Client callback

[supratim chaudhury]

Can anyone plz tell what is client side callback?
Can anyone give some code to illustrate the explanation?
actually,what i have come to know that client acts as the server
and it extends unicastremote...or uses exportobject...
also client class needs to implement another remote interface....
But after that i miss the obvious point...
Where actually does the server invoke a method on the client just as client
explicitly calls a method on remote object of server???

[Nathan Pruett]

If both Client and Server are Remote objects -

That means they both have "Remote" interfaces - let's call them "Client" and "Server".

The actual implementations, let's call "ClientImpl" and "ServerImpl".

One of the methods on the "Server" should be something like -

 
public void register(Client client) throws RemoteException;

When ClientImpl gets a reference to Server (through Naming.lookup()) it can them call register() and pass a reference to itself in:

Server server = (Server)Naming.lookup(<server lookup name>  :wink:  ;
server.register(this);

Then, inside ServerImpl, the register() method will receive the stub to "ClientImpl" - hidden behind a "Client" remote interface - and can save a reference to it...

 
private Vector clients = new Vector();
 
public void register(Client client) throws RemoteException {
    clients.add(client);
}

assuming the "Client" interface has the following method -

public void sendMessage(String message) throws RemoteException;

Later - the ServerImpl can use these references to make calls to all the clients - something like -

private void sendToAllClients(String message) {
    for (Iterator clientIter = clients.iterator(); clientIter.hasNext(); ) {
        Client c = (Client)clientIter.next();
        try {
            c.sendMessage(message);
        } catch (RemoteException x) {
            System.err.println("RemoteException caught when sending message - a client is down");
        }
    }
}

[ May 03, 2006: Message edited by: Nathan Pruett ]