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Working with message-driven beans
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Multithreading Your business application may require multithreaded message consumers that can process messages concurrently. MDBs help you avoid complexity because they handle multithreading right out of the box, without any additional code. They manage incoming messages among multiple instances of beans (in a pool) that have no special multithreading code themselves. As soon as a new message reaches the destination, an MDB instance is retrieved from the pool to handle the message, as figure 4.7 shows. This is popularly known as MDB pooling, which you ll learn about when we discuss the MDB lifecycle later in this chapter.
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Figure 4.7 As soon as a message arrives at its destination, the container retrieves it and assigns a servicing MDB instance from the pool.
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Simplified messaging code In addition, MDBs relieve you from coding the mechanical aspects of processing messages tasks such as looking up connection factories or destinations, creating connections, opening sessions, creating consumers, and attaching listeners. As you ll see when we build the Turtle message consumer MDB, all these tasks are handled behind the scenes for you. In EJB 3, using sensible defaults for common circumstances eliminates most of the configuration. In the worst-case scenario, you ll have to supply configuration information using simple annotations or through the deployment descriptor. Starting message consumption To start picking up messages from the shipping request queue, someone needs to invoke the appropriate method in your code. In a production environment, it is not clear how this will be accomplished. Starting message consumption through a user-driven manual process obviously is not desirable. In a server environment, almost every means of executing the method on server startup is highly system dependent, not to mention awkward. The same is true about stopping message
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Messaging and developing MDBs
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receipt manually. On the other hand, registered MDBs would be bootstrapped or torn down gracefully by the container when the server is started or stopped. We ll continue consolidating these three points as we start investigating a real example of developing MDBs soon. Before we do that, though, let s list the simple rules for developing an MDB.
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4.3.2 Programming rules
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Like all EJBs, MDBs are plain Java objects that follow a simple set of rules and sometimes have annotations. Don t take these rules too seriously yet; simply note them in preparation for going through the code-intensive sections that follow.
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The MDB class must directly (by using the implements keyword in the class declaration) or indirectly (through annotations or descriptors) implement a message listener interface. The MDB class must be concrete. It cannot be either a final or an abstract class. The MDB must be a POJO class and not a subclass of another MDB. The MDB class must be declared public. The bean class must have a no-argument constructor. If you don t have any constructors in your Java class, the compiler will create a default constructor. The container uses this constructor to create a bean instance. You cannot define a finalize method in the bean class. If any cleanup code is necessary, it should be defined in a method designated as PreDestroy. You must implement the methods defined in the message listener interface. These methods must be public and cannot be static or final. You must not throw the javax.rmi.RemoteException or any runtime exceptions. If a RuntimeException is thrown, the MDB instance is terminated.
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We ll apply these rules next in developing our example MDB.
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4.3.3 Developing a message consumer with MDB
Let s now explore developing an MDB by reworking the Turtle server JMS message consumer as an MDB. To make the code a bit more interesting, we ll actually implement the processShippingRequest method mentioned in the JMS code. Listing 4.2 shows the MDB code that first retrieves shipping requests sent to the queue and then saves each request in the Turtle database table named SHIPPING_REQUEST. Note that we re using JDBC for simplicity s sake and because it lets us demonstrate the MDB lifecycle methods for opening and closing JDBC
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