Java class file format : Java Glossary

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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 (Central Processing Unit) chip. Such chips do exist, but most of the time they are simulated with interpreters, JIT (Just In Time), Hotspots or AOT (Ahead Of Time) 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 (Java Development Kit) 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

JLS (Java Language Specification): class file format specification

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