Developing a Simple Example The Chat Application

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Developing a Simple Example The Chat Application

• Peng-Sheng,Chen

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Overview

• Using the JMS pub/sub API to build a simple chat
  application.
• The requirements of Internet chat map neatly onto
  the publish-and-subscribe messaging model.
• In this model, a producer can send a message to
  many consumers by delivering the message to a
  single topic.

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Cont..

• A message producer is also called a publisher and
  a message consumer is also called a subscriber.
• Every participant in a chat session uses this
  Chat program to join a specific chat room
  (topic), and deliver and receive messages to and
  from that room

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Chat Application
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JNDI JMS

• In JMS, JNDI is used mostly as a naming service
  to locate administered objects.
• Administered objects are JMS objects that are
  created and configured by the system
• administrator.
• Administered objects include JMS
  ConnectionFactory and Destination objects like
  topics and queues.

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Cont.

• Using JNDI, JMS clients can browse a naming
  service and obtain references to administered
  objects without knowing the details of the naming
  service or how it is implemented.
• JMS servers are usually be used in combination
  with a standard JNDI driver and directory service
  like LDAP, or provide a proprietary JNDI service
  provider and directory service.

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Obtaining a JNDI connection

• The chat client starts by obtaining a JNDI
  connection to the JMS messaging server.
• Using JNDI, a JMS client can obtain access to a
  JMS provider by first looking up a
  ConnectionFactory.
• JMS servers will either work with a separate
  directory service (e.g., LDAP) or provide
• their own directory service that supports the
  JNDI API.

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Cont.

• The constructor of the Chat class starts by
  obtaining a connection to the JNDI naming
• service used by the JMS server
• Creating a connection to a JNDI naming service
  requires that a javax.naming.InitialContext
  object be created.

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Sessions and Threading

• The Chat application uses a separate session for
  the publisher and subscriber, pubSession and
  subSession, respectively.
• According to the JMS specification, a session may
  not be operated on by more than one thread at a
  time.
• If both the publisher and subscriber had been
  created by the same session, the two threads
  could operate on these methods at the same time

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Constructor. Establish JMS publisher and
subscriber

• public class Chat implements javax.jms.MessageList
  ener
• private TopicSession pubSession
• private TopicSession subSession
• private TopicPublisher publisher
• private TopicConnection connection
• private String username
• public Chat(String topicName, String
  username, String password) throws Exception
• .

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Initialize the instance variables

• public void set(TopicConnection con, TopicSession
  pubSess,
• TopicSession subSess, TopicPublisher
  pub,String username)
• ..

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Receive message from topic subscriber

• public void onMessage(Message message)
• try
• TextMessage textMessage (TextMessage) message
• String text textMessage.getText( )
• System.out.println(text)
• catch (JMSException jmse) jmse.printStackTrace
  ( )

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Create and send message using topic publisher

• protected void writeMessage(String text) throws
  JMSException
• TextMessage message pubSession.createTextMessag
  e( )
• message.setText(username" "text)
• publisher.publish(message)

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Close the JMS connection

• public void close( ) throws JMSException
• connection.close( )

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Run the Chat client

• public static void main(String args)
• try
• .
• Chat chat new Chat(args0,args1,args2)
• // args0topicName args1username
  args2password
• BufferedReader commandLine new
• java.io.BufferedReader(new InputStreamReader(Syst
  em.in)) // Read from command line

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Getting Started with the Chat Example

• Compile it like any other Java program.
• Start your JMS server, setting up whatever
  topics, usernames, and passwords you want.
• It can be run as a standalone Java
• application. It's executed from the command line
  as follows
• java chap2.chat.Chat topic username password

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Cont.

• Run at least two chat clients in separate command
  windows and try typing into one you should see
  the text you type displayed by the other client.

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Cont.