length;
    u1 code[code.sub.-- length];
    u2 exception.sub.-- table.sub.-- length;
    {        u2 start.sub.-- pc;
             u2 end.sub.-- pc;
             u2 handler.sub.-- pc;
             u2 catch.sub.-- type;
    }        exception.sub.-- table[exception.sub.-- table.sub.-- length];
    u2 attributes.sub.-- count;
    attribute.sub.-- info attributes[attributes.sub.-- count];
    ______________________________________

    ______________________________________
    Exceptions.sub.-- attribute {
    u2 attribute.sub.-- name.sub.-- index;
    u4 attribute.sub.-- length;
    u2 number.sub.-- of.sub.-- exceptions;
    u2 exception.sub.-- index.sub.-- table[number.sub.-- of.sub.-- exceptions]
    }
    ______________________________________

    ______________________________________
    LineNumberTable.sub.-- attributes {
    u2 attribute.sub.-- name.sub.-- index;
    u4 attribute.sub.-- length;
    u2 line.sub.-- number.sub.-- table.sub.-- length;
    {       u2 start.sub.-- pc;
            u2 line.sub.-- number
    }       line.sub.-- number.sub.-- table[line.sub.-- number.sub.--
            table.sub.-- length];
    ______________________________________

    ______________________________________
    LocalVariableTable.sub.-- attribute {
    u2 attribute.sub.-- name.sub.-- index;
    u4 attribute.sub.-- length;
    u2 local.sub.-- variable.sub.-- table.sub.-- length;
    {       u2 start.sub.-- pc;
            u2 length;
            u2 name.sub.-- index;
            u2 descriptor.sub.-- index;
            u2 index;
    }      local.sub.-- variable.sub.-- table [local.sub.-- variable.sub.--
           table.sub.-- length];
    ______________________________________