The Byte Code Engineering Library 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, code generators and analysis tools