Casting to an abstract class from a different classloader

Question

I have a class Special that extends an abstract class Base. In my code I use a URLClassLoader to load the class Special and I then want to cast Special to Base. If I do this I get a ClassCastException because the classes are loaded from different classloaders. I can't have the URLClassLoader and the class that performs the cast extend a parent ClassLoader that knows about the Base class. What I want to be able to do is something like this:
URLClassLoader loader = new URLClassLoader(codebase, null);
Class baseClass = loader.loadClass(className);
Base baseObj = (Base)baseClass.newInstance();
I have seen some post that suggest I can achieve this using Reflection but I am not sure how to go about this. Any help would be appreciated.
Thanks
Jim.

Answer 1

Thanks for your help so far but I still can't do the casting, consider this example:
//Base.java
package classTest;
public interface Base
     public abstract void execute();
//ConcBase.java
package classTest;
public class ConcBase implements Base
     public void execute()
          System.out.println("execute in ConcBase called");
I compile these files and jar them into work.jar
I now have my application:
//Test.java
import java.net.*;
import java.io.*;
import classTest.*;
public class Test
public static void main(String[] args)
          Test t = new Test();
          t.test();
     public void test()
          try
               File file = new File("D:/Projects/classloadTest/work.jar");
               URL[] codebase = {file.toURL()};
               ClassLoader ccl = getClass().getClassLoader();
               ccl.loadClass("classTest.Base");
               URLClassLoader ucl = new URLClassLoader(codebase,ccl);
               Class conClass = ucl.loadClass("classTest.ConcBase");
               classTest.Base b = (classTest.Base)conClass.newInstance();
               b.execute();
          catch(Exception t)
               System.out.println("thowable caught");
               t.printStackTrace(System.out);
I compile this and run it with this command:
java -classpath D:\Projects\classloadTest\work.jar;. Test
This runs as I would expect, however I have set the parent class loader of my custom URLClassLoader to the one that does the cast, this means that Base and ConcBase are both being picked up by the application class loader as my custom class loader delegates to its parent. This is the current behaviour I have in my proper application and it is causing problems, I don't want the class that implements Base to delegate to any class on the main applications classpath. If I change the line:
URLClassLoader ucl = new URLClassLoader(codebase,ccl);
In Test.java to:
URLClassLoader ucl = new URLClassLoader(codebase,null);
I get a ClassCastException, this is because the class that does the cast (Test) loads Base from it's classpath and ConcBase is loaded from the URLClassLoader. After spending more time looking at this problem I don't think there is anyway to resolve but if anyone thinks there is please tell me.
Many thanks
Jim.