BCEL

The Byte Code Engineering Library (BCEL) is a project sponsored by the Apache Foundation previously under their Jakarta charter to provide a simple API for decomposing, modifying, and recomposing binary Java classes (I.e. bytecode). The project was conceived and developed by Markus Dahm prior to officially being donated to the Apache Jakarta foundation on 27 October 2001.

BCEL provides a simple library that exposes the internal aggregate components of a given Java class through its API as object constructs (as opposed to the disassembly of the lower-level opcodes). These objects also expose operations for modifying the binary bytecode, as well as generating new bytecode (via injection of new code into the existing code, or through generation of new classes altogether.) The BCEL library has been used in several diverse applications, such as: