如何使用MethodHandles在实现Java 8接口的类上调用方法？

Question

public interface Foo {
    int add(int x, int y);
}

public class Bar implements Foo {
    int add(int x, int y) {
        return x+y;
    }
}

Now Imagine I have Foo.class and Bar.class in /usr/local

Below is the class that loads both Bar and Foo and want want to call the add method dynamically.

import java.io.File;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.net.URL;
import java.net.URLClassLoader;

public class Test {

    public static void main(String... args) throws Throwable {
        File classFile = new File("/usr/local");
        ClassLoader classLoader = new URLClassLoader(new URL[]{classFile.toURI().toURL()});
        Class fooClass = classLoader.loadClass("Foo");
        MethodHandle mh = MethodHandles
                            .lookup()
                            .findVirtual(fooClass, "add", MethodType.methodType(int.class, int.class, int.class));
        System.out.println((int)mh.invoke(2, 3));
    }
}

Below is the error I get

Exception in thread "main" java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle(Foo,int,int)int to (int,int)int

Questions:

1. How to use MethodHandles to invoke a method of a class that implements an interface?
2. Is there a way to avoid crazy casting in (int)mh.invoke(2, 3) ?

Answer 1

You forgot to include one more argument - an instance, on which the method will be invoked:

Class<?> barClass = classLoader.loadClass("Bar");
Object barInstance = barClass.getConstructor().newInstance();
System.out.println(mh.invoke(barInstance, 2, 3));

Also, the return type of the MethodHandle.invoke is Object , thus you can't avoid cast in this case.