How can I intercept a method call to a library class in Java or Android?

Question

I have a library class that is a singleton and does NOT implement any interface (So I'm assuming I cannot use dynamic proxies). But, I need the same functionality as the dynamic proxy. I need to hijack the call, log some data and pass on the call to the library as is.

Is it possible to do this without having to use AspectJ or Spring AOP? (I'm not sure how to use these frameworks, but I will have to look into it if there is no other way).

Answer 1

You can provide your own implementation of the same class, with the same name and package, and try to put it into classpath first. The calling code with such classpath will pick the intercepting class first, and the intercepting class can call others classes of the actual package to provide its actual functionality.

Best would be to remove the overridden class from the library .jar with archive tool.

Answer 2

You could change the import statements in the classes that call f() so that they see a different class that implements f(). The implementation of that class would do the logging and call the real f().

Still requires some editing, but less than changing every call.

BTW: Depending on the size of the project, you may want to consider making wrappers to that "black box" anyway, if this type of requirement will be ongoing.

Answer 3

Like I started in my comment wrapper all the classes you implement from the external library. Then call the external library from your own classes this way you can log in the function(s) you want. If you use the same function name then you don't have to change what you call you only have to change your import(s). Most IDEs provide mass name replace so it shouldn't be too big of a burden it will be tedious however.