Is it possible (how) to get the name of a method reference at Runtime Java?

John :

I've been using a lot of method references and lambdas recently, and wanted to know at runtime if i could print to screen the source of the lambda ie its name, simply for debugging reasons. I figured it might be possible using reflection, by calling getClass() within getName(), but I couldn't find a method with which to find the original source reference's name.

I have a functional interface such as:

@FunctionalInterface
public interface FooInterface {
    // function etc etc irrelevant
    public void method();

    public default String getName() {
        // returns the name of the method reference which this is used to define
    }
}

then lets say i wish to test run the interface, and print the source of the functional interface to the screen.

public static void doStuff(FooInterface f) {
    // prints the lambda name that is used to create f
    System.out.println(f.getName());

    // runs the method itself
    f.method();
}

So that if i do this:

doStuff(Foo::aMethodReference);

it should print something like: "aMethodReference" to the screen, that way i can know, at runtime which methods are being run, in what order etc.

I'm quite doubtful that this is possible, considering that lambdas are not-quite-objects, but hey, i figured there could be a workaround. Furthermore, the eclipse debug tool just says its a lambda, without any other information, do lambda's retain any of this information? or is it all lost at Runtime?

Cheers. (I'm using JDK 11 if that makes any difference)

Federico Peralta Schaffner :

As you're saying that you only need this for debugging purposes, here is a trick (i.e. a dirty hack) that will allow you to do what you want.

First of all, your functional interface must be Serializable:

@FunctionalInterface
public interface FooInterface extends Serializable {

    void method();
}

Now, you can use this undocumented, internal-implementation-dependent and extremely risky code to print some information about the method reference targeted to your FooInterface functional interface:

@FunctionalInterface
public interface FooInterface extends Serializable {

    void method();

    default String getName() {
        try {
            Method writeReplace = this.getClass().getDeclaredMethod("writeReplace");
            writeReplace.setAccessible(true);
            SerializedLambda sl = (SerializedLambda) writeReplace.invoke(this);
            return sl.getImplClass() + "::" + sl.getImplMethodName();
        } catch (Exception e) {
            return null;
        }
    }
}

When you call this method:

doStuff(Foo::aMethodReference);

You'll see the following output:

package/to/the/class/Foo::aMethodReference

Note 1: I've seen this approach in this article by Peter Lawrey.

Note 2: I've tested this with openjdk version "11" 2018-09-25 and also with java version "1.8.0_192".