bcel

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.