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RMI - Example

RMI, which stands for Remote Method Invocation, is a mechanism in Java that allows an object running in one Java Virtual Machine (JVM)—potentially on a different machine—to invoke methods on an object residing in another JVM. It is used to build distributed applications in Java.

RMI makes calling a method on a remote object seem virtually the same as calling a method on a local object. Key components facilitate this:

  • Remote Interface: An interface that declares the methods that can be invoked remotely. It must extend the java.rmi.Remote interface, and all methods must throw java.rmi.RemoteException.
  • Remote Object (Server-side): The actual implementation of the remote interface. It handles the method logic and runs on the server JVM.
  • Stub (Client-side Proxy): An object on the client machine that acts as a proxy for the remote object. When the client calls a method, the stub handles the communication:
    • It establishes a connection to the remote JVM.
    • It marshals (packages) the parameters into a message.
    • It sends the message to the server.
    • It waits for the result.
    • It unmarshals (unpackages) the result or exception and returns it to the client.
  • RMI Registry: A simple naming service on the server where the remote object is registered under a specific name. The client uses this registry to look up the remote object's reference (the stub). 

Both Server and client are in same machine and in same directory

Search.java // Interface
import java.rmi.*;
public interface Search extends Remote
{
	// Declaring the method prototype
	public String find(String search) throws RemoteException;
}
////SearchQuery class implementing Search Interface
import java.rmi.*;
import java.rmi.server.*;
public class SearchQuery extends UnicastRemoteObject
						implements Search
{
	// Default constructor to throw RemoteException
	// from its parent constructor
	SearchQuery() throws RemoteException
	{
		super();
	}

	// Implementation of the query interface
	public String find(String search)
					throws RemoteException
	{
		String result;
		if (search.equals("Java is Coffee"))
			result = "Yes, Java is Coffee";
		else
			result = "No, Java is not coffee";

		return result;
	}
}
///SearchServer -- waiting for client request
import java.rmi.*;
import java.rmi.registry.*;
import java.rmi.server.UnicastRemoteObject;
public class SearchServer
{
    public SearchServer(){}
	public static void main(String args[])
	{
		try
		{
			Search stub = new SearchQuery();
            Naming.bind("rmi://localhost:1099/find",stub);  
            System.out.println("Search Server is listening...");
		}
		catch(Exception ae)
		{
			System.out.println("SearchServer: "+ae);
		}
	}
}
///Client asking for service from server

import java.rmi.*;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
public class ClientRequest
{
    private static Search stub = null;
    private ClientRequest(){}
	public static void main(String args[])
	{
		String answer,value="Java is Coffee";
		try
		{
            Registry reg = LocateRegistry.getRegistry("localhost");
            stub = (Search) reg.lookup("find");
            answer = stub.find(value);
			// lookup method to find reference of remote object
            System.out.println("Answer: "+answer);
		}
		catch(Exception ae)
		{
			System.out.println("Client Request: "+ae.getMessage());
		}
	}
}

Executing : 

E:\rmi>javac *.java

E:\rmi>start rmiregistry

E:\rmi>javac SearchServer.java

E:\rmi>start rmiregistry
//Starting Server
E:\rmi>java SearchServer
Search Server is listening...

//Client
E:\rmi>java ClientRequest
Answer: Yes, Java is Coffee