Java Reflection and bytecode ...

I found a series of very useful articles about Java. Here is the address:http://www.ibm.com/developerworks/java/library/j-dyn0429/.
In this series, the author introduces classloading, bytecode, reflection, Class-transformation on-the-fly...

Javassist --- manipulation of bytecode
Introduction from official site:
"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. They 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). This ease of use is a unique feature of Javassit against other tools."
http://labs.jboss.com/javassist/
http://www.csg.is.titech.ac.jp/~chiba/javassist/