Can't use custom ClassLoader with ObjectInputStream

Question

Hi all,
I want to transmit an object whose class is unknown to the receiver and whose class may not be loaded. Of course when the inputstream's readObject method tries to receive this class, it fails with a ClassNotFoundException. I have a custom FileClassLoader to dynamically load classes, this is tested and works.
I can dynamically load this class and prove that the class was loaded by calling class.newInstance() from the loader.loadClass method. But then when I try to read that same class from the input stream, it says ClassNotFoundException!
The code snippet looks like this:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FileClassLoader loader = new FileClassLoader(classFolder);
Class L = loader.loadClass(className); // no .class at end
System.out.println("New Instance "+L.newInstance().toString()); // works
ObjectInputStream eventStream = ... // stream from a socket
eventStream.readObject(); // throws class not found exception!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The only thing I can think is that the input stream only looks for classes loaded by the system classloader, could that be true? What other possibilities are there?
If this is more appropriate for another forum just let me know and I'll post it there.
Thanks in advance for any helpful hints anybody has!

Answer 1

I was thinking of something along the lines of
ObjectInputStream eventStream = (ObjectInputStream)Class.forName("java.io.ObjectInputStream", true, loader).newInstance();Of course, you would really have to get the proper Constructor for the class - and that just gets too long and stupid. Besides, I actually tried my suggestion, and it didn't work. I also tried setting the ContextClassLoader for the current thread (and creating a new thread with the ContextClassLoader set to "loader"). None of that works. Just setting the ContextClassLoader does not make the thread use it implicitly, it's apparently just a way to pass another ClassLoader to a thread, but it still needs to be used explicitly in a Class.forName call.
I did, however, try David's suggestions of overriding resolveClass in ObjectInputStream. That worked fine! Here's what I created:
import java.net.URLClassLoader;
import java.io.InputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamClass;
import java.io.StreamCorruptedException;
public class MyObjectInputStream extends java.io.ObjectInputStream
    URLClassLoader myLoader = null;
    public MyObjectInputStream(URLClassLoader newLoader, InputStream theStream) throws IOException, StreamCorruptedException
        super(theStream);
        myLoader = newLoader;
    protected Class resolveClass(ObjectStreamClass osc) throws IOException, ClassNotFoundException
        System.out.println("Hey, I'm in my resolveClass");
        Class theClass = null;
        try
            theClass = Class.forName(osc.getName(), true, myLoader);
        catch (Exception e)
            System.err.println("An Error in my ResolveClass:");
            e.printStackTrace();
        return theClass;
}You then just create your ObjectInputStream with
ObjectInputStream eventStream = new MyObjectInputStream(loader, theSocket.getInputStream());