Java: Using a foreign class which conforms to my interface

Question

Hello!

Can I take a foreign class in Java which conforms to one of my custom interfaces and "cast" it to that interface?

Example:

public class YouCanNotChangeMe_IAmNotYours{
    public void doSomethingSpecial();
}

public interface MyInterface{
    void doSomethingSpecial();
}

Can I use that foreign class in places where my interface is expected?

Why do I need this at all?

I want to dynamically reload newer versions of some JAR at runtime, as I've mentioned here.

Now, the server knows all interfaces (knowing the former version of the JAR at compile time). The problem is, after loading classes from newer versions of that JAR at runtime, these "new" classes act as if they don't "know" the compile-time known interface.

Example: An old class, named HelloClass (known at compile time) does implement the interface HelloInterface, but the newly loaded class HelloClass does not implement that same interface!

Answer 1

You could extend the foreign class (assuming it's non-final) and make that class implement the interface.

Alternatively you could implement your own wrapper around that interface:

public class ForeignClassWrapper implements MyInterface {
  private final YouCanNotChangeMe_IAmNotYours delegate;

  public ForeignClassWrapper(YouCanNotChangeMe_IAmNotYours delegate) {
    this.delegate = delegate;
  }

  public void doSomethingSpecial() {
    delegate.doSomethingSpecial();
  }
}

More generally you could get the same effect using reflection (so you wouldn't need to hardcode YouCanNotChangeMe_IAmNotYours in your wrapper class).

Answer 2

Unless I'm mistaken, it sounds like you need to rethink your container and ClassLoader design. You're trying to achieve the same sort of functionality that your average servlet container supports, so it might be an idea to look at that for inspiration.

As to the specific problem: instance x of type A is only an instance of class A if x.getClass().getClassLoader() == A.class.getClassLoader(). So, even though two or more objects might be of class A, Java treats them as different types if they have different ClassLoaders.

It sounds like the interface you're trying to cast to should be in a common base (or the system) ClassLoader.