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?

alicederyn · Accepted Answer · 2015-01-13 14:59:06Z

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, 2015 at 14:59

alicederyn

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answered Jan 11, 2013 at 11:52

Ian Roberts

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2 Comments

SkyWalker Over a year ago

then the OP question would be: how CGLIB does it in terms of pure Java :)

Ian Roberts Over a year ago

@GiovanniAzua it dynamically generates the bytecode for the proxy subclass using the ASM bytecode manipulation library.

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