regarding RMI architecture

[Lucas Smith]

Hi!

I want to implement a project which will be on the basis of RMI.
I have one server and a couple of clients. When one client sends a screen to a server - server forwards it to other client.

I think that both client and server have to register their services in rmiregistry.
1). client invokes server's method to send a screen
2). server invokes client's method to forward a screen

Is it a good idea?

Thanks!

[Karthik Shiraly]

Didn't understand why all the clients need to register in rmiregistry. Each client can implement a client interface (which should also be remote) and pass a reference to that while registering with server. Server uses the received reference to communicate with client.

[Lucas Smith]

OK, so all of the clients implement Client remote interface and server implements Server remote interface.
During client creation reference is passed to Server.
Is it good?

[Lucas Smith]

I have written some code. Please, analyze that:

RemoteClient

import java.rmi.*;

public interface RemoteClient extends Remote {
	public void sendString(String string) throws RemoteException;
}    

Client

import java.net.*;
import java.rmi.*;
import java.rmi.server.*;

public class Client extends UnicastRemoteObject implements RemoteClient {
	public Client() throws RemoteException{}
	
	public void sendString(String string) throws RemoteException{
		System.out.println(string);
	}
	
	public static void main(String[] args) throws RemoteException, MalformedURLException, NotBoundException{
		RemoteServer remoteServer = (RemoteServer)Naming.lookup("rmi://127.0.0.1/server");
		RemoteClient remoteClient = new Client();
		remoteServer.writeReference(remoteClient);
	}
}    

RemoteServer

import java.rmi.*;

public interface RemoteServer extends Remote {
	public void writeReference(RemoteClient remoteClient) throws RemoteException;
}    

Server

import java.rmi.*;
import java.rmi.server.*;
import java.net.*;
import java.util.*;

public class Server extends UnicastRemoteObject implements RemoteServer {
	private final List<RemoteClient> remoteClients = new ArrayList<RemoteClient>();
	
	private static final long serialVersionUID = 1L;

	public Server() throws RemoteException{}
	
	public void writeReference(RemoteClient remoteClient) throws RemoteException{
		remoteClients.add(remoteClient);
		System.out.println("reference written");
		for(RemoteClient client : remoteClients){
			client.sendString("info from server");
		}
	}
	
	public static void main(String[] args) throws RemoteException, MalformedURLException{
		RemoteServer remoteServer = new Server();
		Naming.rebind("server", remoteServer);
	}
}    

Please, notice that I do not use ClientRemote interface - I do not register it in rmiregistry.
I just pass a client's reference to Server and then Server invokes methods on that object. I tried it on localmachine and it works.
What about distributed machines? Will that work? I do not think so.

Please give me an answer.

[Karthik Shiraly]

Hi,

Sorry, am seeing this only now since I didn't get an email alert.

Yes, it works even in distributed case - I know because I've them working. Why do you feel it won't work?
Registering with naming registry is simply to give a label to a java reference so that other objects can get it. In the above code, RemoteClient reference is given directly to server, so it is being used, but just not registered.

Cheers
Karthik

[Lucas Smith]

Got it! Thanks.

[Stephan King]

Sorry to barge in on this thread - haven't really used RMI - does RMI have a default port or how is that allocated in this particular code?

[Karthik Shiraly]

By default, RMI naming registry is reachable on port 1099. In this code, the line

Naming.lookup("rmi://127.0.0.1/server");  

does not specify a port, so it defaults to 1099.

But that's not the only port in picture. Once lookup is done, client and server setup a TCP connection over random ports, unless server specifies specific port on which it'll communicate. This code is not doing it, but it can be done in the RemoteServer constructor as:

public class Server extends UnicastRemoteObject implements RemoteServer {  
  public Server() throws RemoteException{
     super(<yourport>);
  }
  ...

[Stephan King]

Thanks