The Byte Code Engineering Library (formerly known as JavaClass) is intended
to give users a convenient possibility to analyze, create, and manipulate
(binary) Java class files (those ending with .class). Classes are represented
by objects which 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, be transformed
by a program (e.g. a class loader at run-time) and dumped to a file again.
An even more interesting application is the creation of classes from scratch
at run-time. The Byte Code Engineering Library (BCEL) may be also 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, obsfuscators and analysis tools, the most
popular probably being the Xalan XSLT processor at Apache.