Implementing Callbacks in Java RMI

14 Sep 2023

Callbacks

In this blog post, we will explore how to implement Callbacks in Java RMI, enabling the server to notify the client asynchronously.

Setting Up the RMI Environment

Before diving into implementing Callbacks, let’s set up the basic RMI environment.

To start, we need to define the remote interface that both the client and server will use. This interface defines the methods that the client can call on the server object remotely. Let’s call it RemoteInterface.

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface RemoteInterface extends Remote {
    void registerCallback(CallbackInterface callback) throws RemoteException;
    // other remote methods
}

Next, we need to define the callback interface that the server will use to notify the client. Let’s call it CallbackInterface.

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface CallbackInterface extends Remote {
    void notifyClient(String message) throws RemoteException;
}

Now, we can proceed with the implementation of the server and client classes.

Implementing the Server

The server class will implement both the RemoteInterface and CallbackInterface. It will maintain a list of registered callbacks and invoke them when necessary.

import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
import java.util.ArrayList;
import java.util.List;

public class ServerImpl extends UnicastRemoteObject implements RemoteInterface, CallbackInterface {
    private List<CallbackInterface> callbacks;

    public ServerImpl() throws RemoteException {
        callbacks = new ArrayList<>();
    }

    @Override
    public void registerCallback(CallbackInterface callback) {
        callbacks.add(callback);
    }

    // Implement other remote methods

    @Override
    public void notifyClient(String message) {
        for (CallbackInterface callback : callbacks) {
            try {
                callback.notifyClient(message);
            } catch (RemoteException e) {
                // Handle remote exception if necessary
            }
        }
    }
}

Implementing the Client

The client class will implement the CallbackInterface and will be responsible for registering itself with the server’s callback list.

import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;

public class ClientImpl extends UnicastRemoteObject implements CallbackInterface {

    protected ClientImpl() throws RemoteException {
        // Constructor
    }

    @Override
    public void notifyClient(String message) {
        // Handle the notification from the server
        System.out.println("Received notification: " + message);
    }

    public static void main(String[] args) {
        try {
            RemoteInterface server = (RemoteInterface) java.rmi.Naming.lookup("//localhost/Server");
            ClientImpl client = new ClientImpl();
            server.registerCallback(client);
            // Invoke other remote methods if needed
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Running the Example

To run the example, follow these steps:

Start the RMI registry: rmiregistry
Start the server: java ServerImpl
Start the client: java ClientImpl
Interact with the client and server, following the defined remote methods and callback notifications.

Conclusion

In this blog post, we learned how to implement Callbacks in Java RMI, allowing the server to notify clients asynchronously. By leveraging the Java RMI technology and using remote and callback interfaces, we can design and develop distributed systems that benefit from asynchronous communication.

#Java #RMI #Callbacks