how to access instance that 'owns' a method in java?

in java, is it possible to access the instance to which a method belongs, given only the method?

for example:

public class ClassA {
    private 开发者_JAVA技巧ClassB instanceB = new ClassB();
    // ...
    private void sendMethod () {
        instanceB.receiveMethod(foo);
    }
    public void foo () {}
}

public class ClassB {
    public void receiveMethod (Method method) {
        Object o = foo.getInstanceOwner();  // just made that part up...
    }
}

my feeling is that methods belong to classes, not instances of a class, so the answer is no, but maybe there's some sneaky reflection technique i don't know about. i could always pass 'this' along with method foo, but that seems like extra baggage.

Taken from

A Method provides information about, and access to, a single method on a class or interface. The reflected method may be a class method or an instance method (including an abstract method).

A Method permits widening conversions to occur when matching the actual parameters to invoke with the underlying method's formal parameters, but it throws an IllegalArgumentException if a narrowing conversion would occur.

You can call Method#invoke but you will need the instance of the object you want to call the method on, from the method doc:

Invokes the underlying method represented by this Method object, on the specified object with the specified parameters. Individual parameters are automatically unwrapped to match primitive formal parameters, and both primitive and reference parameters are subject to method invocation conversions as necessary. If the underlying method is static, then the specified obj argument is ignored. It may be null.

If the number of formal parameters required by the underlying method is 0, the supplied args array may be of length 0 or null.

If the underlying method is an instance method, it is invoked using dynamic method lookup as documented in The Java Language Specification, Second Edition, section 15.12.4.4; in particular, overriding based on the runtime type of the target object will occur.

If the underlying method is static, the class that declared the method is initialized if it has not already been initialized.

If the method completes normally, the value it returns is returned to the caller of invoke; if the value has a primitive type, it is first appropriately wrapped in an object. However, if the value has the type of an array of a primitive type, the elements of the array are not wrapped in objects; in other words, an array of primitive type is returned. If the underlying method return type is void, the invocation returns null.

So the TL:DR is unless you have the actual object you want you call the method on, it is not possible.

public class ClassA {
    private ClassB instanceB = new ClassB();
    // ...
    private void sendMethod () {
        Method m = ClassA.class.getMethod("foo", null);
        instanceB.receiveMethod(m);
    }
    public void foo () {}
} 

public class ClassB {
    public void receiveMethod (Method method) {
        Class c = method.getDeclaringClass();
    }

}

gives you the owning Class. An instance doesn't own methods.

You can do this, but the proper way in your example would be the use of an interface, because that seems to be what you want: You want to pass in an object that ClassB knows how to operate on.

   interface Callback {
      void foo();
   }

   public class ClassA implements Callback {...}

   public class ClassB {
     public void receiveMethod(Callback cb) {...}
   }

This is like asking:

"Given an apple from an Apple orchard, which tree owns this apple?"

The answer to which is:

"No idea, since all apple trees produce apples, it could belong to any tree".

... in other words - you must supply an instance from which the method will be called

EDIT

From one of your comments, I gather you are looking for an alternative of the Observer pattern. You say you don't like the messiness of the Observer pattern and that it is not "generic" enough for you.

I would argue that it is probably one of the least messiest patterns in existence, AND interfaces are by definition as generic as things get!

So, perhaps its an implementation problem you're having. Luckily, I have already posted on SO an Observer implementation in JAVA, to demonstrate how powerful and elegant it is.

Polymorphism and Interfaces in Java (can polymorphism be used to implement interfaces...why?)

In fact: reflection is messier than using an interface, since you can't guarantee at compile time that the type of Object you are invoking an instance of a Method on, even supports that method! (without some error checking code). Versus with interfaces, its not possible to even have that problem.