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If relational theory is the bedrock of the relational database, SQL is the crown jewels. Java developers with strong OO roots may find SQL s verbose syntax and unmistakably relational feel less than ideal. The truth is that even O/R solutions such as the EJB 3 Persistence API generate SQL under the hood. The fact that you use O/R is no excuse not to have a solid understanding of SQL, particularly during debugging and fine-tuning. SQL (which stands for Structured Query Language) arose as a result of the initial relational research conducted at IBM. The American National Standards Institute (ANSI) has since standardized SQL. Almost all major databases comply with the SQL -92 standard for the most part. Even then, writing portable SQL is a tricky business at best. Luckily, O/R relieves us from this meticulous work to some degree by automatically generating SQL suited to a particular database. SQL statements include the familiar CREATE, INSERT, DELETE, UPDATE, and, of course, everyone s favorite, SELECT. As a testament to the power of the SELECT statement, some elements of it have been ported over into the O/R world through EJB-QL (which we cover in chapter 10).

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public interface EquityHome extends EJBHome { public Equity create(String symbol, String description) throws CreateException, RemoteException; public Equity findByPrimaryKey(String primaryKey) throws FinderException, RemoteException;

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.

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.

//home method public int getCountOfSymbols() throws RemoteException; }

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.

server.registerMBean(statusMonitor,statusMonitorName); statusMonitor.setNotifyDiffer(true); statusMonitor.setNotifyMatch(true); statusMonitor.setStringToCompare("OK"); statusMonitor.setObservedObject(new ObjectName("OrderProcess:name=stats")); statusMonitor.setObservedAttribute("Status"); statusMonitor.setGranularityPeriod(100L); statusMonitor.addNotificationListener( new NotificationListener() { public void handleNotification(Notification notification, Object handback) { if (notification.getType().equals("jmx.monitor.string.differs")) { System.out.println("JMX notification - Abnormal process "); } else { System.out.println("JMX notification - Process OK"); } } }, null, null); statusMonitor.start();

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.

javax.annotation.Resource; javax.ejb.Remove; javax.ejb.Stateful; javax.jms.*;