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Working with message-driven beans
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connections. We recommend that you consider EJB 3 Java Persistence API (discussed in part 3) for persisting your data instead of using straight JDBC.
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Listing 4.2 Turtle server shipping request processor MDB
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package ejb3inaction.example.buslogic; import import import import import import import import import import javax.ejb.MessageDriven; javax.ejb.ActivationConfigProperty; javax.annotation.PostConstruct; javax.annotation.PreDestroy; javax.annotation.Resource; javax.jms.JMSException; javax.jms.Message; javax.jms.MessageListener; java.sql.*; javax.sql.*;
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@MessageDriven( name="ShippingRequestProcessor", activationConfig = { @ActivationConfigProperty( propertyName="destinationType", Defines propertyValue="javax.jms.Queue"), @MessageDriven @ActivationConfigProperty( annotation propertyName="destinationName", propertyValue="jms/ShippingRequestQueue") } ) public class ShippingRequestProcessorMDB implements MessageListener { Implements message listener private java.sql.Connection connection; private DataSource dataSource;
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@Resource private MessageDrivenContext context;
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Injects MessageDrivenContext
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Uses resource
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@Resource(name="jdbc/TurtleDS") injection public void setDataSource(DataSource dataSource) { this.dataSource = dataSource; } @PostConstruct public void initialize() { try { connection = dataSource.getConnection(); } catch (SQLException sqle) { sqle.printStackTrace(); } } @PreDestroy
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Implements onMessage method
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Messaging and developing MDBs
public void cleanup() { try { connection.close(); connection = null; } catch (SQLException sqle) { sqle.printStackTrace(); } } public void onMessage(Message message) { Defines lifecycle callbacks try { ObjectMessage objectMessage = (ObjectMessage)message; ShippingRequest shippingRequest = (ShippingRequest)objectMessage.getObject(); processShippingRequest(shippingRequest); } catch (JMSException jmse) { jmse.printStackTrace(); context.setRollBackOnly();(); } catch (SQLException sqle) { sqle.printStackTrace(); context.setRollBackOnly(); } }
Processes business logic
private void processShippingRequest(ShippingRequest request) throws SQLException { Statement statement = connection.createStatement(); statement.execute( "INSERT INTO " + "SHIPPING_REQUEST (" + "ITEM, " + "SHIPPING_ADDRESS, " + "SHIPPING_METHOD, " + "INSURANCE_AMOUNT ) " + "VALUES ( " + request.getItem() + ", " + "\'" + request.getShippingAddress() + "\', " + "\' " + request.getShippingMethod() + "\', " + request.getInsuranceAmount() + " )"); } }
The @MessageDriven annotation b identifies this object as an MDB and specifies the MDB configuration, including the fact that we are listening on the shipping request queue. Our code then marks this MDB as a JMS message listener C. The onMessage method implements the message listener interface F and processes incoming messages. A message-driven context is injected D and used inside the
Working with message-driven beans
F to roll back transactions as needed. A database resource is injected E. The lifecycle callbacks G open and close a connection derived from
onMessage method
the database resource. Finally, the shared JDBC connection is used by the business logic H called in onMessage to save each shipping request into the database. Next, we ll examine the major MDB features by analyzing this code in greater detail, starting with the @MessageDriven annotation.
4.3.4 Using the @MessageDriven annotation
MDBs are one of the simplest kinds of EJBs to develop, and they support the smallest number of annotations. In fact, the @MessageDriven annotation and the @ActivationConfigProperty annotation nested inside it are the only MDB -specific annotations. The @MessageDriven annotation in our example represents what
you ll typically use most of the time. The annotation is defined as follows:
@Target(TYPE) @Retention(RUNTIME) public @interface MessageDriven { String name() default ""; Class messageListenerInterface default Object.class; ActivationConfigProperty[] activationConfig() default {}; String mappedName(); String description(); }
Notice that all three of the annotation s arguments are optional. If you are a minimalist, you can keep the annotation as simple as this:
@MessageDriven public class ShippingRequestProcessorMDB
and leave any details to be added elsewhere, such as in the deployment descriptor. The first element, name, specifies the name of the MDB in our case, ShippingRequestProcessor. If the name element is omitted, the code uses the name of the class, ShippingRequestProcessorMDB, in our example. The second parameter, messageListenerInterface, specifies which message listener the MDB implements. The last parameter, activationConfig, is used to specify listener-specific configuration properties. Let s take a closer look at the two last parameters.
4.3.5 Implementing the MessageListener
An MDB implements a message listener interface for the very same reason our plain JMS consumer implemented the javax.jms.MessageListener interface. The container uses the listener interface to register the MDB with the message provider
Messaging and developing MDBs
and to pass incoming messages by invoking implemented message listener methods. Using the messageListenerInterface parameter of the @MessageDriven annotation is just one way to specify a message listener; we could have done the following instead:
@MessageDriven( name="ShippingRequestJMSProcessor", messageListenerInterface="javax.jms.MessageListener") public class ShippingRequestProcessorMDB {
However, we chose to omit this parameter and specified the interface using the implements keyword:
public class ShippingRequestProcessorMDB implements MessageListener {
Yet another option is to specify the listener interface through the deployment descriptor and leave this detail out of our code altogether. The approach you choose is largely a matter of taste. We prefer the second approach because it resembles our JMS example. MDBs let you specify a message listener with relative flexibility, which is especially cool if you consider the following scenario: suppose that we decide to switch messaging technologies and use Java API for XML Messaging (JAXM) to send shipping requests instead of JMS. (JAXM is essentially a SOAP-based XML messaging API. For more information, visit http://java.sun.com/webservices/jaxm/.) Thanks to JCA support, we can use still use MDBs to receive shipping requests (see the sidebar JCA Connectors and Messaging to learn how this might be done). All we have to do is switch to the JAXM message listener interface, javax.jaxm. OneWayMessageListener, instead of using javax.jms.MessageListener. We can reuse most of the MDB code and configuration:
public class ShippingRequestProcessorMDB implements javax.jaxm.OneWayMessageListener {
However you choose to specify the message listener, make sure you provide a valid implementation of all methods required by your message listener especially when using the deployment descriptor approach, where there are no compiletime checks to watch your back. Next, let s take a look at the last (but definitely not least) parameter of the @MessageDriven annotation: activationConfig.
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