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

Such objects can be read from an existing file, transformed by a
program (such as a class loader at runtime), and dumped to a file
again. An even more interesting application is the creation of classes
from scratch at runtime. The Byte Code Engineering Library (BCEL) may
also be useful if you want to learn about the Java Virtual Machine
(JVM) and the format of Java .class files.

BCEL is already being used successfully in several projects, such as
compilers, optimizers, obfuscators, code generators, and analysis
tools.

It contains a byte code verifier named JustIce, which usually gives you
much better information about what is wrong with your code than the
standard JVM message.