Remote method invocation allows applications to call object methods located remotely, sharing resources and processing load across systems. Unlike other systems for remote execution which require that only simple data types or defined structures be passed to and from methods, RMI allows any Java object type to be used - even if the client or server has never encountered it before. RMI allows both client and server to dynamically load new object types as required.

Overview

Remote Method Invocation (RMI) facilitates object function calls between Java Virtual Machines (JVMs). JVMs can be located on separate computers - yet one JVM can invoke methods belonging to an object stored in another JVM. Methods can even pass objects that a foreign virtual machine has never encountered before, allowing dynamic loading of new classes as required. This is a powerful feature!
Consider the follow scenario :

• Developer A writes a service that performs some useful function. He regularly updates this service, adding new features and improving existing ones.
• Developer B wishes to use the service provided by Developer A. However, it's inconvenient for A to supply B with an update every time.

Java RMI provides a very easy solution! Since RMI can dynamically load new classes, Developer B can let RMI handle updates automatically for him. Developer A places the new classes in a web directory, where RMI can fetch the new updates as they are required.

Figure 1 - Connections made when client uses RMI

Figure 1 shows the connections made by the client when using RMI. Firstly, the client must contact an RMI registry, and request the name of the service. Developer B won't know the exact location of the RMI service, but he knows enough to contact Developer A's registry. This will point him in the direction of the service he wants to call..
Developer A's service changes regularly, so Developer B doesn't have a copy of the class. Not to worry, because the client automatically fetches the new subclass from a webserver where the two developers share classes. The new class is loaded into memory, and the client is ready to use the new class. This happens transparently for Developer B - no extra code need to be written to fetch the class.

Writing RMI services

Writing your own RMI services can be a little difficult at first, so we'll start off with an example which isn't too ambitious. We'll create a service that can calculate the square of a number, and the power of two numbers (2³⁸ for example). Due to the large size of the numbers, we'll use the java.math.BigInteger class for returning values rather than an integer or a long.

Writing an interface

The first thing we need to do is to agree upon an interface, An interface is a description of the methods we will allow remote clients to invoke. Let's consider exactly what we'll need.

1. A method that accepts as a parameter an integer, squares it, and returns a BigInteger
   public BigInteger square ( int number_to_square );
2. A method that accepts as a parameter two integers, calculates their power, and returns a BigInteger
   public BigInteger power ( int num1, int num2 );

Once we've decided on the methods that will compose our service, we have to create a Java interface. An interface is a method which contains abstract methods; these methods must be implemented by another class. Here's the source code for our service that calculates powers.

import java.math.BigInteger;
import java.rmi.*;

//
// PowerService Interface
//
// Interface for a RMI service that calculates powers
//
public interface PowerService extends java.rmi.Remote
{
    // Calculate the square of a number
    public BigInteger square ( int number )
        throws RemoteException;

    // Calculate the power of a number
    public BigInteger power  ( int num1, int num2) 
        throws RemoteException;
}

Our interface extends java.rmi.Remote, which indicates that this is a remote service. We provide method definitions for our two methods (square and power), and the interface is complete. The next step is to implement the interface, and provide methods for the square and power functions.

Implementing the interface

Implementing the interface is a little more tricky - we actually have to write the square and power methods! Don't worry if you're not sure how to calculate squares and powers, this isn't a math lesson. The real code we need to be concerned about is the constructor and main method.
We have to declare a default constructor, even when we don't have any initialization code for our service. This is because our default constructor can throw a java.rmi.RemoteException, from its parent constructor in UnicastRemoteObject. Sound confusing? Don't worry, because our constructor is extremely simple.
public PowerServiceServer () throws RemoteException
    {
        super();
    }

Our implementation of the service also needs to have a main method. The main method will be responsible for creating an instance of our PowerServiceServer, and registering (or binding) the service with the RMI Registry. Our main method will also assign a security manager to the JVM, to prevent any nasty surprises from remotely loaded classes. In this case, a security manager isn't really needed, but in more complex systems where untrusted clients will be using the service, it is critical.
    public static void main ( String args[] ) throws Exception
    {
        // Assign a security manager, in the event that dynamic
       // classes are loaded
       if (System.getSecurityManager() == null)
            System.setSecurityManager ( new RMISecurityManager() );

       // Create an instance of our power service server ...
       PowerServiceServer svr = new PowerServiceServer();

       // ... and bind it with the RMI Registry
       Naming.bind ("PowerService", svr);

       System.out.println ("Service bound....");
    }
Once the square and power methods are added, our server is complete. Here's the full source code for the PowerServiceServer.

import java.math.*;
import java.rmi.*;
import java.rmi.server.*;

//
// PowerServiceServer
//
// Server for a RMI service that calculates powers
//
public class PowerServiceServer extends UnicastRemoteObject
implements PowerService
{
    public PowerServiceServer () throws RemoteException
    {
        super();
    }

    // Calculate the square of a number
    public BigInteger square ( int number ) 
    throws RemoteException
    {
        String numrep = String.valueOf(number);
        BigInteger bi = new BigInteger (numrep);
        
        // Square the number
        bi.multiply(bi);

        return (bi);
    }

    // Calculate the power of a number
    public BigInteger power ( int num1, int num2)
    throws RemoteException
    {
        String numrep = String.valueOf(num1);
        BigInteger bi = new BigInteger (numrep);

 bi = bi.pow(num2);

 return bi;
    }

    public static void main ( String args[] ) throws Exception
    {
        // Assign a security manager, in the event that dynamic
 // classes are loaded
        if (System.getSecurityManager() == null)
            System.setSecurityManager ( new RMISecurityManager() );

        // Create an instance of our power service server ...
        PowerServiceServer svr = new PowerServiceServer();

        // ... and bind it with the RMI Registry
        Naming.bind ("PowerService", svr);

        System.out.println ("Service bound....");
    }
}

Writing a RMI client

What good is a service, if you don't write a client that uses it? Writing clients is the easy part - all a client has to do is call the registry to obtain a reference to the remote object, and call its methods. All the underlying network communication is hidden from view, which makes RMI clients simple.
Our client must first assign a security manager, and then obtain a reference to the service. Note that the client receives an instance of the interface we defined earlier, and not the actual implementation. Some behind-the-scenes work is going on, but this is completely transparent to the client.
    // Assign security manager
    if (System.getSecurityManager() == null)
    {
        System.setSecurityManager   (new RMISecurityManager());
    }

    // Call registry for PowerService
    PowerService service = (PowerService) Naming.lookup
        ("rmi://" + args[0] + "/PowerService");
To identify a service, we specify an RMI URL. The URL contains the hostname on which the service is located, and the logical name of the service. This returns a PowerService instance, which can then be used just like a local object reference. We can call the methods just as if we'd created an instance of the remote PowerServiceServer ourselves.

// Call remote method
System.out.println    ("Answer : " + service.square(value));

        // Call remote method
        System.out.println    ("Answer : " + service.power(value,power));
Writing RMI clients is the easiest part of building distributed services. In fact, there's more code for the user interface menu in the client than there is for the RMI components! To keep things simple, there's no data validation, so be careful when entering numbers. Here's the full source code for the RMI client.

import java.rmi.*;
import java.rmi.Naming;
import java.io.*;

//
//
// PowerServiceClient
//
//
public class PowerServiceClient 
{
 public static void main(String args[]) throws Exception
 {
  // Check for hostname argument
  if (args.length != 1)
  {
   System.out.println
   ("Syntax - PowerServiceClient host");
   System.exit(1);
  }

  // Assign security manager
  if (System.getSecurityManager() == null)
  {
   System.setSecurityManager
   (new RMISecurityManager());
  }

  // Call registry for PowerService
  PowerService service = (PowerService) Naming.lookup
   ("rmi://" + args[0] + "/PowerService");

  DataInputStream din = new 
   DataInputStream (System.in);

  for (;;)
  {
   System.out.println 
     ("1 - Calculate square");
   System.out.println 
     ("2 - Calculate power");
   System.out.println 
     ("3 - Exit"); System.out.println();
   System.out.print ("Choice : ");

   String line = din.readLine();
   Integer choice = new Integer(line);
   
   int value = choice.intValue();

   switch (value)
   {
   case 1:
     System.out.print ("Number : ");
     line = din.readLine();System.out.println();
     choice = new Integer (line);
     value  = choice.intValue();
    
     // Call remote method
     System.out.println 
     ("Answer : " + service.square(value));
    
     break;
   case 2:
     System.out.print ("Number : ");
     line = din.readLine();     
     choice = new Integer (line);
     value  = choice.intValue();
    
     System.out.print ("Power  : ");
     line = din.readLine();
     choice = new Integer (line);
     int power = choice.intValue();

     // Call remote method
     System.out.println 
  ("Answer : " + service.power(value, power));

     break;
   case 3:
     System.exit(0);
   default :
     System.out.println ("Invalid option");
   break;
   }
  }
 }

}

Running the client and server

Our example was extremely simple. More complex systems, however, might contain interfaces that change, or whose implementation changes. To run this article's examples, both the client and server will have a copy of the classfiles, but more advanced systems might share the code of the server on a webserver, for downloading as required. If your systems do this, don't forget to set the system property java.rmi.server.codebase to the webserver directory in which your classes are stored!
You can download all the source and class files together as a single ZIP file. Unpack the files into a directory, and then perform the following steps.

1. Start the rmiregistry

   To start the registry, Windows users should do the following (assuming that your java\bin directory is in the current path):-
   

   start rmiregistry 

   To start the registry, Unix users should do the following:-
   rmiregistry &
   

2. Compile the server

   Compile the server, and use the rmic tool to create stub files.
   

3. Start the server

   From the directory in which the classes are located, type the following:-
   

   java PowerServiceServer

4. Start the client

   You can run the client locally, or from a different machine. In either case, you'll need to specify the hostname of the machine where you are running the server. If you're running it locally, use localhost as the hostname.
   

   java PowerServiceClient localhost 

TIP - If you running the client or server with JDK1.2, then you'll need to change the security settings. You'll need to specify a security policy file (a sample is included with the source code and classes) when you run the client and server.The following changes should be made when running the server java -Djava.security.policy=java.policy PowerServiceServer The following changes should be made when running the client java -Djava.security.policy=java.policy PowerServiceClient localhost

Summary

Java RMI is a useful mechanism for invoking methods of remote objects. Java RMI allows one Java Virtual Machine to invoke methods of another, and to share any Java object type, even if client or server has never come across that object type before.