Byteman is a tool which simplifies tracing and testing of Java programs.
Byteman allows you to insert extra Java code into your application,
either as it is loaded during JVM startup or even after it has already
started running. The injected code is allowed to access any of your data
and call any application methods, including where they are private.
You can inject code almost anywhere you want and there is no need to
prepare the original source code in advance nor do you have to recompile,
repackage or redeploy your application. In fact you can remove injected
code and reinstall different code while the application continues to execute.