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2.4 Messaging with message-driven beans
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Just as session beans process direct business requests from the client, MDBs process indirect messages. In enterprise systems, messaging has numerous uses, including system integration, asynchronous processing, and distributed system communication. If you ve been working on enterprise development for some time, you re probably familiar with at least the idea of messaging. In the most basic terms, messaging involves communicating between two separate processes, usually across different machines. Java EE messaging follows this same idea just on steroids. Most significantly, Java EE makes messaging robust by adding a reliable middleman between the message sender and receiver. This idea is illustrated in figure 2.5. In Java EE terms, the reliable middleman is called a messaging destination, powered by message-oriented middleware (MOM) servers like IBM s MQSeries or Progress Software s SonicMQ. Java EE standardizes messaging through a well-known API, Java Messaging Service (JMS), upon which MDBs are heavily dependent.
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Figure 2.5 The Java EE pony express messaging model. Java EE adds reliability to messaging by adding a middleman that guarantees the delivery of messages despite network outages, even if the receiver is not present on the other end when the message is sent. In this sense, Java EE messaging has much more in common with the postal service than it does with common RPC protocols like RMI. We ll discuss this model in much greater detail in chapter 4.
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Messaging with message-driven beans
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We ll discuss messaging, JMS, and MDBs in much greater detail in chapter 4. For the moment, this is all you really need to know. Next, we ll build a simple example of message producer and an MBD. In our ActionBazaar example, we enable asynchronous order billing through messaging. To see how this is done, let s revisit the parts of the PlaceOrderBean introduced in listing 2.3 that we deliberately left hidden, namely the implementation of the billOrder method.
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2.4.1 Producing a billing message
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As we discussed in our high-level solution schematic in section 2.2, the PlaceOrderBean accomplishes asynchronous or out-of-process order billing by generating a message in the confirmOrder method to request that the order billing be started in parallel. As soon as this billing request message is sent to the messaging middleman, the confirmOrder method returns with the order confirmation to the user. We ll now take a look at exactly how this piece is implemented. As you can see in listing 2.5, the billing request message is sent to a messaging destination named jms/OrderBillingQueue. Since you have already seen most of the implementation of the PlaceOrder bean, we won t repeat a lot of the code shown in listing 2.3 here.
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Listing 2.5 PlaceOrderBean that produces the JMS message
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package ejb3inaction.example.buslogic; ... import import import import ...
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javax.annotation.Resource; javax.ejb.Remove; javax.ejb.Stateful; javax.jms.*;
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@Stateful public class PlaceOrderBean implements PlaceOrder { @Resource(name="jms/QueueConnectionFactory") private ConnectionFactory connectionFactory; @Resource(name="jms/OrderBillingQueue") private Destination billingQueue; ... @Remove public Long confirmOrder() { Order order = new Order(); order.setBidderId(bidderId); order.setItems(items);
Injects JMS resources
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order.setShippingInfo(shippingInfo); order.setBillingInfo(billingInfo); saveOrder(order); billOrder(order); return order.getOrderId(); } ... private billOrder(Order order) { try { Connection connection = connectionFactory.createConnection(); Session session = Contains JMS connection.createSession(false, setup code Session.AUTO_ACKNOWLEDGE); MessageProducer producer = session.createProducer(billingQueue); ObjectMessage message = session.createObjectMessage(); Creates and sends the message.setObject(order); message producer.send(message); producer.close(); session.close(); Releases JMS resources connection.close(); } catch(Exception e){ e.printStackTrace(); } }
Not surprisingly, the code to send the message in listing 2.5 is heavily dependent on the JMS API. In fact, that s all that the code in the billOrder method consists of. If you re familiar with JDBC, the flavor of the code in the method might seem familiar. The end result of the code is that the newly created Order object is sent as a message to a JMS destination named jms/OrderBillingQueue. We won t deal with the intricacies of JMS immediately, but we ll save a detailed discussion of this essential messaging API for chapter 4. It is important to note a few things right now, though. The first thing is that two JMS resources, including the message destination, are injected using the @Resource annotation b instead of being looked up.
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