Question

According to the documentation:

[java.lang.reflect.]Proxy provides static methods for
creating dynamic proxy classes and
instances, and it is also the
superclass of all dynamic proxy
classes created by those methods.

The newProxyMethod method (responsible for generating the dynamic proxies) has the following signature:

public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
                             throws IllegalArgumentException

Unfortunately, this prevents one from generating a dynamic proxy that extends a specific abstract class (rather than implementing specific interfaces). This makes sense, considering java.lang.reflect.Proxy is “the superclass of all dynamic proxies”, thereby preventing another class from being the superclass.

Therefore, are there any alternatives to java.lang.reflect.Proxy that can generate dynamic proxies that inherit from a specific abstract class, redirecting all calls to the abstract methods to the invocation handler?

For example, suppose I have an abstract class Dog:

public abstract class Dog {

    public void bark() {
        System.out.println("Woof!");
    }

    public abstract void fetch();

}

Is there a class that allows me to do the following?

Dog dog = SomeOtherProxy.newProxyInstance(classLoader, Dog.class, h);

dog.fetch(); // Will be handled by the invocation handler
dog.bark();  // Will NOT be handled by the invocation handler

Answer 1

It can be done using Javassist (see ProxyFactory) or CGLIB.

Adam’s example using Javassist:

I (Adam Paynter) wrote this code using Javassist:

ProxyFactory factory = new ProxyFactory();
factory.setSuperclass(Dog.class);
factory.setFilter(
    new MethodFilter() {
        @Override
        public boolean isHandled(Method method) {
            return Modifier.isAbstract(method.getModifiers());
        }
    }
);

MethodHandler handler = new MethodHandler() {
    @Override
    public Object invoke(Object self, Method thisMethod, Method proceed, Object[] args) throws Throwable {
        System.out.println("Handling " + thisMethod + " via the method handler");
        return null;
    }
};

Dog dog = (Dog) factory.create(new Class<?>[0], new Object[0], handler);
dog.bark();
dog.fetch();

Which produces this output:

Woof!
Handling public abstract void mock.Dog.fetch() via the method handler