How to use a MethodHandle to invoke a private instance method?

Question

5.00/5 (1 vote)

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I have this class:

package com.gmail.inverseconduit.jchatexchange.assets;

import jchatexchange.util.User;

public class UserFactory implements jchatexchange.factory.Factory {

    private boolean cnu(User user) {
        [implementation dependant on #cnuz(User)]
    }

    private int cnuz(User user) {
        [my implementation]
    }

    @Override
    public User createNewUser(int id) {
        [my implementation]
    }

    // Other methods

}

I want to obtain a MethodHandle of each of the three methods in another class, so I used:

Java

class Baz {

    public static void main(String..$) {
        Lookup myLookup = MethodHandles.lookup();
        UserFactory foo = new UserFactory(); // test
    
        try {
            MethodType signature = MethodType.methodType(boolean.class, User.class);
            MethodHandle pmh = lookup.findVirtual(UserFactory.class, "cnu", signature);
            // ??? what now? I can't find any method in Java API that does what I need -
            // to actually invoke a private method with this.
    
            // I would have expected this to work:
            boolean cnuResult = pmh.bindTo(foo).invoke();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

I need Lookup.IMPL_LOOKUP, but there is no public way that allows it. What should I use in order for this to work?

Additional note: MethodHandles are added in Java 6, but I'm using Java 8, so any tricks related to Java 8 will be appreciated as well.

Posted 13-Dec-14 23:01pm

Unihedron

Updated 13-Dec-14 23:06pm

v4

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Thomas Daniels · Accepted Answer · 2014-12-13T23:53:00

You can use Lookup.unreflect[^]:

Java

public static void main(String..$) {
    Lookup myLookup = MethodHandles.lookup();
    UserFactory foo = new UserFactory(); // test
    User baz = new User();
    
    try {
        Method m = UserFactory.class.getDeclaredMethod("cnu", User.class);
        m.setAccessible(true);
        MethodHandle pmh = myLookup.unreflect(m);
        boolean b = (boolean)pmh.invoke(foo, baz);
           
    } catch (Exception e) {
        e.printStackTrace();
    } catch (Throwable e) {
      	e.printStackTrace();
    }
}