Javassist (Java Programming Assistant) makes Java bytecode manipulation
simple. 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 when the JVM loads it. Unlike other similar bytecode editors,
Javassist provides two levels of API: source level and bytecode level.
If the users use the source-level API, they can edit a class file
without knowledge of the specifications of the Java bytecode. The whole
API is designed with only the vocabulary of the Java language. You can
even specify inserted bytecode in the form of source text; Javassist
compiles it on the fly. On the other hand, the bytecode-level API
allows the users to directly edit a class file as other editors.