BCEL
The Byte Code Engineering Library is intended to give users a convenient possibility to analyze, create, and manipulate (binary) Java class files (those ending with .class). Classes are represented by objects which contain all the symbolic information of the given class: methods, fields and byte code instructions, in particular.

Cglib
Cglib is a powerful, high performance and quality Code Generation Library, It is used to extend JAVA classes and implements interfaces at runtime.

Classfile Reader and Writer
This package makes it easy to read and write java classfiles. It doesn't, however, provide any help with displaying the contents of a classfile to the user (unless you count debug output), or disassembling the bytecodes.

Cojen
Dynamic Java bytecode generation and disassembly tools. A builder allows code to be generated more easily, instead of having to select exact instructions. Utility classes are provided which demonstrate the code generator and loader. Cojen is a fork of the Trove class file API.

Javassist
Javassist (Java programming assistant) is a load-time reflective system for Java. It is a class library for editing bytecodes in Java; it enables Java programs to define a new class at runtime and to modify a class file before the JVM loads it. Unlike other similar systems, Javassist provides source-level abstraction; programmers can modify a class file without detailed knowledge of the Java bytecode.

JBET
The Java Binary Enhancement Tool (JBET) is a general Java program analysis and manipulation tool. Existing class files can be disassembled, reassembled, or edited programmatically through the JBET API.

JClassLib
JClassLib bytecode is a viewer but it also contains a library that enables developers to read, modify and write Java class files and bytecode.

Jiapi
Jiapi is a tool which can be used to alter the normal class loading behavior. Instead of letting the class to be loaded as it is, Jiapi is used to manipulate classes' bytecode.

ObjectWeb ASM
ASM is a Java bytecode manipulation framework. It can be used to dynamically generate stub classes or other proxy classes, directly in binary form, or to dynamically modify classes at load time, i.e., just before they are loaded into the Java Virtual Machine.

Package gnu.bytecode
Contains classes to generate, read, write, and print Java bytecode (.class) files. Part of the Kawa project.