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Advantages of Using Message-Driven Beans
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One of the main benefits of the EJB architecture is that the EJB container vendor implements the infrastructure for your application, so you deal only with the applicationlevel details, such as your specialized business logic This approach is also reflected in the way message-driven beans are implemented The container automatically performs several actions on behalf of the bean: It creates a message consumer to receive messages, based on the settings configured at deployment time It registers the message listeners (in fact, you must not call setMessageListener yourself) The container also specifies a message acknowledgement mode, based on the bean deployment parameters Most important, the container takes care of pooling bean instances In other words, it creates a single bean instance for handling messages, and if the message load is high, it can create several such instances that serve incoming message concurrently The container also provides support for container-managed transactions It is important that you realize that these are not nearly as powerful as container-managed transactions for session beans or for entity beans The main difference here is that the message handler cannot expect to execute as part of the same distributed transaction as the party that sends the message The rationale for this is, of course, that asynchronous message processing is all about decoupling the sender and the receiver, so using the same transaction pretty much brings us back to synchronous communication The practical implication of a single transaction context is that if you allow one transaction to include both the sender and the recipient of a message, the application may run into a deadlock because the sender will not actually place the message at the destination (such as the queue) until the transaction commits The receiver will not commit until the message appears in the destination, and this might cause the whole application to hang 566
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So what does the container do when you ask for container-managed transactions It uses a single transaction scope for both reading the message and performing the onMessage business logic In case of failures of any kind, the transaction is rolled back, and in particular, an acknowledgement is not sent to the sending party Container-managed transactions for message-driven beans are similar to transactions for other EJBs in the sense that you do not need to include explicit calls to rollback and commit in your code In addition to container-managed transactions, you can use either bean-managed transactions or no transactions at all With bean-managed transactions, the container lets you implement your own transaction bracketing The transaction begins after the message-driven bean has received the message, and you can choose the acknowledgement mode in the deployment descriptor The transaction begins when you explicitly invoke the begin method of the UserTransaction object, which can be obtained from the bean context Because the transaction can begin only after the message is received, rolling back the transaction does not cause the message to be redelivered This is in contrast to the situation with container-managed transactions, in which an invocation of setRollbackOnly in the MessageDrivenContext method (or a rollback due to any external reason) causes the message to be redelivered When asking for no transactional support, the behavior of the container with respect to message acknowledgement is undefined The EJB specification does guarantee that the container will issue a message acknowledgement However, the container acknowledges the message at its own discretion; it does not provide any timing guarantees (That is, the acknowledgment may or may not be sent after the bean's onMessage method returns)
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Message-Driven Bean Example
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The following simple example illustrates the concepts of message-driven beans This bean receives text messages and prints out the message, preceded by the message provided b y the "Hello World" EJB from 5 The bean implements each of the required methods for message-driven beans: The ejbCreate method looks up the home interface of the Hello bean and creates one remote reference to that bean The ejbRemove method removes the session bean The setMessageDrivenContext method stores the context in the member variable ctx The onMessage method receives the message If the message is not a text message, an error message is printed Otherwise, the remote method is invoked to retrieve its text message, and a concatenation of that text message and the text of the input message is printed out The following example is somewhat artificial in that the remote bean provides a constant message, but it illustrates the idea of a message bean that invokes a session bean This is a common combination in that the session bean often serves as an entry point into the application logic, whereas the message-driven bean provides a managed way to expose this functionality in an asynchronous messaging environment
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import javaxjms*; import javaxejb*; import bookwebspherechapter5; 567
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public class HelloMsgBean implements MessageDrivenBean, MessageListener { private MessageDrivenContext ctx; private Hello helloBean; public void ejbCreate() { try { Properties env = SystemgetProperties(); envput(javaxnamingContextINITIAL_CONTEXT_FACTORY, "comibmejsnsjndiCNInitialContextFactory"); envput(javaxnamingContextPROVIDER_URL, "iiop://localhost"); InitialContext ic = new InitialContext(env); HelloHome home = (HelloHome) javaxrmiPortableRemoteObjectnarrow (iclookup("HelloBean")); helloBean = homecreate(); } catch (Exception ex) { exprintStackTrace(); } } public void ejbRemove() { try { Properties env = SystemgetProperties(); envput(javaxnamingContextINITIAL_CONTEXT_FACTORY, "comibmejsnsjndiCNInitialContextFactory"); envput(javaxnamingContextPROVIDER_URL, "iiop://localhost"); InitialContext ic = new InitialContext(env); HelloHome home = (HelloHome) javaxrmiPortableRemoteObjectnarrow( iclookup("HelloBean")); homeremove(helloBean); } catch (Exception ex) { exprintStackTrace(); } } 568
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public void setMessageDrivenContext(MessageDrivenContext c) { ctx = c; } public void onMessage(Message msg) { if (!(msg instanceOf TextMessage)) { Systemoutprintln("HelloMsgBean received wrong message type" + msggetClass()getName()); return; } try { TextMessage textMsg = (TextMessage) msg; String msgText = msggetText(); String helloText = null; helloText = helloBeangetTextMessage(); Systemoutprintln(helloText + textMsg); } catch (Exception ex) { exprintStackTrace(); } }
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