Java class file format : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
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Java class file format

Compiled Java is typically distributed in a very compact format called Class files. It is the assembler code for a virtual stack-oriented Java-friendly CPU chip. Such chips do exist, but most of the time they are simulated with interpreters, JIT, Hotspots or AOT compilation on the target machines which have quite different architectures.

Class files are usually compressed and bundled into jar files.

Each class file is marked with a class file format version, which lets you know which JDK it was compiled for. At offset 6 in the class file is a big-endian short that gives the major version number. You can display for the classes is in a jar with JarCheck.

Class File Versions
JDK	Major Version
6.0	50
5.0	49
1.4	48
1.3	47
1.2	46
1.1	45
1.0	44

Learning More

Sun’s VM (Virtual Machine) Specification: Specification

ANTLR: byte code parser
AOT
BCEL (Jakarta Byte Code Engineering Library)
Class File Format Table
Class.forName
ClassLoader
compiler
decompiler
disassembler
HotSpot
interpreter
jar
JASM: byte code assemblers
javac.exe
JIT
JVM
on the fly compose, compile and run
Ram.Jar

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