How to change method behaviour through reflection?

I have a a static method in some legacy code, which is called by multiple clients. I obviously have no options to override it, or change behaviour through dependency injection. I am not allowed to modify the existing class.

What I want to do now is change the behaviour (that method - with the same signature and return type) using reflection.

Is it possible ? If not, can any design pattern rescue me ?

Thanks !

EDIT : There is some confusion on what can I change/modify. I cannot change any existing class/method - but I can add more classes to the project. The best I can do with the existing classes is annotate them. This is all done to avoid breaking anything in the existing code - which means a complete round of testing for a big project.

EDIT 2 : java.lang.Instrumentation is not available for Android - or else it sounds like a good fit !

Sounds like a weird requirement...

Anyway, reflection does not allow you to change code behaviour, it can only explore current code, invoke methods and constuctors, change fields values, that kind of things.

If you want to actually change the behaviour of a method you would have to use a bytecode manipulation library such as ASM. But this will not be very easy, probably not a good idea...

Patterns that might help you :

• If the class is not final and you can modify the clients, extend the existing class and overload the method, with your desired behaviour. Edit : that would work only if the method were not static !
• Aspect programming : add interceptors to the method using AspectJ

Anyway, the most logical thing to do would be to find a way to modify the existing class, work-arounds will just make your code more complicated and harder to maintain.

Good luck.