The Java EE 6 Tutorial

Writing the Application Components for the clientsessionmdb Example

This application demonstrates how to send messages from an enterprise bean (in this case, a session bean) rather than from an application client, as in the example in Chapter 17, A Message-Driven Bean Example. Figure 31–3 illustrates the structure of this application.

Figure 31–3 An Enterprise Bean Application: Client to Session Bean to Message-Driven Bean

Diagram of application showing an application client
calling a session bean, which publishes a message that is consumed by a message-driven

The Publisher enterprise bean in this example is the enterprise-application equivalent of a wire-service news feed that categorizes news events into six news categories. The message-driven bean could represent a newsroom, where the sports desk, for example, would set up a subscription for all news events pertaining to sports.

The application client in the example injects the Publisher enterprise bean’s remote home interface and then calls the bean’s business method. The enterprise bean creates 18 text messages. For each message, it sets a String property randomly to one of six values representing the news categories and then publishes the message to a topic. The message-driven bean uses a message selector for the property to limit which of the published messages it receives.

Coding the Application Client:

The application client, clientsessionmdb-app-client/src/java/, performs no JMS API operations and so is simpler than the client in Chapter 17, A Message-Driven Bean Example. The client uses dependency injection to obtain the Publisher enterprise bean’s business interface:

static private PublisherRemote publisher;

The client then calls the bean’s business method twice.

Coding the Publisher Session Bean

The Publisher bean is a stateless session bean that has one business method. The Publisher bean uses a remote interface rather than a local interface because it is accessed from the application client.

The remote interface, clientsessionmdb-ejb/src/java/sb/, declares a single business method, publishNews.

The bean class, clientsessionmdb-ejb/src/java/sb/, implements the publishNews method and its helper method chooseType. The bean class also injects SessionContext, ConnectionFactory, and Topic resources and implements @PostConstruct and @PreDestroy callback methods. The bean class begins as follows:

public class PublisherBean implements PublisherRemote {

    private SessionContext sc;

    @Resource(lookup = "jms/ConnectionFactory")
    private ConnectionFactory connectionFactory;

    @Resource(lookup = "jms/Topic")
    private Topic topic;

The @PostConstruct callback method of the bean class, makeConnection, creates the Connection used by the bean. The business method publishNews creates a Session and a MessageProducer and publishes the messages.

The @PreDestroy callback method, endConnection, deallocates the resources that were allocated by the @PostConstruct callback method. In this case, the method closes the Connection.

Coding the Message-Driven Bean:

The message-driven bean class, clientsessionmdb-ejb/src/java/mdb/, is almost identical to the one in Chapter 17, A Message-Driven Bean Example. However, the @MessageDriven annotation is different, because instead of a queue the bean is using a topic with a durable subscription, and it is also using a message selector. Therefore, the annotation sets the activation config properties messageSelector, subscriptionDurability, clientId, and subscriptionName, as follows:

@MessageDriven(mappedName = "jms/Topic", activationConfig =  {
    @ActivationConfigProperty(propertyName = "messageSelector",
            propertyValue = "NewsType = 'Sports' OR NewsType = 'Opinion'")
    , @ActivationConfigProperty(propertyName = "subscriptionDurability",
            propertyValue = "Durable")
    , @ActivationConfigProperty(propertyName = "clientId",
            propertyValue = "MyID")
    , @ActivationConfigProperty(propertyName = "subscriptionName",
            propertyValue = "MySub")

Note –

For a message-driven bean, the destination is specified with the mappedName element instead of the lookup element.

The JMS resource adapter uses these properties to create a connection factory for the message-driven bean that allows the bean to use a durable subscriber.