Viva - Open Source Core Java Code Libraries

Byte Code Engineering Library (BCEL)
http://jakarta.apache.org/bcel
Toolkit that lets you analyze, create, and manipulate (binary) Java class files (those ending with .class and holding the Java bytecode) For example, you can create Java classes from scratch at run-time; licensed under Apache license; headed by Markus Dahm

ASM
http://asm.objectweb.org
Java bytecode toolkit that lets you dynamically generate stub classes or other proxy classes, directly in binary form, or that lets you dynamically modify classes at load time, that is, just before they get loaded into the Java runtime; licensed under the GNU Library GPL (LPGL); headed by Eric Bruneton

Javassist
http://www.javassist.org
Javassist (Java programming assistant) is a load-time reflective system for Java that is, a Java class library for editing bytecodes that lets you define new classes at runtime or modify class files before the Java runtime loads them. Unlike other Java bytecode toolkits Javassist provides source-level abstraction; you can modify a class file without detailed knowledge of the Java bytecode and you do not have to even write an inserted bytecode sequence; Javassist instead can compile a fragment of source text on line (for example, just a single statement); dual-licensed under Mozilla Public License (MPL) and GNU Library GPL (LGPL); headed by Shigeru Chiba

Code Generation Library (cgl)
http://cglib.sourceforge.net | http://sourceforge.net/projects/cglib
Toolkit that lets you extend Java classes and implements interfaces at runtime; licensed under Apache-style license

XDoclet
http://xdoclet.sourceforge.net | http://sourceforge.net/projects/xdoclet
XDoclet is a code generation engine that lets you use predefined Javadoc @tags or your very own for generating source code or docs or any other files (such as xml deployment descriptors and more) using templates and more; licensed under a BSD-style license

QDox
http://qdox.codehaus.org
QDox is a high speed, small footprint parser for extracting class/interface/method definitions from source files complete with JavaDoc @tags. QDox is designed for code generators or documentation tools; licensed under an Apache-style license

AspectWerkz
http://aspectwerkz.codehaus.org
AspectWerkz is a simple, dynamic, lightweight and high-speed AOP toolkit for Java that uses bytecode modification to weave your classes at runtime. AspectWerkz lets you define aspects using either XML files or runtime attributes; licensed under GNU Library GPL (LGPL); headed by Jonas Boner

Nanning
http://nanning.codehaus.org
Nanning Aspects is an Aspect Oriented Programming (AOP) toolkit for Java using dynamic proxies and aspects on plain old Java objects (POJOs); licensed under GNU Library GPL (LGPL); headed by Jon Tirsen

AspectJ
http://www.eclipse.org/aspectj
Aspect Orientend Programming (AOP) language for Java; licensed under Eclipse Public License (EPL)