Description:

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.