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The Structure of the Java Virtual Machine

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This excerpt specifies an abstract machine and covers the class file format, data types, primitive types and values, reference types and values, run-time data areas, frames, representation of objects, floating-point arithmetic, special methods, exceptions, instruction set summary, class libraries, and private implementation.

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This document specifies an abstract machine. It does not describe any particular implementation of the Java Virtual Machine.

To implement the Java Virtual Machine correctly, you need only be able to read the class file format and correctly perform the operations specified therein. Implementation details that are not part of the Java Virtual Machine’s specification would unnecessarily constrain the creativity of implementors. For example, the memory layout of run-time data areas, the garbage-collection algorithm used, and any internal optimization of the Java Virtual Machine instructions (for example, translating them into machine code) are left to the discretion of the implementor.

All references to Unicode in this specification are given with respect to The Unicode Standard, Version 6.0.0, available at http://www.unicode.org/.

2.1. The class File Format

Compiled code to be executed by the Java Virtual Machine is represented using a hardware- and operating system-independent binary format, typically (but not necessarily) stored in a file, known as the class file format. The class file format precisely defines the representation of a class or interface, including details such as byte ordering that might be taken for granted in a platform-specific object file format.

Chapter 4, “The class File Format”, covers the class file format in detail.

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