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Messaging and developing MDBs
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Figure 4.5 The publish-subscribe messaging model with one producer and three consumers. Each topic subscriber receives a copy of the message.
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At this point, you should have a good conceptual foundation of messaging and are perhaps eager to get a taste of some code. Next, we take a brief look at JMS and implement the ActionBazaar message producer for sending the message.
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The request-reply model
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In the ActionBazaar example, you might want a receipt confirmation from Turtle once they have the shipping request you sent to the queue. A third kind of model called request-reply comes in handy in these kinds of situations. In this model, we give the message receiver enough information so that they might call us back. This is known as an overlay model because it is typically implemented on top of either the PTP or pub-sub models. For example, in the PTP model the sender specifies a queue to be used to send a reply to (in JMS, this is called the reply to queue) as well as a unique ID shared by both the outgoing and incoming messages (known as the correlation ID in JMS). The receiver receives the message and sends a reply to the reply queue, copying the correlation ID. The sender receives the message in the reply queue and determines which message received a reply by matching the correlation ID.
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Introducing Java Messaging Service
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4.2 Introducing Java Messaging Service
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In this section we provide an overview to JMS API by building a basic message producer. JMS is a deceptively simple and small API to a very powerful technology. The JMS API is to messaging what the Java Database Connectivity (JDBC) API is to database access. JMS provides a uniform, standard way of accessing MOM in Java and is therefore an alternative to using product-specific APIs. With the exception of Microsoft Message Queuing (MSMQ), most major MOM products support JMS. The easiest way to learn JMS might be by looking at code in action. We re going to explore JMS by first developing the ActionBazaar code that sends out the shipping request. We develop a message producer using JMS and learn about structure of the message interface; then in the next section, we develop the message consumer using MDBs.
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4.2.1 Developing the JMS message producer
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As we described in our scenario in section 4.1.2, when a user places an order for an ActionBazaar Special, a shipping request is sent to a queue shared between ActionBazaar and Turtle. The code in listing 4.1 sends the message out and could be part of a method in a simple Java object invoked by the ActionBazaar application. All relevant shipping information such as the item number, shipping address, shipping method, and insurance amount is packed into a message and sent out to ShippingRequestQueue.
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Listing 4.1 JMS code that sends out shipping requests from ActionBazaar
@Resource(name="jms/QueueConnectionFactory") private ConnectionFactory connectionFactory; @Resource(name="jms/ShippingRequestQueue") private Destination destination;
Injects connection factory and destination
Connection connection = connectionFactory.createConnection(); Session session = connection.createSession(true, Session.AUTO_ACKNOWLEDGE); MessageProducer producer = session.createProducer(destination); ObjectMessage message = session.createObjectMessage(); ShippingRequest shippingRequest = new ShippingRequest(); shippingRequest.setItem(item); Creates shippingRequest.setShippingAddress(address); payload shippingRequest.setShippingMethod(method); shippingRequest.setInsuranceAmount(amount);
Connects, creates session, producer
Creates message
Messaging and developing MDBs
message.setObject(shippingRequest); producer.send(message); session.close(); connection.close();
Sets payload
Sends message
As we explain each logical step of this code in the following sections, we ll go through a large subset of the JMS API components and see usage patterns. Note that for simplicity we have removed the code for exception handling. Retrieving the connection factory and destination In JMS, administrative objects are similar to JDBC javax.sql.DataSource objects. These are resources that are created and configured outside the code and stored in JNDI. JMS has two administrative objects: javax.jms.ConnectionFactory and javax.jms.Destination, both of which we use in listing 4.1. We then retrieve the connection factory using dependency injection with the @Resource annotation, and the connection factory encapsulates all the configuration information needed to connect to MOM. We also inject the queue to forward the shipping request to, aptly named, ShippingRequestQueue. With EJB 3, using resources is much easier; you don t have to deal with the complexity of JNDI or configure resource references in deployment descriptors. 5 discusses dependency injection in greater detail. The next step in listing 4.1 is creating a connection to MOM and getting a new JMS session. Opening the connection and session The javax.jms.Connection object represents a live MOM connection, which we create using the createConnection method of the connection factory. Connections are thread-safe and designed to be sharable because opening a new connection is resource intensive. A JMS session (javax.jms.Session), on the other hand, provides a single-threaded, task-oriented context for sending and receiving messages. We create a session from the connection using the createSession method. The first parameter of the method specifies whether the session is transactional. We ve decided that our session should be transactional and therefore set the parameter to true. This means that the requests for messages to be sent won t be realized until either the session s commit method is called or the session is closed. (If the session isn t transactional, messages will be sent as soon as the send method is invoked.) The second parameter of the createSession method specifies the
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