Thursday, March 6, 2003

Distributed Notification with Java RMI

Abstract

Java’s implementation of Remote Method Invocation (RMI) is very easy to use and very power. Java makes the task of setting up an RMI server an almost trivial one. Once running, connecting client applications to the RMI server is also a breeze. There are examples and how-tos for client to server communication everywhere. But what about the other way, server to client communication? Is it possible for an RMI server to actively communicate with all the clients that are connected to it without the client initiating the conversation? In other words, is distributed notification possible? The purpose of this post is to demonstrate that yes, it is possible.

Disclaimer

This post is solely informative. Critically think before using any information presented. Learn from it but ultimately make your own decisions at your own risk.

Problem

Consider development of a small test application that demonstrates an RMI server notifying its clients of events. A very simple example is an RMI “time server”. Clients register with the server (client to server communication) and then every second all registered clients are notified of an event, the new date and time (server to client communication). The client implements some kind of listener and that listener can be used to handle the event any way the client sees fit.

Proposed Solution

The difficulty that I have with implementing distributed notification is that the server has to have some knowledge of the client application. Just as the client needs an Xxx_Stub.class to talk to the server, the server needs a similar Xxx_Stub.class to talk to the client.

The first, and most obvious (to me anyway) solution was for the client to develop its own remote class, generate an Xxx_Stub.class for it, then give that Xxx_Stub.class file to the server. Not a very attractive solution.

After struggling with the problem for a while I narrowed down the root of the problem. On the server side, I had interfaces and objects like listing 1 and listing 2.

Listing 1 - ClientCallback interface

public interface ClientCallback extends Remote {
    public void informClient(String msg)
    throws RemoteException;
}

Listing 2 - Remote service allowing clients to register

class SomeServiceImpl extends UnicastRemoteObject implements SomeService {
    public void registerWithService(ClientCallback cc)
    throws RemoteException {
        //...
    }
}

Continuing with the root of the problem, on the client side I want to make a remote call that looks something like listing 3.

Listing 3 - What client wants to do

serverObject.registerWithService(this);

The root of the problems caused by this code is that the client would then have to become a ‘server’ itself, generating Xxx_Stub.class files for the classes that implement ClientCallback and somehow getting those Xxx_Stub classes to the server. Not very pretty.

When considering a solution to this problem, realization struck. The server would have no problems if the class that implemented ClientCallback was a class that the server had created! For example, let us say that the server gives to the client the class in listing 4.

Listing 4 - Server gives client this class

class ServerListener implements ClientCallback {
   /**
    * constructor
    */
   public ServerListener(String serverName, int port)
   throws Exception {
      UnicastRemoteObject.exportObject( this );

      SomeService ss = (SomeService)Naming.lookup("rmi://<server>:<port>/SomeService");

      ss.registerWithService(this);
   }


   /**
    * MyCustomListener interface method
    */
   public void informClient(String msg) {
     // do something
   }
}

When the code for the RMI server is compiled, a ServerListener_Stub.class is also created and bundled in the jar file for the client to use. Basically, this is an object that a client can create an instance of just like any other Java object.

ServerListener ls = new ServerListener("localhost", 1099);

This object knows how to contact the RMI server, do the appropriate lookup, and register itself with the server. And because of the line:

UnicastRemoteObject.exportObject(this); 

The server is able to call the informClient() method on the client’s machine! Bingo! that’s exactly what I wanted!

Well not quite. The server can now inform the client of something, but that code is buried inside the ServerListener object that the client really has no access to. Extending the ServerListener object won’t work because that gets you back to the previous problem of the server having to know what the client classes are. Here’s my solution, another listener interface provided by the server!

Let’s say the ClientCallback interface had an analogous, non-remote interface called ClientListener. Also let’s say that the ServerListener class that the server provides as an additional method that looks like listing 5.

Listing 5 - Method to register client-side listener

Vector listeners = new Vector();

public void addClientListener(ClientListener cl) {
    if (!listeners.contains(cl)) {
        listeners.add(cl);
    }
}

Now let ClientListener be just a simple, non-remote interface as in listing 6.

Listing 6 - Non-remote, client-side interface

public interface ClientListener() {
    public void statusChanged(String s);
}

Now the client side code just falls into place as in listing 7.

Listing 7 - Client code

// create an object that can "listen" for events on the server
ServerListener sl = new ServerListener("localhost", 1099);

// add as many classes as you want to handle those events
sl.addClientListener(new ClientListenerA());
sl.addClientListener(new ClientListenerB());
sl.addClientListener(new ClientListenerC());
sl.addClientListener(new ClientListenerD());

The implementation of the ServerListener.informClient() object is nothing more than looping over the list of ClientListener objects and calling the changeStatus() method on each.

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