How do I intercept a method invocation with standard java features (no AspectJ etc)?

Question

I want to intercept all method invocations to some class MyClass to be able to react on some setter-invocations.

I tried to use dynamic proxies, but as far as I know, this only works for classes implementing some interface. But MyClass does not have such an interface.

Is there any other way, besides implementing a wrapper class, that delegates all invocations to a member, which is an instance of the MyClass or besided using AOP?

Answer 1

As you note, you cannot use JDK dynamic proxies (no interface), but using Spring and CGLIB (JAR included with Spring), you can do the following:

public class Foo {     public void setBar()     {         throw new UnsupportedOperationException("should not go here");     }      public void redirected()     {         System.out.println("Yiha");     } }  Foo foo = new Foo(); ProxyFactory pf = new ProxyFactory(foo);  pf.addAdvice(new MethodInterceptor() {     public Object invoke(MethodInvocation mi) throws Throwable     {         if (mi.getMethod().getName().startsWith("set"))         {             Method redirect = mi.getThis().getClass().getMethod("redirected");             redirect.invoke(mi.getThis());         }         return null;     } });  Foo proxy = (Foo) pf.getProxy(); proxy.setBar(); // prints "Yiha" 

Answer 2

If you are prepared to do something really ugly, have a look at:

http://docs.oracle.com/javase/7/docs/technotes/guides/jpda/

Basically the debugger interface ought to allow you to attach like a debugger, and hence intercept calls. Bear in mind I think this is a really bad idea, but you asked if it was possible.