The Byte Code Engineering Library (Apache Commons BCEL) is intended to give users a convenient way to analyze, create, and manipulate (binary) Java class files. 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 written 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.