Question

I am currently working on an assignment that deals with RMI. Essentially, it is just a chatroom. The problem that I am running into, though, is that I want to be able to get a username variable from my client, but that variable is not part of its remote interface:

public interface MulticastClient extends Remote {

    public void deliver( MulticastClient sender, String channel, 
        Serializable message) throws RemoteException;
}

From what I understand, since sender is really a remote object, I can only access the methods and variables defined in the interface. Since this is a homework assignment I cannot change the interface (it was given to us).

Some code from my MulticastClient implementation:

public class ChatClient implements MulticastClient {

       ...
       private String username;

    @Override
    public void deliver(MulticastClient sender, String channel,
        Serializable message) throws RemoteException {

    String senderName = ((ChatClient)sender).getUsername();
    System.out.println("\r" + senderName + ": " + message.toString());
    }

    public String getUsername() {
        return username;
    }

    ...

    public static void main(String[] args) throws Exception{
            ...

    ChatClient client = new ChatClient();
    MulticastClient stub = (MulticastClient)UnicastRemoteObject.exportObject(client, 0);

    Registry reg = LocateRegistry.getRegistry(hostname, port);
    MulticastService server = (MulticastService)reg.lookup(SERVER_NAME);

            ...
    }
}

When I attempt to user the deliver method, as it is now, the error I get is:

Exception in thread "Thread-2" java.lang.ClassCastException: $Proxy0 cannot
be cast to csci4401.mc.ChatClient

I know that the cast in deliver() is causing the problem, but I am not sure what the correct way to do this is. Is there another way that I can get the ChatClient’s username?

Answer 1

What you can access remotely is defined by the remote interface, by definition. What you have at the client isn’t the remote object itself, it is a proxy for it that implements the same remote interface. Hence your exception.