Java RMI Tutorial

Introduction

This is a brief introduction to Java Remote Method Invocation (RMI). 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. CORBA is another object-oriented RPC mechanism. CORBA differs from Java RMI in a number of ways:

  1. CORBA is a language-independent standard.
  2. CORBA includes many other mechanisms in its standard (such as a standard for TP monitors) none of which are part of Java RMI.
  3. There is also no notion of an "object request broker" in Java RMI.

This tutorial attempts to show the essence of RMI, without discussing any extraneous features. Sun has provided a Guide to RMI, but it includes a lot of material that is not relevant to RMI itself. For example, it discusses how to incorporate RMI into an Applet, how to use packages and how to place compiled classes in a different directory than the source code. All of these are interesting in themselves, but they have nothing at all to do with RMI. As a result, Sun's guide is unnecessarily confusing. Moreover, Sun's guide and examples omit a number of details that are important for RMI.

There are three processes that participate in supporting remote method invocation.

  1. The Client is the process that is invoking a method on a remote object.
  2. The Server is the process that owns the remote object. The remote object is an ordinary object in the address space of the server process.
  3. The Object Registry is a name server that relates objects with names. Objects are registered with the Object Registry. Once an object has been registered, one can use the Object Registry to obtain access to a remote object using the name of the object.

In this tutorial, we will give an example of a Client and a Server that solve the classical "Hello, world!" problem. You should try extracting the code that is presented and running it on your own computer.

There are two kinds of classes that can be used in Java RMI.

  1. A Remote class is one whose instances can be used remotely. An object of such a class can be referenced in two different ways:
    1. Within the address space where the object was constructed, the object is an ordinary object which can be used like any other object.
    2. Within other address spaces, the object can be referenced using an object handle or proxy. While there are limitations on how one can use an object handle compared to an object, for the most part one can use object handles in the same way as an ordinary object.
    For simplicity, an instance of a Remote class will be called a remote object.
  2. A Serializable class is one whose instances can be copied from one address space to another. An instance of a Serializable class will be called a serializable object. In other words, a serializable object is one that can be marshaled. Note that this concept has no connection to the concept of serializability in database management systems.

    If a serializable object is passed as a parameter (or return value) of a remote method invocation, then the value of the object will be copied from one address space to the other. By contrast if a remote object is passed as a parameter (or return value), then the object handle will be copied from one address space to the other.

    Serializable objects are something like the concrete objects of C++, but only with respect to remote method invocations. With respect to ordinary method invocations, serializable objects are ordinary objects, not concrete objects. This is confusing, and so in Java one must carefully distinguish remote method invocations from ordinary method invocations.

Serializable Classes

We now consider how to design Remote and Serializable classes. The easier of the two is a Serializable class. A class is Serializable if it implements the java.io.Serializable interface. This interface has no methods; it serves only to mark the class as serializable to the compiler. Subclasses of a Serializable class are also Serializable. Many of the standard classes are Serializable, so a subclass of one of these is automatically also Serializable. Normally, any data within a Serializable class should also be Serializable. Although there are ways to include non-serializable objects within a serializable object, it is awkward to do so. See the documentation of java.io.Serializable for more information about this.

Using a serializable object in a remote method invocation is straightforward. One simply passes the object using a parameter or as the return value. The type of the parameter or return value is the Serializable class. Note that both the Client and Server programs must have access to the definition of any Serializable class that is being used. The only Serializable class that will be used in the "Hello, world!" example is the String class.

Remote Classes and Interfaces

Next consider how to define a Remote class. This is more difficult than defining a Serializable class. A Remote class has two parts: the interface and the class itself. The Remote interface must have the following properties:

  1. The interface must be public.
  2. The interface must extend the interface java.rmi.Remote. The Remote interface has not methods; it serves only to mark the interface so that classes implementing the interface can be used to create remote objects.
  3. Every method in the interface must declare that it throws java.rmi.RemoteException. Other exceptions may also be thrown.
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. Objects of such a class exist in the address space of the server and can be invoked remotely. While there are other ways to define a Remote class, this is the simplest way to ensure that objects of a class can be used as remote objects. See the documentation of the java.rmi.server package for more information.
  3. It can have methods that are not in its Remote interface. These can only be invoked locally.
Unlike the case of a Serializable class, it is not necessary for both the Client and the Server to have access to the definition of the Remote class. The Server requires the definition of both the Remote class and the Remote interface, but the Client only uses the Remote interface. Roughly speaking, the Remote interface represents the type of an object handle, while the Remote class represents the type of an object. If a remote object is being used remotely, its type must be declared to be the type of the Remote interface, not the type of the Remote class.

In the example program, we need a Remote class and its corresponding Remote interface. We call these Hello and HelloInterface, respectively. Here is the the file HelloInterface.java:

import java.rmi.*;
/**
 * Remote Interface for the "Hello, world!" example.
 */
public interface HelloInterface extends Remote {
  /**
   * Remotely invocable method.
   * @return the message of the remote object, such as "Hello, world!".
   * @exception RemoteException if the remote invocation fails.
   */
  public String say() throws RemoteException;
}
Here is the the file Hello.java:
import java.rmi.*;
import java.rmi.server.*;
/**
 * Remote Class for the "Hello, world!" example.
 */
public class Hello extends UnicastRemoteObject implements HelloInterface {
  private String message;
  /**
   * Construct a remote object
   * @param msg the message of the remote object, such as "Hello, world!".
   * @exception RemoteException if the remote method fails.
   */
  public Hello(String msg) throws RemoteException {
    message = msg;
  }
  /**
   * Implementation of the remotely invocable method.
   * @return the message of the remote object, such as "Hello, world!".
   * @exception RemoteException if the remote invocation fails.
   */
  public String say() throws RemoteException {
    return message;
  }
}

All of the Remote interfaces and classes should be compiled using javac. Once this has been completed, the stubs and skeletons for the Remote interfaces should be compiled by using the rmic stub compiler. The stub and skeleton of the example Remote interface are compiled with the command:

  rmic Hello

The only problem one might encounter with this command is that rmic might not be able to find the files Hello.java and HelloInterface.java even though they are in the same directory where rmic is being executed. If this happens to you, then try setting the CLASSPATH environment variable to the current directory, as in the following command:

  setenv CLASSPATH .
If your CLASSPATH variable already has some directories in it, then you might want to add the current directory to the others.

Programming a Client

Having described how to define Remote and Serializable classes, we now discuss how to program the Client and Server. The Client itself is just a Java program. It need not be part of a Remote or Serializable class, although it will use Remote and Serializable classes.

A remote method invocation can return a remote object as its return value, but one must have a remote object in order to perform a remote method invocation. So to obtain a remote object one must already have one. Accordingly, there must be a separate mechanism for obtaining the first remote object. The Object Registry fulfills this requirement. It allows one to obtain a remote object using only the name of the remote object.

The name of a remote object includes the following information:

  1. The Internet name (or address) of the machine that is running the Object Registry with which the remote object is being registered. If the Object Registry is running on the same machine as the one that is making the request, then the name of the machine can be omitted.
  2. The port to which the Object Registry is listening. If the Object Registry is listening to the default port, 1099, then this does not have to be included in the name.
  3. The local name of the remote object within the Object Registry.
Here is the example Client program:
  /**
   * Client program for the "Hello, world!" example.
   * @param argv The command line arguments which are ignored.
   */
  public static void main(String[] argv) {
    try {
      HelloInterface hello = 
        (HelloInterface) Naming.lookup("rmi://maasim.ccs.neu.edu/Hello");
      System.out.println(hello.say());
    } catch (Exception e) {
      System.out.println("HelloClient exception: " + e);
    }
  }

The Naming.lookup method obtains an object handle from the Object Registry running on maasim.ccs.neu.edu and listening to the default port. Note that the result of Naming.lookup must be cast to the type of the Remote interface.

The remote method invocation in the example Client is hello.say(). It returns a String which is then printed. A remote method invocation can return a String object because String is a Serializable class.

The code for the Client can be placed in any convenient class. In the example Client, it was placed in a class HelloClient that contains only the program above.

Programming a Server

The Server itself is just a Java program. It need not be a Remote or Serializable class, although it will use them. The Server does have some responsibilities:

  1. At least one remote object must be registered with the Object Registry. The statement for this is: Naming.rebind(objectName, object); where object is the remote object being registered, and objectName is the String that names the remote object.
Here is the example Server:
  /**
   * Server program for the "Hello, world!" example.
   * @param argv The command line arguments which are ignored.
   */
  public static void main(String[] argv) {
    try {
      Naming.rebind("Hello", new Hello ("Hello, world!"));
      System.out.println("Hello Server is ready.");
    } catch (Exception e) {
      System.out.println("Hello Server failed: " + e);
    }
  }
Java RMI limits binding and unbinding requests to Object Registries running on the same machine, so it is never necessary to specify the name of the machine when one is registering an object.

The code for the Server can be placed in any convenient class. In the example Server, it was placed in a class HelloServer that contains only the program above.

Starting the Server

Before starting the Server, one should first start the Object Registry, and leave it running in the background. One performs this by using the command:

  rmiregistry &
It takes a second or so for the Object Registry to start running and to start listening on its socket. If one is using a script, then one should program a pause after starting the Object Registry. If one is typing at the command line, it is unlikely that one could type fast enough to get ahead of the Object Registry.

The Server should then be started; and, like the Object Registry, left running in the background. The example Server is started using the command:

  java HelloServer &
The Server will take a few seconds to start running and to construct and register remote objects. So one should wait a few seconds before running any Clients. Printing a suitable message, as in the example Server, is helpful for determining when the Server is ready.

Running a Client

Th Client is run like any other java program. The example Client is executed using:

  java HelloClient


Ken Baclawski
207 Cullinane Hall
College of Computer Science
Northeastern University
360 Huntington Avenue
Boston, MA 02115
kenb@ccs.neu.edu
(617) 373-4631 / Fax: (617) 373-5121

Copyright © 1998, 2008 by Kenneth Baclawski. All rights reserved.