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 RMIBySwarup Kulkarni(MCA-II, VIT, Pune)

2. Remote Method InvocationRMI - Remote Method InvocationAllows 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 itIn this req. two numbers are specifiedServer add these numbers and returns it3/11/20092RMI - Swarup Kulkarni

3. Step 1: Enter and Compile the Source CodeWe use four source files -First source file:RmiDemoIntf.javaDefines remote interface, a remote method,All remote interfaces must inherit Remote interfaceRemote interface is a part of java.rmi.remoteDefines no members ,purpose is to indicate that interface uses remote methods.All remote methods can throw RemoteException3/11/20093RMI - Swarup Kulkarni

4. Step 1: (Continued..)The first source file - Code:RmiDemoIntf.javaimport java.rmi.*;public interface RmiDemoIntf extends Remote { double add(double d1, double d2) throws RemoteException;}3/11/20094RMI - Swarup Kulkarni

5. Step 1: (Continued..)The second source file:RmiDemoImpl.javaImplements Remote interfaceimplementation of the add( ) method is straightforwardAll remote objects must extend UnicastRemoteObjectUnicastRemoteObjectprovides functionality that is needed to make objects available from remote machines3/11/20095RMI - Swarup Kulkarni

6. Step 1: (Continued..)The second source file - Code:RmiDemoImpl.javaimport 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;}}3/11/20096RMI - Swarup Kulkarni

7. Step 1: (Continued..)The third source file:RmiServer.javaContains main program for the server machineThis modifies the RMI registry of the machine using rebind() method of Naming class(found in java.rmi)Naming.rebind()Associates name with the object reference

8. First argument names server as “RmiServer”

9. Second argument is reference to object of RmiDemoImpl3/11/20097RMI - Swarup Kulkarni

10. Step 1: (Continued..)The third source file - Code:RmiServer.javaimport 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);}}}3/11/20098RMI - Swarup Kulkarni

11. Step 1: (Continued..)The forth source file:RmiClient.javaImplements 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.3/11/20099RMI - Swarup Kulkarni

12. Step 1: (Continued..)The forth source file - Code:RmiClient.javaimport 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/nameRmiDemoIntf obj = (RmiDemoIntf)Naming.lookup(RmiDemoURL);3/11/200910RMI - 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 class3/11/200911RMI - Swarup Kulkarni

14. Step 2: Generate Stubs and SkeletonsBefore 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.3/11/200912RMI - 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.3/11/200913RMI - Swarup Kulkarni

16. Step 3: Install Files on Client and ServerCopy 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.3/11/200914RMI - Swarup Kulkarni

17. Step 4: Start the RMI Registry on the ServerThe 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 rmiregistryWhen this command runs, it creates a new window. Leave that window open until the experimentation finishes.3/11/200915RMI - Swarup Kulkarni