How to explicitly invoke default method from a dynamic Proxy?

Question

Since Java 8 interfaces could have default methods. I know how to invoke the method explicitly from the implementing method, i.e. (see Explicitly calling a default method i

Answer 1

If all you have is an interface, and all you have access to is a class object is an interface that extends your base interface, and you want to call the default method without a real instance of a class that implements the interface, you can:

Object target = Proxy.newProxyInstance(classLoader,
      new Class[]{exampleInterface}, (Object p, Method m, Object[] a) -> null);

Create an instance of the interface, and then construct the MethodHandles.Lookup using reflection:

Constructor lookupConstructor = 
    MethodHandles.Lookup.class.getDeclaredConstructor(Class.class, Integer.TYPE);
if (!lookupConstructor.isAccessible()) {
    lookupConstructor.setAccessible(true);
}

And then use that lookupConstructor to create a new instance of your interface that will allow private access to invokespecial. Then invoke the method on the fake proxy target you made earlier.

lookupConstructor.newInstance(exampleInterface,
        MethodHandles.Lookup.PRIVATE)
        .unreflectSpecial(method, declaringClass)
        .bindTo(target)
        .invokeWithArguments(args);