creating Java Proxy instance for a class type?

Question

I have the following code that works for creating a Proxy instance for an interface type backed by an InvocationHandler implementation, however when I use a concrete class type it doesn't work and this is well known and documented in Proxy.newProxyInstance:

    // NOTE: does not work because SomeConcreteClass is not an interface type
    final ClassLoader myClassLoader = SomeConcreteClass.getClassLoader();
    SomeConcreteClass myProxy = (SomeConcreteClass) Proxy.newProxyInstance(myClassLoader, new Class[] {
        SomeConcreteClass.class }, new InvocationHandler() { /* TODO */ });        

However, if I recall correctly I have seen this use-case work in some mock frameworks where it is possible to mock a concrete class type e.g. EasyMock. Before checking up their source code can anyone indicate what needs to be done to proxy also concrete class types and not only interfaces?

Answer 1

JDK dynamic proxies only work with interfaces. If you want to create a proxy with a concrete superclass you need to use something like CGLIB instead.

Enhancer e = new Enhancer();
e.setClassLoader(myClassLoader);
e.setSuperclass(SomeConcreteClass.class);
e.setCallback(new MethodInterceptor() {
  public Object intercept(Object obj, Method method, Object[] args,
        MethodProxy proxy) throws Throwable {
    return proxy.invokeSuper(obj, args);
  }
});
// create proxy using SomeConcreteClass() no-arg constructor
SomeConcreteClass myProxy = (SomeConcreteClass)e.create();
// create proxy using SomeConcreteClass(String) constructor
SomeConcreteClass myProxy2 = (SomeConcreteClass)e.create(
    new Class<?>[] {String.class},
    new Object[] { "hello" });