Java RMI

Jussi Pohjolainen

TAMK University of Applied Sciences » ICT

Intro to RMI

• RMI: Java Remote Method Invication
• Provides remote communication between programs written in Java
• Method calls between JVM 1 and JVM 2

Overview

• RMI connection needs 1) client, 2) server and 3) remote object
• Server app:
  • Creates remote objects
  • Makes references to them accessible
  • Waits clients to invoke the remote object's methods
• Client app:
  • Gets reference to the remote object
  • Invokes methods on remote objects

Locating and Communicating

• To locate the remote object, rmiregistry is used
• Rmiregistry is a simple naming facility which is used to locate remoteobjects
• Provides remote communication between programs written in Java
• Communicating looks like normal method invication!

Locating and Communicating

• To locate the remote object, rmiregistry is used
• Rmiregistry is a simple naming facility which is used to locate remoteobjects
• Provides remote communication between programs written in Java
• Communicating looks like normal method invication!

RMI Overview

Implementing RMI System

• Client locates remote object from rmiregistry and invokes remote object's methods.
• Server binds remote object to rmiregistry
• Remote object implements Remote interface

Class Diagram of the Server - side

UnicastRemoteObject and Remote - interface

• RemoteObject must:
  • inherit java.rmi.server.UnicastRemoteObject
  • implement interface that inherites java.rmi.Remote
• java.rmi.Remote interface does not have any methods!
• Every remote method must throw java.rmi.RemoteException

Parameter passing

• Java: Every object is passed by reference
• Exception! When calling remote method, object's are passed by copy
• Parameter must be serializable. Object is transferred to bits and moved from one JVM to another

Stub and Skeleton

• Stub and Skeleton: classes that are used to create the real communication between client and server.
• Stub class: acts as client's representive for the remote object
  • Initiative the connection, marshals the parameter, waits result, unmarhals return value, return the value to the caller
• Skeleton class: Each remote object has corresponding skeleton class
  • unmarhals the parameter, invokes the method, marhals the result to the caller
• Stub and Skeleton classes are created by using rmic compiler.
• The creation of stub and skeleton are not needed in Java 5 or later

In Practice: Creating a simple RMI system

1. Define a interface that holds remote methods. Remember that every method must throw java.rmi.RemoteException and the interface must inherit java.rmi.Remote
2. Create RemoteObject class that implements the interface implemented above. Remember that this class must inherit java.rmi.UnicastRemoteObject and it's default constructor must throw java.rmi.RemoteException
3. Create Server class that creates the RemoteObject and binds it to the rmiregistry
4. Create a client that locates RemoteObject from rmiregistry and calls it's remote methods
5. (Optional) create stub and skeleton classes using rmic compiler
6. Start rmiregistry, server and client

1) Define interface

import java.rmi.*;

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

2) Create Remote Object

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

public class RemoteObject extends UnicastRemoteObject 
     implements Hello {
    
    public RemoteObject() throws RemoteException { }
    
    public String sayHello() throws RemoteException {
        return "Hello from another JVM!"; 
    }
}

3) Create Server

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

public class HelloServer {
    public static void main(String [] args) {
        try {
            RemoteObject remoteobject = new RemoteObject();
            Naming.rebind("donaldduck", remoteobject);
            System.out.println("Remote object is now 
                bind to the rmi-registry");
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

4) Create Client

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

public class HelloClient {
    public static void main(String [] args) {
        try {
            Hello external = 
                (Hello)Naming.lookup("//localhost/donaldduck");
            String message = external.sayHello();
            System.out.println(message);
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

5) Create Stub and Skeleton classes

> rmic -v1.1 RemoteObject
// creates RemoteObject_Skel.class and RemoteObject_Stub.class

6) Start the application

> rmiregistry
> java HelloServer
> java HelloClient