Puzzled about RMI

Question

Folks,
I'm just playing with RMI now and have followed the tutorial but I am a little puzzled about some behaviour I'm observing. This may be that I am missing a piece of information about RMI or how it works so if anyone can fill in the blanks then I would be most appreciative!!!
So I've got the following Remote interface definition:
import java.rmi.*;
public interface RMIInterface extends Remote
    public int getValue () throws RemoteException;
    public void setValue (int val) throws RemoteException;
}And I have some code that implements that interface. This code also contains a main method that performs some stuff with the RMI object. It is given below:
import java.rmi.*;
import java.io.*;
public class RMIObject implements Remote, RMIInterface, Serializable
    private int value = 0;
    public int getValue ()
                         throws RemoteException
     return this.value;
    public void setValue (int    val)
                          throws RemoteException
     this.value = val;
    static public void main (String argv[])
     try
         RMIObject o = new RMIObject ();
         Naming.rebind ("//localhost/RMIObject", o);
         o = (RMIObject) Naming.lookup ("//localhost/RMIObject");
         o.setValue (100);
         System.out.println (o.getValue ());
         o = (RMIObject) Naming.lookup ("//localhost/RMIObject");
         System.out.println (o.getValue ());
     } catch (Exception e) {
         e.printStackTrace ();
}As you can see, I bind the object, then get a new reference to the object, set a value then get the reference again and see what the value is.
The output I get from this code is:
100
0
Which indicates that setValue is working when the reference is local but it's not updating the remote object. Now the question is, is this the way that RMI is supposed to work? i.e. should you NOT use RMI objects for holding state? If so what's the point because effectively the object would be isolated...can anyone shed any light on this?
BTW, I am NOT a student and this is NOT related to any exam/coursework question!!!

Answer 1

Hi!
Naming.lookup(..) actually returns a local reference to the server's stub object (which is locally present in the RMI client's machine). This stub takes care of the remote method invocations. Actually I don't understand how could you do the typecasting to the RMIObject but this is not really important right now!
Let's see what happened in your example:
First you registered your RMIObject in the registry.
Then you retrieved a remote reference to it (a local stub, which implements the remote interface of the server object) and you set some variables of it.
Then you retrieved again that stub (the java system probably instantiated an other stub object) and its variables were initialized also.
This is not the way as the RMI should be used!
The Naming.lookup(..) method always returns a Remote interface (if it can, or throws a RemoteException). This return value must be typecasted to the server object's Remote interface (in your example it's RMIInterface).
Then you can use this object's (interface's) methods as you want to. (actually you use the stub object's methods which converts your calls to remote calls...)
I hope it's clear now,
Sany