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

I've been troubled by similar issues as well when using MethodHandle.Lookup in JDK 8 - 10, which behave differently. I've blogged about the correct solution here in detail.

This approach works in Java 8

In Java 8, the ideal approach uses a hack that accesses a package-private constructor from Lookup:

import java.lang.invoke.MethodHandles.Lookup;
import java.lang.reflect.Constructor;
import java.lang.reflect.Proxy;

interface Duck {
    default void quack() {
        System.out.println("Quack");
    }
}

public class ProxyDemo {
    public static void main(String[] a) {
        Duck duck = (Duck) Proxy.newProxyInstance(
            Thread.currentThread().getContextClassLoader(),
            new Class[] { Duck.class },
            (proxy, method, args) -> {
                Constructor constructor = Lookup.class
                    .getDeclaredConstructor(Class.class);
                constructor.setAccessible(true);
                constructor.newInstance(Duck.class)
                    .in(Duck.class)
                    .unreflectSpecial(method, Duck.class)
                    .bindTo(proxy)
                    .invokeWithArguments(args);
                return null;
            }
        );

        duck.quack();
    }
}

This is the only approach that works with both private-accessible and private-inaccessible interfaces. However, the above approach does illegal reflective access to JDK internals, which will no longer work in a future JDK version, or if --illegal-access=deny is specified on the JVM.

This approach works on Java 9 and 10, but not 8

import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.reflect.Proxy;

interface Duck {
    default void quack() {
        System.out.println("Quack");
    }
}

public class ProxyDemo {
    public static void main(String[] a) {
        Duck duck = (Duck) Proxy.newProxyInstance(
            Thread.currentThread().getContextClassLoader(),
            new Class[] { Duck.class },
            (proxy, method, args) -> {
                MethodHandles.lookup()
                    .findSpecial( 
                         Duck.class, 
                         "quack",  
                         MethodType.methodType(void.class, new Class[0]),  
                         Duck.class)
                    .bindTo(proxy)
                    .invokeWithArguments(args);
                return null;
            }
        );

        duck.quack();
    }
}

Solution

Simply implement both of the above solutions and check if your code is running on JDK 8 or on a later JDK and you'll be fine. Until you're not :)