This document provides an introduction and overview of Remote Method Invocation (RMI) in Java. It describes building a simple client-server application using RMI that allows a client to invoke methods on a Java object running on a remote server machine. The application involves defining a remote interface, implementing that interface on the server, generating stubs and skeletons, running an RMI registry on the server, and having the client lookup and invoke methods on the remote object.

1. Introduction To RMI By Swarup Kulkarni (MCA-II, VIT, Pune)

2. Remote Method Invocation RMI - Remote Method Invocation Allows to invoke a method of a Java object that executes on another machine. Important feature, because it allows to build distributed applications. Lets discus a simple Client/Server application using RMI. The server receives a request from client and processes it In this req. two numbers are specified Server add these numbers and returns it 3/11/2009 2 RMI - Swarup Kulkarni

3. Step 1: Enter and Compile the Source Code We use four source files - First source file: RmiDemoIntf.java Defines remote interface, a remote method, All remote interfaces must inherit Remote interface Remote interface is a part of java.rmi.remote Defines no members , purpose is to indicate that interface uses remote methods. All remote methods can throw RemoteException 3/11/2009 3 RMI - Swarup Kulkarni

4. Step 1: (Continued..) The first source file - Code: RmiDemoIntf.java import java.rmi.*; public interface RmiDemoIntf extends Remote { double add(double d1, double d2) throws RemoteException; } 3/11/2009 4 RMI - Swarup Kulkarni

5. Step 1: (Continued..) The second source file: RmiDemoImpl.java Implements Remote interface implementation of the add( ) method is straightforward All remote objects must extend UnicastRemoteObject UnicastRemoteObject provides functionality that is needed to make objects available from remote machines 3/11/2009 5 RMI - Swarup Kulkarni

6. Step 1: (Continued..) The second source file - Code: RmiDemoImpl.java import java.rmi.*; import java.rmi.server.*; public class RmiDemoImpl extends UnicastRemoteObject implements RmiDemoIntf{ public double add(double d1, double d2) throws RemoteException { return d1 + d2; } } 3/11/2009 6 RMI - Swarup Kulkarni

8. First argument names server as “RmiServer”

9. Second argument is reference to object of RmiDemoImpl3/11/2009 7 RMI - Swarup Kulkarni

10. Step 1: (Continued..) The third source file - Code: RmiServer.java import java.net.*; import java.rmi.*; public class RmiServer{ public static void main(String args[]) { try { RmiDemoImpl obj_RmiDemoImpl = new RmiDemoImpl(); Naming.rebind(“RmiServer", obj_RmiDemoImpl); } catch(Exception e) { System.out.println("Exception: " + e); } } } 3/11/2009 8 RMI - Swarup Kulkarni

11. Step 1: (Continued..) The forth source file: RmiClient.java Implements client side of this distributed application. Requires thee main things – IP address or name of server, and the two numbers to be added. We create a string that follows URL syntax. The URL includes IP or Name of the server and the string “RmiServer”. The program invokes lookup() method from Naming class which accepts rmi URL as argument and returns a reference of object of RmiDemoIntf . Now all the remote method invocations can be directed to this object. 3/11/2009 9 RMI - Swarup Kulkarni

12. Step 1: (Continued..) The forth source file - Code: RmiClient.java import java.rmi.*; public class RmiClient { public static void main(String args[]) { try { double d1 = 24.73; double d2 = 11.12; String RmiDemoURL = "rmi://" + “127.0.0.1” + "/RmiServer"; //127.0.0.1 is loopback address replace it with server IP/name RmiDemoIntf obj = (RmiDemoIntf)Naming.lookup(RmiDemoURL); 3/11/2009 10 RMI - Swarup Kulkarni

13. Step 1: (Continued..) RmiClient.java(Contd..) System.out.println("The first number is: " + d1); System.out.println("The second number is: " + d2); System.out.println("The sum is: " + RmiDemoIntf.add(d1, d2)); }//end of try catch(Exception e) { System.out.println("Exception: " + e); }//end of catch }//end of main }//end of class 3/11/2009 11 RMI - Swarup Kulkarni

14. Step 2: Generate Stubs and Skeletons Before using client and server we have to generate stub and skeleton. Stub resides on client machine. Remote method calls initiated by client are directed to stub. A remote method can have simple arguments or objects as arguments. Objects may have reference to other objects, to send whole information to server the objects in arguments must be serialized. Skeletons are not required by Java 2.(Needed for compatibility with java1.1) Skeleton resides on sever machine, when it receives request, it performs deserialization and invokes appropriate method. In case of response to client, the process works reverse. 3/11/2009 12 RMI - Swarup Kulkarni

15. Step 2: (Continued..) How to generate stub and Skeleton? Use a tool called RMI Compiler, which is invoked from command line: rmic RmiDemoImpl; This command generates two new files: RmiDemoImpl_Skel.class (skeleton) and RmiDemoImpl_Stub.class (stub). Before using rmic make sure that CLASSPATH is set to include current directories. rmic by default generates stub as well as skeleton, we have option to suppress it. 3/11/2009 13 RMI - Swarup Kulkarni

16. Step 3: Install Files on Client and Server Copy RmiDemo.class, RmiDemoImpl_Stub.class, and RmiDemoImpl.class on the client machine. Copy RmiDemontf.class, RmiDemoImpl.class, RmiDemompl_Skel.class, RmiDemompl_Stub.class, and RmiDemo.class on the server machine. 3/11/2009 14 RMI - Swarup Kulkarni

17. Step 4: Start the RMI Registry on the Server The Java 2 SDK provides a program called rmiregistry, which executes on server. The rmiregistry maps names to object reference. First, check that the CLASSPATH environment variable includes the directory in which your files are located and start RMI Registry: start rmiregistry When this command runs, it creates a new window. Leave that window open until the experimentation finishes. 3/11/2009 15 RMI - Swarup Kulkarni

18. Step 5: The Experiment The server code is started from the command line, as shown here: java RmiServer Recall that the RmiServer code instantiates RmiDemoImpl and registers that object with the name “RmiServer”. The client code is started from command line as shown here: java RmiClient Output of progarm: The first number is: 24.73 The second number is: 11.12 The sum is: 35.85 3/11/2009 16 RMI - Swarup Kulkarni

19. 3/11/2009 17 RMI - Swarup Kulkarni Thank You