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6.1 Sending a publish/subscribe JMS message 6.3 Creating a message-driven Enterprise JavaBean 7.8 Securing a message-driven bean
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6.3 Creating a message-driven Enterprise JavaBean
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You want to create a message-driven bean to contain business logic that will be triggered by a JMS message.
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Message-driven beans (added to the EJB 2.0 specification) are assigned to receive messages from a particular JMS message destination. These EJBs are ideal for executing business logic asynchronously and for exposing EJB applications to enterprise messaging systems. Message-driven beans use the same transaction models (see chapter 5) and declarative security (see chapter 7) as do session and entity beans. Another advantage of message-driven beans is that they can be used to process messages concurrently. EJB containers can create a pool of identical messagedriven beans that are able to process messages at the same time, generating a great deal of processing power.
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This recipe illustrates how to build a simple message-driven bean and create its XML descriptor. The class in listing 6.3 defines a message-driven bean. It implements the required MessageDrivenBean interface and the necessary MessageListener interface that allows the bean to receive JMS messages.
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Creating a message-driven Enterprise JavaBean
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Listing 6.3
SampleMDB.java
public class SampleMDB implements MessageDrivenBean, MessageListener { Implements the private MessageDrivenContext ctx; public void ejbRemove() { } public void ejbPassivate() { }
MessageDrivenBean and MessageListener interfaces
public void setMessageDrivenContext(MessageDrivenContext ctx) { this.ctx = ctx; } public void ejbCreate () throws CreateException { public void ejbActivate() { } }
public void onMessage( Message msg ) { MapMessage map = ( MapMessage ) msg; try { processMessage( map ); } catch(Exception ex) { ex.printStackTrace(); } }
Handles incoming messages
private void processMessage( MapMessage map ) throws Exception { //implementation not shown } }
Listing 6.4 contains the partial deployment XML file for the bean; notice how it indicates the source type of messages for the bean (either point-to-point or publish/subscribe).
Listing 6.4 Deployment descriptor
<ejb-jar> <enterprise-beans> <message-driven> <ejb-name>SampleMDB</ejb-name> <ejb-class>SampleMDB</ejb-class> <transaction-type>Container</transaction-type>
Messaging
<message-driven-destination> <destination-type>javax.jms.Topic</destination-type> </message-driven-destination> </message-driven> </enterprise-beans> <assembly-descriptor> </assembly-descriptor> </ejb-jar>
Describes the messaging type for this bean
Finally, you must perform the vendor-specific steps to assign the bean to an actual JMS message destination. The deployment XML describes only the type of messaging used by the message-driven bean, not the actual name of a topic or queue. Consult your application server documentation for more information. For example, the following XML could be used for the Weblogic application server:
<weblogic-ejb-jar> <weblogic-enterprise-bean> <ejb-name>SampleMDB</ejb-name> <message-driven-descriptor> <destination-jndi-name>BookJMSTopic</destination-jndi-name> </message-driven-descriptor> <jndi-name>ejb/SampleMDB</jndi-name> </weblogic-enterprise-bean> </weblogic-ejb-jar>
Discussion
As with all other types of EJBs, security and transaction control is implemented in the usual way. In some cases, transactions and security do have special considerations that you must take into account when dealing with message-driven beans. For example, you need a good way to prevent unwanted clients from sending messages to your message-driven EJBs and triggering business logic, and you also need to know how to handle rollbacks in the onMessage() method. In addition, you should keep in mind that message-driven beans are stateless, and you should therefore not attempt to keep any state information stored at a class level inbetween onMessage() invocations. The MessageDriveBean interface must be implemented in order to provide the bean with the appropriate bean methods, such as ejbRemove() and ejbCreate(). The Context object set in the bean is an instance of the MessageDrivenContext, which provides many of the methods found in the session and entity bean context classes. However, due to the nature of the message-driven bean, many of the
Processing messages in a FIFO manner from a message queue
context methods will throw an exception if used. Since a message-driven bean has no real EJB client (only the container that delivers the message), the getCallerPrincipal() and isCallerInRole() methods throw a runtime exception. In addition, message-driven beans have no home interfaces (and therefore have no home objects), so getEJBHome() and getEJBLocalHome() also throw runtime exceptions if used. Finally, since no EJB clients exist for a message-driven bean, the transaction context for the start of the onMessage() method is started by the container in the case of container-managed transactions, or by the bean itself in the case of bean-managed transactions.
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