Java RMI (Remote Method Invocation)

Java RMI (Remote Method Invocation) allows invocation of methods that are located in different JVMs (Java Virtual Machines). This makes it possible to create distributed applications where objects can communicate across networks.

• **Remote Object**: Any object that can be invoked from a different JVM.
• **Remote Interface**: An interface that defines the methods that can be invoked remotely.
• **Stub**: A client-side representation of a remote object that forwards requests to the actual remote object.
• **Skeleton**: A server-side representation that receives calls from the stub and forwards them to the actual remote object (Note: skeletons are no longer required in Java 2.0 and later).
• **RMI Registry**: A simple server that allows remote clients to obtain a reference to a remote object.

To set up a Java RMI application, follow these steps:

1. Ensure you have Java Development Kit (JDK) installed.
2. Create a remote interface.
3. Implement the remote interface in a remote object.
4. Start the RMI registry.
5. Bind the remote object to the registry.
6. Create the client to invoke methods on the remote object.

4.1 Create Remote Interface

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

public interface HelloWorld extends Remote {
    String sayHello() throws RemoteException;
}

4.2 Implement Remote Interface

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

public class HelloWorldImpl extends UnicastRemoteObject implements HelloWorld {
    public HelloWorldImpl() throws RemoteException {
        super();
    }

    public String sayHello() throws RemoteException {
        return "Hello, World!";
    }
}

4.3 Create Server

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;

public class Server {
    public static void main(String[] args) {
        try {
            HelloWorld hello = new HelloWorldImpl();
            Registry registry = LocateRegistry.createRegistry(1099);
            registry.bind("HelloWorld", hello);
            System.out.println("Server is ready.");
        } catch (Exception e) {
            System.err.println("Server exception: " + e.toString());
            e.printStackTrace();
        }
    }
}

4.4 Create Client

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;

public class Client {
    public static void main(String[] args) {
        try {
            Registry registry = LocateRegistry.getRegistry("localhost", 1099);
            HelloWorld stub = (HelloWorld) registry.lookup("HelloWorld");
            String response = stub.sayHello();
            System.out.println("Response: " + response);
        } catch (Exception e) {
            System.err.println("Client exception: " + e.toString());
            e.printStackTrace();
        }
    }
}

• Always handle RemoteException in remote methods.
• Use interfaces for remote objects to ensure loose coupling.
• Secure RMI applications with authentication and encryption.
• Consider using RMI over IIOP for interoperability with CORBA.