Java reflection: implement interface and pass to another class

Question

I'm not sure if this is possible with Java, but I'm trying to implement an interface that's unavailable at compile time** and pass it to another class as an object of that interface. Let's say I have an interface like:

public interface MyInterface {
    void onReceive(int i);
}

and another class like:

public void MyClass {
    ArrayList<MyInterface> listenerList = new ArrayList<MyInterface>();

    public void add(MyInterface m) {
        listenerList.add(m);
    }
}

If they were available at compile time, I would be using them like:

blah = new MyInterface() {
    public void onReceive(int i) {
        System.out.println("BLAH");
    }
}

MyClass mc = new MyClass();
myClass.add(blah);

I'm wondering if there is a way to write code that does the same as above if the first two classes are only available at runtime.

Thanks in advance!

**I'm trying to use a framework library from Android's ROM, but it is in dalvik bytecode so I can't use it for compilation.

UPDATE: Here's some sample code I used to test the solution:

File a/IIMSListener.java

// Excerpt from decompiled class

public interface IIMSListener
{
    void onReceive(int p0, int p1/*, IMSParameter p2*/);
}

File a/IMSRemoteListenerStub.java

// Excerpt from decompiled class

import java.util.concurrent.*;
import java.util.*;

public class IMSRemoteListenerStub
{
    public List<IIMSListener> mListenerList = new CopyOnWriteArrayList<IIMSListener>();

    public boolean addListener(final IIMSListener iimsListener) {
        if (iimsListener != null && !this.mListenerList.contains(iimsListener)) {
            this.mListenerList.add(iimsListener);
            return true;
        }
        return false;
    }

    public boolean removeListener(final IIMSListener iimsListener) {
        if (iimsListener != null && this.mListenerList.contains(iimsListener)) {
            this.mListenerList.remove(iimsListener);
            return true;
        }
        return false;
    }
}

File b/test.java

import java.lang.reflect.; import java.util.;

public class test {
  public static void main(String[] args) throws IllegalAccessException,
                                                IllegalArgumentException,
                                                InvocationTargetException,
                                                NoSuchMethodException,
                                                SecurityException,
                                                ClassNotFoundException {

    // Implement interface
    Class<?> IIMSListener = Class.forName("IIMSListener");

    Object listenerInstance = Proxy.newProxyInstance(IIMSListener.getClassLoader(), new Class<?>[]{IIMSListener}, new InvocationHandler() {
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if(method.getName().equals("onReceive")){
          System.out.println("ARGS: " + (Integer)args[0] + ", " + (Integer)args[1]);
          return 1;
        }
        else return -1;
      }
    }); 

    // Test
    Method onReceive = listenerInstance.getClass().getDeclaredMethod("onReceive", new Class[] { int.class, int.class });
    onReceive.invoke(listenerInstance, new Object[] { 1, 2 });

    try {
      // Pass to another class
      Class IMSRemoteListenerStub = Class.forName("IMSRemoteListenerStub");
      Constructor ctor = IMSRemoteListenerStub.getConstructor();
      Object stubInstance = ctor.newInstance(new Object[] {});
      Method addListener = stubInstance.getClass().getDeclaredMethod("addListener", new Class[] { IIMSListener });
      addListener.invoke(stubInstance, new Object[] { listenerInstance });

      // Test
      Field mListenerList = IMSRemoteListenerStub.getField("mListenerList");
      List<?> list = (List<?>)mListenerList.get(stubInstance);
      onReceive.invoke(list.get(0), new Object[] { 3, 4 });
    }
    catch (InstantiationException e) {}
    catch (NoSuchFieldException e) {}
  }
}

Execution:

$ cd b
$ CLASSPATH=".:../a" java test
ARGS: 1, 2
ARGS: 3, 4

Is it always going to be same interface or interface is going to change? — Narendra Pathai
– Narendra Pathai, Commented Oct 18, 2013 at 1:40
If you don't know the types at compile time, you will have to use reflection to get a list of constructors and methods. You'll then need to find a way to resolve any arguments so that you can invoke any of the methods or constructors. All in all, not a simple task. You can use JDK proxies to generate interface instances with an InvocationHandler, but it won't be much use without the a variable of that type or reflection. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Oct 18, 2013 at 1:43

Narendra Pathai · Accepted Answer · 2013-10-19 03:53:13Z

12

If it is going to be same interface then use Dynamic Proxies

//Loading the class at runtime
public static void main(String[] args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException, ClassNotFoundException {
    Class<?> someInterface = Class.forName("SomeInterface");

    Object instance = Proxy.newProxyInstance(someInterface.getClassLoader(), new Class<?>[]{someInterface}, new InvocationHandler() {

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

            //Handle the invocations
            if(method.getName().equals("someMethod")){
                return 1;
            }
            else return -1;
        }
    }); 
    System.out.println(instance.getClass().getDeclaredMethod("someMethod", (Class<?>[])null).invoke(instance, new Object[]{}));
}

edited Oct 19, 2013 at 3:53

answered Oct 18, 2013 at 1:43

Narendra Pathai

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5 Comments

Andrew Gunnerson Over a year ago

Thanks for the answer! The interface never changes. The one thing I'm wondering is: if SomeInterface is not available at compile time, how can it be used as a type for the line: SomeInterface instance = ...?

Narendra Pathai Over a year ago

@ChenXiao-Long Ohh yeah that was an error. I have edited the answer to totally use reflection. Take a look. Nice catch though.

Narendra Pathai Over a year ago

But just a general wondering how will you plug in the classes that you are loading and whose interface you dont know. I mean you will have to use reflection all the way.

Andrew Gunnerson Over a year ago

To give a little more background, I'm trying to make an Android IMS protocol library from the manufacturer firmware work with AOSP. I can decompile the library to see the classes and interfaces to see what functions I'm dealing with. EDIT: By the way, your solution works perfectly! I'll update my question with some sample code I tested.

Narendra Pathai Over a year ago

So in that case this can help.

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