byteman

Byteman is a tool which makes it easy to trace, monitor and test the behaviour of Java application and JDK runtime code. It injects Java code into your application methods or into Java runtime methods without the need for you to recompile, repackage or even redeploy your application. Injection can be performed at JVM startup or after startup while the application is still running. Injected code can 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. You can even remove injected code and reinstall different changes while the application continues to execute.