The last week, i got a chance to experiment on couple of bytecode manipulation APIs/tools and thought i can share my learning here.
Javassist is one of bytecode manipulation APIs that helps you to manipulate bytecode and it is simpler to use. I wrote a simple method profiler that takes class files as input and instruments the bytecodes in each method. Added, method start time, complete time and measured the time spent in the method. One good thing about Javassist is that you can directly add code written in Java that compiles on the fly while instrumentation. Seems like bytecode instrumentation is bit easier in Javassist. With Javassist, you can also do on the fly instrumentation during classloading phase.
Retroweaver is another bytecode manipulation tool that back ports the code compiled by Java 5 to Java 1.4. If you have written classes (or a third party classes/libraries) in Java 5 and if your production system still runs with Java 1.4/1.3/1.2, you may want to check out this tool. It is really cool to use latest APIs/libraries in old JVM. Retrotranslator is a similar tool.
Bytecode engineering seems to be very interesting.
