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Java Messaging Service(JMS)

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Distributed Object Technologies like RMI, CORBA and DCOM areadvanced variants of RPC(Remote-Procedure-Call).EJB( SessionBeans % Entity Beans)is essentially built on the RMI-IIOP model,( a combination of RMI with CORBA style). All these systems are Synchronous in nature. (ie) the client invokes a method on the remote object and if for some reason , there isdelay in response from the server end, the line getsblocked.This is sometimes referred to as ‘Tight Coupling’.In Enterprise level computing, especially, B2B(Business to Business) a number of applications are integrated together.Ifeither one of the applications crashes or if we want to add/remove a section from this integrated scheme,it leads to a mess. This is what led to the development of Messaging systems. Such systems are based on Asynchronous processing. (ie) aprogramsendsamessage to another program but does not wait for response. This ensures that there is no ‘blocking’.

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Fundamentally, the idea is similar to ’email’.Such systems are said to be ‘loosely coupled’ systems. Wherever there are huge number of players , loose coupling is beneficial. For this reason, the Enterprise world isnowadaysveering round to the Messaging protocol . It is in response to this shift in requirements, that Sun addedthe JMS ( Java Messaging System) API andthenMessage-driven bean as a third type of EJBean in addition to Session Bean & Entity bean. .

A number ofsuch Messaging Systems had been developed by Enterprise-world majors.These are known as ‘Message-oriented-Middleware’. (MOM).Also known as JMS-Provider.

A list of such MOM products is given below.

  1. WebLogicfrom BEA
  2. MQSeriesfromIBM
  3. JBossMQfrom JBoss
  4. SoniqMQfrom Progres (David Chappel’s company)
  5. FiaranoMQfrom Fiarano
  6. MSMQfrom Microsoft

JMS is not a messaging system by itself. It is an abstraction of the underlying principles ofthe major Messaging systems in industry. This enables the programmer to write code as if there is just one generic MOM system..( Microsoftdoes not follow JMS).

There are two types of messaging:

a) Publish&Subscribe

b) PointtoPoint

In Publish & Subscribe method, the sender sends the message toJMS-Provider.. The JMS-Provider broadcasts the message. The message will be received by any ‘listener ‘who’ tunes’himself to the broadcast by registering himself as a listener. In such a method ,the message is known as a ‘Topic’.The sender is known as ‘publisher’ and the receiver is known as ‘subscriber‘.This is sometimes referred to as ‘one to many’.

In the Point to Point method, the ‘Sender‘ sends the message as before to the MOM server. These message are meant for a specific ‘Receiver‘.Such a message is known as a ‘Queue’. The listener registers himself with the JMS-Provider for a particular message and as soon as a message arrives at the provider,it is sent to the listener.This method is referred to as ‘one to one’.

For writing messaging programs, we need to import jms package. ( This is available in weblogic by default.). Thecodingconcepts are almost identical for Topics and Queues. So we will illustrate the steps forTopic method only.

steps for ‘publishdemo’ program.

  1.  connecttothe JMS PROVIDER.
  2.  locateTopicConnectionFactory
  3. create aTopicConnection
  4.  create aTopicSession
  5. locate theTopic
  6. create aTopicPublisher
  7.  create aTextMessage or other types of message.
  8.  start the connection
  9. publish the message

The ‘publishdemo’programthussends the message toaJMS-Provider(ie) WebLogic.,for publication.

A separate ‘subscribedemo’ programconnects to the sameJMS-Provider, and follows a similar procedure,but this time as a recipient of message.

  1. connect to theJMS PROVIDER.
  2.  locateTopicConnectionFactory
  3.  create aTopicConnection from the above factory
  4. create aTopicSession
  5. locate theTopic
  6. create aTopicSubscriber
  7.  Register the subscriber as a messageListener
  8. Wait to get message from JMS
  9.  Get the message as and when sent by the sender.

This time , we will create our programs as ‘stand-alone’ frames .There are no fancy items in the GUI (Graphical User Interface). TheGUI for the ‘publishdemo”program has just a textfield and a button to ‘send’ the message.Another button is provided for ‘exit’.

Similarly, the GUI for the ‘subscribedemo’ program has justatextarea .

We start the ‘subscribedemo’ program first. ( to start listening!). Then we start the ‘publishdemo’ program . We type some message in text1of’ publishdemo’ window and click the ‘send’ button. The message goes to the JMS-Provider and it broadcasts the message to all registered listeners. So our ‘subscribedemo’ programgets the message and it is displayed in textarea. We can close the sender and restart . The client side is not affected. You will remember that in socket method , if you close the sender, the client will quit because it was synchronous.

Just to make it interesting, we start another receiver program in a separate window. We will find that this client also gets the message.Withall this description, it should not be difficult to follow the coding.

If we want to make use of QUEUE, the steps are almost identical.

  1. connect toJMS-Provider
  2. locateQueueConnectionFactory
  3. createQueueConnection
  4. createQueueSession
  5. locateQueue
  6. createaQueueSender
  7.  create the message
  8. send the message.

Similar steps for the QueueReceiver. The following demo illustrates a Publish-Subscribemodel.

You will find reference to ‘ourfactory’& ‘ourtopic’in the code. These are known as ‘Administered objects’. (ie) we set up these names in the relevant section of the Weblogic.propertiesfile. How it is done has been explained below.

//***************publishdemo.java**************************


import javax.ejb.*;

import javax.jms.*;

import javax.naming.*;

import java.rmi.*;

import java.util.*;

import java.awt.*;

public class publishdemoextends Frame

{

TextFieldtext1;

Buttonbutton1;

Buttonbutton2;

TopicPublisher
 publisher;

TextMessagetm;

public
 static void main(String args[])

{

publishdemoapp=new publishdemo();

app.resize(300,200);

app.show();

}

public
 publishdemo()

{

setLayout(new FlowLayout());

setBackground(Color.red);

text1=new TextField(40);

button1=new Button("send");

button2=new Button("Exit");

add(text1);

add(button1);

add(button2);

System.out.println("Please wait");// diagnostic

Propertiesprops=new Properties();

props.put(Context.INITIAL_CONTEXT_FACTORY,

"weblogic.jndi.WLInitialContextFactory");

props.put(Context.PROVIDER_URL,

"t3://127.0.0.1:7001");

Contextcontext=new InitialContext(props);

System.out.println("context ok");

//--------------------------------------------------------

TopicConnectionFactoryfactory=

(TopicConnectionFactory)context.lookup("ourfactory");

TopicConnectionconnection=

factory.createTopicConnection();

TopicSessionsession=

connection.createTopicSession

(false,Session.AUTO_ACKNOWLEDGE);

Topictopic=

(Topic)context.lookup("ourtopic");

publisher=session.createPublisher(topic);

tm=session.createTextMessage();

}catch(Exception e1){System.out.println(""+e1);}

}

public
 boolean action(Event e,Object c)

{

if(e.target==button1)

{

try

{

String text=text1.getText();

tm.setText(text);

publisher.publish(tm);

System.out.println("message published");

}catch(Exception e1){System.out.println(""+e1); }

}

if(e.target==button2)

{

System.exit(0);

}

return
 true;

}

}

============================================================================

//subscribedemo.java

import javax.ejb.*;

import javax.jms.*;

import javax.naming.*;

import java.rmi.*;

import java.util.*;

import java.awt.*;

public class subscribedemoextends FrameimplementsMessageListener

{

TextAreaarea1;

public
 static void main(String args[])

{

subscribedemoapp = new
 subscribedemo();

app.resize(300,300);

app.show();

}

publicsubscribedemo()

{

setBackground(Color.orange);

setLayout(new FlowLayout());

area1 =
 new TextArea(10,30);

add(area1);

try

{

area1.append("please waitn");

Properties
 props=new Properties();

props.put(Context.INITIAL_CONTEXT_FACTORY,

"weblogic.jndi.WLInitialContextFactory");

props.put(Context.PROVIDER_URL,

"t3://127.0.0.1:7001");

Context
 context=new InitialContext(props);

area1.append("context
 readyn");

//--------------------------------------------------------

TopicConnectionFactoryfactory=

(TopicConnectionFactory)context.lookup("ourfactory");

TopicConnectionconnection=

factory.createTopicConnection();

TopicSessionsession=

connection.createTopicSession

(false,Session.AUTO_ACKNOWLEDGE);

Topictopic=

(Topic)context.lookup("ourtopic");

TopicSubscribersubscriber=

session.createSubscriber(topic);

subscriber.setMessageListener(this);

area1.append("starting connectionn");

connection.start();

area1.append("connection startedn");

area1.append("waiting for messagen");

area1.append("--------------------n");

}catch(Exception e){area1.setText(""+e);}

}

public
 void onMessage(Message msg)

{

try

{

TextMessage tm=(TextMessage)msg;

String
 s=tm.getText();

area1.append(s+"n");

}catch(Exception
 e){area1.setText(""+e);}

}

}

How shall we test this program?

This program is not connected with EJB but is preliminary to the study of Message-Driven Bean to be takenup shortly.So far, all our examples are being run with Weblogic 5.1. It can be run on Windows-98.

Message-Driven bean is based on EJB2.0 specification and requires Weblogic7.0 . which needs Windows 2000 for proper functioning.

We will now test our JMS program in Weblogic 5.1 installed in Windows- 98.


Create a folderas:c:jmsdemo

cd toc:jmsdemo

> SETJAVA_HOME=C:JDK1.3

>SETWL_HOME=C:WEBLOGIC

> set path=c:windowscommand;
 c:jdk1.3bin;c:weblogicbin

> set classpath=c:jmsdemo;c:weblogicclasses;c:weblogiclibweblogicaux.jar

Edit publishdemo.java and subscribedemo.java in c:jmsdemofolder.


.....jmsdemo
 >javac*.java//compile

Edit c:weblogicweblogic.properties file as follows:

You will find a section named ‘Weblogic JMS DEMOPROPERTIES’. ADD THE FOLLOWING LINES


weblogic.jms.connectionFactoryName.ourfactory=ourfactory

weblogic.jms.topic.ourtopic=ourtopic

From the ‘start’ menu,rememberto start the weblogic server as before. Minimize the window. Run the subscriber program first.

....jmsdemo >javasubscribedemo

You will geta window withjust a textarea. The area will display :


connection started

waiting for message

Now go to another dos prompt and set path, classpath etc as before. Start the publishdemo program


....jmsdemo><b>javapublishdemo

About Krishna Srinivasan

He is Founder and Chief Editor of JavaBeat. He has more than 8+ years of experience on developing Web applications. He writes about Spring, DOJO, JSF, Hibernate and many other emerging technologies in this blog.