1. Advanced Java Programming
Topic: Distributed Programming (RMI)
By
Ravi Kant Sahu
Asst. Professor, LPU

2. Remote method INVoCAtIoN
(RMI)
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

3. Introduction

RMI is a mechanism for communicating (only) between two
machines running Java Virtual Machines.

When Java code on machine A needs a service or a method,
respectively, of (remote Java object) objB on machine B it starts
a remote method invocation.

It does this the same way as invoking a local (Java) object's
method.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

4. Remote Method Invocation

Java RMI is a mechanism that allows one to invoke a method on
an object that exists in another address space.

The other address space could be on the same machine or a
different one.

The RMI mechanism is basically an object-oriented RPC
mechanism.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

5. Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

6. RMI ARchItectuRe
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

7. Remote Communication

RMI allows an object to invoke methods on an object running in
another JVM.

RMI provides remote communication between the applications
using two objects:
1. Stub
2. Skeleton

A remote object is an object whose method can be invoked from
another JVM.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

8. Understanding Stub and Skeleton
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

9. Stub

Stub is an object which acts as a gateway for the client side.
All the outgoing requests are routed through Stub.
It represents the remote object at the client side.

Stub performs the following tasks:


1. It initiates a connection with remote JVM.
2. It writes and transmits the parameters to remote JVM
(Marshaling).
3. It reads the returned value or exception (Unmarshaling).
4. Finally, returns the result to the Caller.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

10. Skeleton

Skeleton is an object which acts as a gateway for the srver side.

All the outgoing requests are routed through Stub.

Skeleton performs the following tasks:
1. It reads the parameters for the remote method.
2. It reads the parameters from caller (Unmarshaling).
3. It invokes the method on the actual remote object.
4. It writes and transmits the result to the caller (Marshaling).
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

11. Writing rMi ApplicAtions
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

12. Writing RMI Programs
Steps to write RMI program:
1. Create a remote Interface.
2. Provide implementation of the remote interface (Remote
class).
3. Compile the implementation class and create the stub and
skeleton objects using rmic tool (rmic remoteclass).
4. Start the registry service by rmiregistry tool (start rmiregistry).
5. Create and start the remote application.
6. Create and start the client application.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

13. Remote InteRface & class
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

14. Remote Class

A Remote class has two parts: the interface and the class itself.
Remote Interface:
The Remote interface must have the following properties:
1. The interface must be public.
2. The interface must extend the interface java.rmi.Remote.
3. Every method in the interface must declare that it throws
java.rmi.RemoteException. Other exceptions may also be
thrown.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

15. Remote Interface
import java.rmi.*;
public interface RemoteInterface extends Remote
{
double sum(double d1, double d2) throws RemoteException;
}
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

16. Remote Class

The Remote class itself has the following properties:
1. It must implement a Remote interface.
2. It should extend the java.rmi.server.UnicastRemoteObject
class.
3. It can have methods that are not in its Remote interface. These
can only be invoked locally.
Note: Objects of such a class exist in the address space of the
server and can be invoked remotely.
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

17. Remote Class
import java.rmi.*;
import java.rmi.server.*;
public class RemoteClass extends UnicastRemoteObject implements
RemoteInterface
{
public RemoteClass() throws RemoteException
{}
public double sum(double d1, double d2) throws RemoteException
{
return d1 + d2;
}
}
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)