Proble with callbacks

Question

This is the usual case...
I am trying to create a client - server application, where the server will be able to perform call backs using methods of an exposed object at the client.
I saw the tutorial and what I did was this:
I created a "register" method at the remote object of the server that has a parameter that is a handle to the client remote object. I store all the handles in a Vector with IDs and stuff so I can Look them up and use them. The look up works fine.
The client remote object DOES NOT extend UnicastRemoteObject. Is that the correct approach?
The problem I'm having is that once the client registers, the server can perform a callback from within the same method (the method that the client uses to register). If the server attempts to perform a callback at a later time I get a NullPointerException ...
I triple checked my code in case I tamper with the handle values. That doesnt seem to happen though.
I read in an older post concerning the same matter that I should extend UnicastRemoteObject at the client. Is that true? Right now its pretty much the same with the tutorial.
Is it possible for the Garbage Collector to erase my handles?
Cheers
John

Answer 1

OK I ve changed the code to make it a bit less messy. Still I get the same errors.
So now I have a Remote interface at the server that accepts calls from the client :
import java.rmi.Remote;
import java.rmi.RemoteException;
import java.util.Vector;
public interface ClientRMICommandHandlerInterface extends Remote{
public User clientGetUserData(String sUserName) throws RemoteException;
public User clientRegisterNewUser(String sUID) throws RemoteException;
Vector clientConnectToServer(ClientReceiverInterface cri,User user) throws RemoteException;
boolean clientDisconnectFromServer(User user) throws RemoteException;
boolean clientSendMessage(User user,String sMessage,String sUIDdestination, int iMessageMethod) throws RemoteException;
boolean clientSetUserData(ClientReceiverInterface cri,User user) throws RemoteException;
boolean clientSetState(ClientReceiverInterface cri,User user) throws RemoteException;
boolean clientSetDefaultState (ClientReceiverInterface cri,User user) throws RemoteException;
User clientAddFriend(ClientReceiverInterface cri,User user,String sUID) throws RemoteException;
boolean clientRemoveFriend(ClientReceiverInterface cri,User user,String sUID) throws RemoteException;
The method clientRegisterNewUser(String sUID) does not accept the client reference to the client remote object ... Most of the other methods have an argument named ClientReceiverInterface cri. This is the interface of the remote object at the client.
Before I get to that I have to say that the RMI calls from the client to the server (to the interface above) work perfectly.
I will include the part of the code I use to implement this interface:
public class LocalClientManager extends UnicastRemoteObject implements ErrorListener,ClientRMICommandHandlerInterface{ ...
public Vector clientConnectToServer(ClientReceiverInterface cri,User user) throws RemoteException{ ...}
} // end class
This is how I instansiate the class:
try {
localClientManager = new LocalClientManager();
catch (RemoteException re) {...
And this is how I bind it...
try {
Naming.rebind("rmi://" + SERVER_IP + ":" + RMI_REGISTRY_PORT + "/LocalClientManagerService", localClientManager);
catch (MalformedURLException mue) {...}
On the client side I have the ClientReceiverInterface. This looks like:
import java.rmi.RemoteException;
import java.util.Vector;
public interface ClientReceiverInterface extends java.rmi.Remote{
public void setFriendData(User uFriend) throws RemoteException;
public void setFriendList(Vector vNewFriendList) throws RemoteException;
public void setMessage(String sMessage,String sUIDorigin, int iMessageMethod) throws RemoteException;
and the Implementation class is :
import java.rmi.server.UnicastRemoteObject;
import java.rmi.RemoteException;
import java.util.Vector;
public class ClientReceiver extends UnicastRemoteObject implements ClientReceiverInterface{ ...
all the method implementations follow...
}// end class
This is how I instansiate the object
try{
clientReceiver = new ClientReceiver();
}catch(RemoteException re){...}
I do call super() on the class above ( I saw just now that I dont on the server side Remote object but I guess thats ok since java calls it anyway...)
The way in which I store the references at the server is a bit complicated to post here since it involves a number of classes (4 actually) and references pasted from one to the other using "events". By the way could that be the problem? I mean I 'm 100% sure that I dont reset the reference value to null and I have a System.out.println before any changes happen to it so I know exactly when and why a change it. But could the problem be that I pass it around the server classes as an argument?
As I said in an older post the funny thing is that if I use the reference immediately after I get it (in one of the methodes of the ClientRMICommandHandlerInterface it works great. Once I store it and try to reuse it I get the exception.
I hope that know you can tell me where I 'm wrong.
In any case I really appresiate your help .
Cheers
John
PS: Sorry about the long listing and the funny names for the classes and the interfaces