How to get the MethodInfo of a Java 8 method reference?

Please have a look at the following code:

Method methodInfo = MyClass.class.getMethod("myMethod");

This works, but the method name is passed as a string, so this will compile even if myMethod does not exist.

On the other hand, Java 8 introduces a method reference feature. It is checked at compile time. It is possible to use this feature to get method info?

printMethodName(MyClass::myMethod);

Full example:

@FunctionalInterface
private interface Action {

    void invoke();
}

private static class MyClass {

    public static void myMethod() {
    }
}

private static void printMethodName(Action action) {
}

public static void main(String[] args) throws NoSuchMethodException {
    // This works, but method name is passed as a string, so this will compile
    // even if myMethod does not exist
    Method methodInfo = MyClass.class.getMethod("myMethod");

    // Here we pass reference to a method. It is somehow possible to
    // obtain java.lang.reflect.Method for myMethod inside printMethodName?
    printMethodName(MyClass::myMethod);
}

In other words I would like to have a code which is the equivalent of the following C# code:

    private static class InnerClass
    {
        public static void MyMethod()
        {
            Console.WriteLine("Hello");
        }
    }

    static void PrintMethodName(Action action)
    {
        // Can I get java.lang.reflect.Method in the same way?
        MethodInfo methodInfo = action.GetMethodInfo();
    }

    static void Main()
    {
        PrintMethodName(InnerClass.MyMethod);
    }

No, there is no reliable, supported way to do this. You assign a method reference to an instance of a functional interface, but that instance is cooked up by LambdaMetaFactory, and there is no way to drill into it to find the method you originally bound to.

Lambdas and method references in Java work quite differently than delegates in C#. For some interesting background, read up on invokedynamic.

Other answers and comments here show that it may currently be possible to retrieve the bound method with some additional work, but make sure you understand the caveats.

In my case I was looking for a way to get rid of this in unit tests:

Point p = getAPoint();
assertEquals(p.getX(), 4, "x");
assertEquals(p.getY(), 6, "x");

As you can see someone is testing Method getAPoint and checks that the coordinates are as expected, but in the description of each assert was copied and is not in sync with what is checked. Better would be to write this only once.

From the ideas by @ddan I built a proxy solution using Mockito:

private<T> void assertPropertyEqual(final T object, final Function<T, ?> getter, final Object expected) {
    final String methodName = getMethodName(object.getClass(), getter);
    assertEquals(getter.apply(object), expected, methodName);
}

@SuppressWarnings("unchecked")
private<T> String getMethodName(final Class<?> clazz, final Function<T, ?> getter) {
    final Method[] method = new Method[1];
    getter.apply((T)Mockito.mock(clazz, Mockito.withSettings().invocationListeners(methodInvocationReport -> {
        method[0] = ((InvocationOnMock) methodInvocationReport.getInvocation()).getMethod();
    })));
    return method[0].getName();
}

No I can simply use

assertPropertyEqual(p, Point::getX, 4);
assertPropertyEqual(p, Point::getY, 6);

and the description of the assert is guaranteed to be in sync with the code.

Downside:

Will be slightly slower than above
Needs Mockito to work
Hardly useful to anything but the usecase above.

However it does show a way how it could be done.

Though I haven't tried it myself, I think the answer is "no," since a method reference is semantically the same as a lambda.

How to get the MethodInfo of a Java 8 method reference?

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