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?

score 18 · Accepted Answer · edited Jan 13 '15 at 14:59

18

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" });

edited Jan 13 '15 at 14:59

Alice Purcell

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answered Jan 11 '13 at 11:52

Ian Roberts

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then the OP question would be: how CGLIB does it in terms of pure Java :) – SkyWalker Jan 11 '13 at 11:55
4

@GiovanniAzua it dynamically generates the bytecode for the proxy subclass using the ASM bytecode manipulation library. – Ian Roberts Jan 11 '13 at 12:05

creating Java Proxy instance for a class type?

1 Answers1