A Java class file library also contains the popular Java byte code viewer. [Open source, GPL]

Compiler front-end for Java, supports static analysis of programs, parses source code and class files, builds full abstract syntax tree from source code files, enriched with name and type analysis information. [Open Source, BSD License]

Implementation of an API for reading, writing, and modifying class files in an object-oriented fashion. Can be used to create (dis)assemblers, (de)compilers, and obfuscators. [Open Source, LGPL]

Contains classes to generate, read, write, and print Java bytecode (.class) files. Part of the Kawa project. [Open Source, GPL]

Contains a Class File API to simplify the creation (and modification!) of Java class files - high-level methods are provided in order to avoid having to write bytecode directly. [Open Source, BSD-like]

A Java bytecode/XML intreroperability framework. Converts a .class file to a XML file. [Open source, GPL]