Network Programming: RMI

Remote method invocation(RMI) allow a java object to invoke method on an object running on another machine. RMI provide remote communication between java program. RMI is used for building distributed application.

Concept of RMI application

A RMI application can be divided into two part,Client program and Server program. A Server program creates some remote object, make their references available for the client to invoke method on it. A Client program make request for remote objects on server and invoke method on them. Stub and Skeleton are two important object used for communication with remote object.

Stub and Skeleton

Stub act as a gateway for Client program. It resides on Client side and communicate with Skeleton object. It establish the connection between remote object and transmit request to it.

Skeleton object resides on server program. It is responsible for passing request from Stub to remote object.

Creating a Simple RMI application involves following steps

Define a remote interface.
Implementing remote interface.
create and start remote application
create and start client application

Define a remote interface

A remote interface specifies the methods that can be invoked remotely by a client. Clients program communicate to remote interfaces, not to classes implementing it. To be a remote interface, a interface must extend the Remote interface of java.rmi package.

import java.rmi.*;
public interface AddServerInterface extends Remote
{
public int sum(int a,int b);
}

Implementation of remote interface

For implementation of remote interface, a class must either extend UnicastRemoteObject or use exportObject() method of UnicastRemoteObject class.

import java.rmi.*;
import java.rmi.server.*;
public class Adder extends UnicastRemoteObject implements AddServerInterface
{
Adder()throws RemoteException{
super();
}
public int sum(int a,int b)
{
return a+b; 
}
}

Create AddServer and host rmi service

You need to create a server application and host rmi service Adder in it. This is done using rebind()method of java.rmi.Naming class. rebind() method take two arguments, first represent the name of the object reference and second argument is reference to instance of Adder

import java.rmi.*;
import java.rmi.registry.*;
public class AddServer{
public static void main(String args[]){
try{
AddServerInterface addService=new Adder();
Naming.rebind("AddService",addService); 
//addService object is hosted with name AddService. 

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

Create client application

Client application contains a java program that invokes the lookup() method of the Naming class. This method accepts one argument, the rmi URL and returns a reference to an object of type AddServerInterface. All remote method invocation is done on this object.

import java.rmi.*;
public class Client{
public static void main(String args[]){
try{
AddServerInterface st=(AddServerInterface)Naming.lookup("rmi://"+args[0]+"/AddService");
System.out.println(st.sum(25,8));
}catch(Exception e){System.out.println(e);}
}
}

Steps to run this RMI application

Save all the above java file into a directory and name it as "rmi"

compile all the java files
```
javac *.java
```
Start RMI registry
```
start rmiregistry
```
Run Server file
```
java AddServer
```
Run Client file in another command prompt abd pass local host port number at run time
```
java Client 127.0.0.1
```