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Java Message Service Before examining the source code in detail, a quick explanation will be helpful. The chat client creates a JMS publisher and subscriber for a specific topic. The topic represents the chat room. The JMS server registers all the JMS clients that want to publish or subscribe to a specific topic. When text is entered at the command line of one of the chat clients, it is published to the messaging server. The messaging server identifies the topic associated with the publisher and delivers the message to all the JMS clients that have subscribed to that topic. As Figure 2.1 illustrates, messages published by any one of the JMS clients are delivered to all the JMS subscribers for that topic. 2.1.2 Examining the Source Code Running the Chat example in a couple of command windows demonstrates what the Chat application does. The rest of this chapter examines the source code for the Chat application so that you can see how the Chat application works. 2.1.2.1 Bootstrapping the JMS client The main( ) method bootstraps the chat client and provides a command-line interface. Once an instance of the Chat class is created, the main( ) method spends the rest of its time reading text typed at the command line and passing it to the Chat instance using the instance's writeMessage( ) method. The Chat instance connects to the topic and receives and delivers messages. The Chat instance starts its life in the constructor, which does all the work to connect to the topic and set up the TopicPublisher and TopicSubscribers for delivering and receiving messages. 2.1.2.2 Obtaining a JNDI connection The chat client starts by obtaining a JNDI connection to the JMS messaging server. JNDI is an implementation-independent API for directory and naming systems. A directory service provides JMS clients with access to ConnectionFactory and Destinations (topics and queues) objects. ConnectionFactory and Destination objects are the only things in JMS that cannot be obtained using the JMS API - unlike connections, sessions, producers, consumers, and messages, which are manufactured using the JMS API. JNDI provides a convenient, location-transparent, configurable, and portable mechanism for obtaining ConnectionFactory and Destination objects, also called JMS administered objects because they are established and configured by a system administrator. Using JNDI, a JMS client can obtain access to a JMS provider by first looking up a ConnectionFactory. The ConnectionFactory is used to create JMS connections, which can then be used for sending and receiving messages. Destination objects, which represent virtual channels (topics and queues) in JMS, are also obtained via JNDI and are used by the JMS client. The directory service can be configured by the system administrator to provide JMS administered objects so that the JMS clients don't need to use proprietary code to access a JMS provider. JMS servers will either work with a separate directory service (e.g., LDAP) or provide their own directory service that supports the JNDI API. For more details on JNDI, see the sidebar Understanding JNDI. 22
Java Message Service The constructor of the Chat class starts by obtaining a connection to the JNDI naming service used by the JMS server: // Obtain a JNDI connection Properties env = new Properties( ); // ... specify the JNDI properties specific to the vendor InitialContext jndi = new InitialContext(env); Creating a connection to a JNDI naming service requires that a javax.naming.InitialContext object be created. An InitialContext is the starting point for any JNDI lookup - it's similar in concept to the root of a filesystem. The InitialContext provides a network connection to the directory service that acts as a root for accessing JMS administered objects. The properties used to create an InitialContext depend on which JMS directory service you are using. The code used to create a JNDI InitialContext in BEA's Weblogic naming service, for example, would look something like this: Properties env = new Properties( ); env.put(Context.SECURITY_PRINCIPAL, "guest"); env.put(Context.SECURITY_CREDENTIALS, "guest"); env.put(Context.INITIAL_CONTEXT_FACTORY, "weblogic.jndi.WLInitialContextFactory"); env.put(Context.PROVIDER_URL, "t3://localhost:7001"); InitialContext jndi = new InitialContext(env); When SonicMQ is used in combination with a third party LDAP directory service, the connection properties would be very different. For example, the following shows how a SonicMQ JMS client would use JNDI to access JMS administered objects stored in a LDAP directory server: Properties env = new Properties( ); env.put(Context.SECURITY_PRINCIPAL, "guest"); env.put(Context.SECURITY_CREDENTIALS, "guest"); env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory"); env.put(Context.PROVIDER_URL, "ldap://localhost:389/o=acme.com"); InitialContext jndi = new InitialContext(env); Alternatively, the InitialContext( ) can be created without properties (no-arg constructor). In this case JNDI will read the vendor-specific JNDI properties from a special file in the classpath named jndi.properties. This eliminates provider-specific code in JMS clients, making them more portable. 23
Page 2 and 3: Java Message Service Richard Monson
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import javax.jms.TopicConnectionFac
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TopicConnectionFactory tFactory = n
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Java Message Service have already p
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} if (message != null){ int orderQt
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Java Message Service Chapter 6. Gua
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6.2.1 AUTO_ACKNOWLEDGE Java Message
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Java Message Service via publish-an
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Java Message Service message. If th
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Java Message Service QWholesaler an
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Java Message Service avoids process
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Figure 6.8. Transactional messages
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Java Message Service uses a JMS tra
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} return; session.commit(); } else
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Else, commit (that's what just happ
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XAResource jmsXA = session.getXARes
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... } { try { Thread.sleep(10000);
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Chapter 7. Deployment Consideration
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Java Message Service A vendor may h
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Java Message Service that your memo
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Java Message Service only really vi
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7.2.5 The Bottom Line Java Message
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Java Message Service There are two
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Java Message Service Figure 7.2. Us
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Chapter 8. J2EE, EJB, and JMS 8.1 J
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Java Message Service JNDI ENC. In a
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8.3 The JMS Resource in J2EE Java M
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Java Message Service Although sessi
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Figure 8.3. Lifecycle of a message-
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Java Message Service With the messa
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Java Message Service It's also supp
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Java Message Service The FioranoMQ
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Java Message Service The iBus//Mobi
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9.6.2 Next Version Java Message Ser
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A.1.2 Connection Java Message Servi
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A.1.8 JMSException Java Message Ser
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} public void setInt(String name, i
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} public Message receiveNoWait( ) t
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} public void writeInt(int value) t
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A.2.6 QueueSender Java Message Serv
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} throws JMSException; public Conne
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Appendix B. Message Headers Java Me
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Java Message Service are delivered
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Java Message Service JMSTimestamp P
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UTC Java Message Service UTC (Unive
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Java Message Service JMSReplyTo Pur
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Java Message Service JMSType Purpos
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} public void setByteProperty(Strin
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Java Message Service Each of the ac
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JMSXAppID Java Message Service This
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Appendix D. Message Selectors Java
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The BETWEEN operator can be used to
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D.5 Declaring a Message Selector Ja
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Colophon Java Message Service Our l
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