Question

So, given the following code:

public MyInterface getMyInterface() {
    return new MyInterface() {
        public SomethingElse getSomethingElse() {
           // ....
        }
    }
}

...
MyInterface obj = getMyInterface();

Is there some way to instrument a call to getSomethingElse() on that obj? To go in and do some bytecode modification or something?

I have production code in there that in a different situation (call it “design time”) I want to add some tracing/logging and such code for help in troubleshooting and analysis. Performance is critical for the production case so I want to leave it without the extra tracing/logging overhead. But in the design time situation, I want to have all the trace info.

Answer 1

Yes, it is possible to do what you’re asking, although there are definitely better ways to accomplish it – the most obvious would be to create a default implementation of MyInterface, and then a “tracing” subclass of it that extends and logs before invoking the superclass version.

If instrumentation is your only option, then when running at design time, you can start your project with a java agent in Java 5 or add a java agent to the classpath at runtime in Java 6. See the instrumentation documentation.

To instrument the class, you will probably want to use a tool like ASM. The steps would be something like this:

1. In your Agent class, implement java.lang.instrument.ClassFileTransformer .
2. In your agentmain() or premain() method, request to transform classes.
3. When you receive a call to the transform method, you can check if the class implements MyInterface by using Class.getInterfaces().
4. Optionally, you can check to see if its Class.getEnclosingClass() is the class in which you wrote/found this code.
5. If the Class passes these sanity checks, then create a ClassWriter that adds logging to the getSomethingElse() method. The ASMifier helps a lot when trying to figure out how to generate the code you want.

Then, in production, none of that code will exist. In development, you would add your Java Agent in your environment, which would enable your debugging.

Again, there are almost certainly better ways to do this, but there are good reasons to use instrumentation, and this is a mini-crash course in doing it.

Hope that helps,