Java remote method invocation

Java remote method invocation
 The Java Remote Method Invocation Application
Programming Interface (API), or Java RMI, is a Java
application programming interface that performs the
object-oriented equivalent of remote procedure calls
(RPC).
 Typically comprise 2 separate programs:-
 Client
 Server
 A remote interface specifies the methods that can be
invoked remotely by a client

RMI Server Interface
import java.rmi.Remote;
import java.rmi.RemoteException;
/*interface must extend Remote to enable access from
another JVM
The methods being remote methods can throw Remote
Exception */

public interface RmiServerIntf extends Remote {
public String getMessage() throws RemoteException;
/*This is the method implemented by server and
accessed by client*/
}

Implementing the interface
 An RMI server program needs to create the initial
remote objects and export them to the RMI runtime

 The setup procedure should do the following:
 Create and install a security manager
 Create and export one or more remote objects
 Register at least one remote object with the RMI
registry (or with another naming service

RMI Server Program
import java.rmi.Naming;
import java.rmi.registry.*;
import java.rmi.RMISecurityManager;
import java.rmi.server.UnicastRemoteObject;

//public class classname implements remote-interface
public class RmiServer extends UnicastRemoteObject implements RmiServerIntf {

public static final String MESSAGE = "Hello world";

public RmiServer() throws RemoteException {
//constructor defn }

public String getMessage() {
//Todo code for the remote method(s)
return MESSAGE;
}

public static void main(String args[]) {

//main method is used to create an instance and make it available to clients.

//Write the todo code here

System.out.println("RMI server started");

// Create and install a security manager

if (System.getSecurityManager() == null) {
System.setSecurityManager(new RMISecurityManager());
System.out.println("Security manager installed.");
} else {
System.out.println("Security manager already exists.");
}

try {

//special exception handler for registry creation
//Registry used for naming services

LocateRegistry.createRegistry(1099);
System.out.println("java RMI registry created.");
}
catch (RemoteException e) {

//do nothing, error means registry already exists
System.out.println("java RMI registry already exists.");
}

try {
//Instantiate RmiServer
//(ClassNAme)UnicastRemoteObject.exportObject(object1, portno);
RmiServer obj = new RmiServer();

// Bind this object instance to the name "RmiServer“

Naming.rebind("//localhost/RmiServer", obj);

System.out.println("PeerServer bound in registry");

}
catch (Exception e) {

System.err.println("RMI server exception:" + e);
e.printStackTrace();
}
}
}

import java.rmi.Naming;
import java.rmi.RMISecurityManager;

public class RmiClient {
// "obj" is the reference of the remote object
RmiServerIntf obj = null;

public String getMessage() {
try {
obj = (RmiServerIntf)Naming.lookup("//localhost/RmiServer");
return obj.getMessage();
} catch (Exception e) {
System.err.println("RmiClient exception: " + e);
return e.getMessage();
}
}

public static void main(String args[]) {

// Create and install a security manager

if (System.getSecurityManager() == null) {

System.setSecurityManager(new RMISecurityManager());

}

RmiClient cli = new RmiClient();

System.out.println(cli.getMessage());
}
}

Implementation:
 Compile the source code files
 javac filename
 Create the stub for client and skeleton for server
 rmic serverfilename
 Start the server
 rmiregistry
 Ensure security permits are in order
 Run the server and client files

Common sources of errors
 Before running the server, make sure you have an
instance of rmiregistry running on the server
 Make sure that your CLASSPATH variable is empty
 Not having a suitable security policy
 Mixing Java versions

Java remote method invocation

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