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.