X-post: Puzzled about RMI

Question

Folks, yes I am cross posting this so try to keep the flames down :) I've also placed it on the RMI forum but no one there seems to want to help and I know everyone in the "Advanced" forum are very helpful (do you like the flattery???).
Anyway, 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
             // Create the object.
         RMIObject o = new RMIObject ();
             // Bind it in.
         Naming.rebind ("//localhost/RMIObject", o);
             // Now get a reference to the object from the name server.
         o = (RMIObject) Naming.lookup ("//localhost/RMIObject");
             // Set the value to 100.
             o.setValue (100);
             // Print out the value.
         System.out.println (o.getValue ());
             // Get ANOTHER reference to the object.
         o = (RMIObject) Naming.lookup ("//localhost/RMIObject");          
             // See what the value is.
         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

Easy folks...my reference to my potential student status was supposed to be "tongue-in-cheek", tis been a LONG time since I could get up when I want and pretend that I was really overworked and complain that I had too many assignments...
Now of course I am overworked and have too many assignments...least I get paid now...
Thanks for the help on this, I like to understand the mechanisms behind things, RMI from the outside has an appearance of "magic" that annoys me, I wasn't able to find out any description of how it actually works behind the scenes. I had to infer that it acts as a "broker", I presume that the RemoteObject object stores some information about where the JVM that actually holds the object is and then the Client uses this information (again behind the scenes) to get a connection.
This tends to worry me a bit though, the implication is that each object that is exported also contains a ServerSocket on a particular port that listens for incoming requests. Now I would presume that when it gets the request it then creates a new thread to handle it to prevent IO blocking. Now in a highly utilized system this means a lot of threads and a lot of port access, which is o.k. but that's a big use of resources. Also, if I have 2 objects listening on the same port, how does the JVM distinguish between the 2 objects when the incoming connection is received. The OS will wake up the JVM and inform it of a new connection (at least that's how Unix does it), but does the JVM have some "magic" that allows it to send it to the correct ServerSocket? I suppose that you also need to take into account Thread concurrency issues as well since you are (by definition) creating objects that will be multi-threaded...
Can anyone enlighten me on some of the above? Are there any books around that describe how RMI actually works...