Software Engineering

Michael Elhadad

Software Engineering - Fall 1999

Lecture 5: Distributed Objects (Part 1) -- Java RMI

Selection of Infrastructure: Platform for development of Distributed Applications.

Motivation

Must select a development platform appropriate for the objective.
Characteristics of the project: distributed and multi-media.
Need: tools to support a distributed architecture and multi-media work.

Candidates for Distributed Architecture Platforms

Sockets
RPC
CORBA
COM/DCOM (Microsoft)
Java RMI
ObjectSpace Voyager

Basic Concepts: Distributed Objects

Distributed Object
Stub (or Proxy)
Skeleton (or Server object)
ORB: Object Request Broker
Serialization: marshalling/unmarshalling.
Object Reference

First RMI Example (Nelson 98)

Objective: a Line Printer server object.

public interface LinePrinter
  {
    public void type(String line)
  }

RMI Requirements:

public interface LinePrinter
    extends java.rmi.Remote
  {
    public void type(String line)
      throws java.rmi.RemoteException;
  }

RMI uses the extends java.rmi.Remote as an indication that an object is distributed. All operations on remote objects can potentially fail on the network. The exception RemoteException indicates this.

How parameters are passed

If parameter is an instance of a distributed object, an object reference is passed. All operations to that object are invoked remotely. If parameter is not a distributed object, it is serialized using Java's standard Object Serialization.

try  {
  printer.type("Hello!");
} 
catch (java.rmi.RemoteException e) {
  System.err.println("Cannot connect.");
  e.printStackTrace();
}

Objects Implementation

public class LinePrinterImpl
  extends java.rmi.server.UnicastRemoteObject
  implements LinePrinter
{
  public LinePrinterImpl()
    throws java.rmi.RemoteException
  {
  }
  
      
  public void type(String s)
    throws java.rmi.RemoteException
  {
    System.out.println(s);
  }
}

Distributed Object Compilation

Compile Implementation as usual (javac LinePrinterImpl).
Generate stub and skeleton automatically: (rmic LinePrinterImpl)

Writing a Server

import java.rmi.*;


public class PrintServer
{
  public static void main(String[] args)
  {
    System.setSecurityManager(new RMISecurityManager());

    try
      {
        LinePrinter printer = new LinePrinterImpl();
        Naming.rebind( "LinePrinter", printer );
        System.out.println("PrintServer is ready.");
      }
    catch (Exception e)
      {
        System.out.println( "Error: " + e );
        e.printStackTrace();
      }
  }
}

Writing a Client


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


public class PrintClient
{
  public static void main(String[] args)
  {
    try
      {
        LinePrinter printer = 
          (LinePrinter)Naming.lookup("//localhost/LinePrinter");
 
        BufferedReader r = new
          BufferedReader(new InputStreamReader(System.in));

        String nextLine = r.readLine();
        
        while (nextLine != null)
          {
            printer.type( nextLine );
            nextLine = r.readLine();
          }
    
      }
    catch (Exception e)
      {
        System.err.println("Error: " + e);
        e.printStackTrace();
      }
  }
}

Naming Service and rmiregistry

Must run rmiregistry for client to locate server.
Client uses URLs of the form: rmi://hostname:port/name
The Naming object connects to rmiregistry to resolve these names.

Running the Example

The files needed are:

Open a window on lead

Make sure you define in your .cshrc (or .tcshrc):

    setenv JAVA_HOME /usr/local/java
    alias java /usr/local/java/bin/java
    alias javac /usr/local/java/bin/javac
    alias rmiregistry /usr/local/java/bin/rmiregistry

Start the registry:
```
    % rmiregistry &
    
```
Start the server:
```
    % java PrintServer &
    
```
Open another window on lead
In the second window, run the PrintClient:
```
    % java PrintClient
    
```
Everything you type in the client window will be echoed on the server window.

References


Java Tutorial on RMI

Java RMI example



        Last modified Apr 25th, 1999
Michael Elhadad