📘 RMI (Remote Method Invocation) in Java

---

🔷 1. Definition

RMI (Remote Method Invocation) is a Java mechanism that allows an object in one Java Virtual Machine (JVM) to invoke methods of an object in another JVM over a network.

👉 In simple words:

RMI allows Java programs to call methods on remote computers as if they were local.

---

🔷 2. Key Idea

• Works in distributed systems
• Uses client-server model
• Hides network complexity from programmer

📊 Concept Diagram (Description) Client JVM → Stub → Network → Skeleton → Server JVM

---

🔷 3. RMI Architecture

🔹 Main Components:

1. Client

• Calls remote method

2. Stub (Client-side proxy)

• Acts as local representative of remote object
• Sends request to server

3. Skeleton (Server-side handler)

• Receives request
• Calls actual method

4. Remote Object (Server)

• Executes method logic

---

🔷 4. Working of RMI (Step-by-Step)

✔ Step-by-step process:

1. Client calls remote method
2. Stub converts request into network format
3. Request sent to server
4. Skeleton receives request
5. Server executes method
6. Result sent back to client
7. Stub returns result to client

---

🔷 5. RMI Architecture Diagram (Description)

📊 Flow:

Client → Stub → Remote Reference Layer → Skeleton → Remote Object → Response back

---

🔷 6. Steps to Create RMI Application

---

🔹 Step 1: Define Remote Interface

import java.rmi.*;

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

👉 Must extend Remote 👉 Methods must throw RemoteException

---

🔹 Step 2: Implement Remote Interface

import java.rmi.server.*;

public class MyRemoteImpl extends UnicastRemoteObject implements MyRemote {

    public MyRemoteImpl() throws RemoteException {
        super();
    }

    public String sayHello() {
        return "Hello from Server";
    }
}

---

🔹 Step 3: Create Server

import java.rmi.*;

public class Server {
    public static void main(String[] args) {
        try {
            MyRemoteImpl obj = new MyRemoteImpl();
            Naming.rebind("rmi://localhost/hello", obj);

            System.out.println("Server started...");
        } catch(Exception e) {
            System.out.println(e);
        }
    }
}

---

🔹 Step 4: Create Client

import java.rmi.*;

public class Client {
    public static void main(String[] args) {
        try {
            MyRemote obj = (MyRemote) Naming.lookup("rmi://localhost/hello");

            System.out.println(obj.sayHello());
        } catch(Exception e) {
            System.out.println(e);
        }
    }
}

---

🔷 7. Key Classes in RMI

Class	Purpose
Remote	Marker interface
RemoteException	Handles network errors
Naming	Registry lookup/binding
UnicastRemoteObject	Remote object support

---

🔷 8. RMI Registry

👉 A registry service that stores remote objects.

Commands:

rmiregistry

• Used to register and locate remote objects

---

🔷 9. Advantages of RMI

• Simple distributed programming
• Object-oriented communication
• Platform independent (Java-to-Java)
• Easy remote method calling

---

🔷 10. Disadvantages

• Only works with Java
• Slower than local method calls
• Complex setup compared to sockets

---

🔷 11. RMI vs Socket Programming

Feature	RMI	Socket
Level	High-level	Low-level
Complexity	Easy	Complex
Communication	Method-based	Data-based
Language	Java only	Any language

---

🔷 12. Important Rules

• Remote interface must extend Remote
• Methods must throw RemoteException
• Objects must be registered in RMI registry
• Use Naming.lookup() in client

---

🔷 13. Diagram Description

📊 RMI Flow Diagram:

Client → Stub → Network → Skeleton → Remote Object → Response

---

📝 Likely Exam Questions

1. Define RMI.
2. Explain RMI architecture.
3. What are stub and skeleton?
4. Steps to create RMI application.
5. Write RMI client-server program.
6. Difference between RMI and socket programming.
7. What is Remote interface?
8. Role of RMI registry.
9. Advantages of RMI.
10. What is RemoteException?

---

📌 Quick Revision Summary

• RMI = Remote Method Invocation

• Calls methods across JVMs

• Components:

  • Client
  • Stub
  • Skeleton
  • Server

• Uses registry for lookup

• Java-to-Java communication

---