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6.1 Sending a publish/subscribe JMS message 6.3 Creating a message-driven Enterprise JavaBean 6.9 Filtering messages for a message-driven EJB

In most enterprise situations, you want your asynchronous business functions to complete as quickly as possible. Since a message-driven bean processes a single message at a time, the waiting time for a single message increases as the number of messages delivered before it increases. In other words, if a single message takes a long period of time to complete, other messages experience a delay before processing. In critical applications, these messages should be processed as quickly as possible.

To speed up the consumption of messages, use a pool of message-driven beans. Each EJB is an instance of the single EJB class. With a pool of message-driven beans, you can consume more messages in a shorter time. Listing 6.16 shows a simple message-driven bean used to consume messages.

public class MessageBean implements MessageDrivenBean, MessageListener { private MessageDrivenContext ctx; public void ejbRemove() { } public void ejbActivate() { } public void ejbPassivate() { } public void setMessageDrivenContext(MessageDrivenContext ctx) { this.ctx = ctx; } public void ejbCreate () throws CreateException { } public void onMessage(Message msg) { MapMessage map= (MapMessage) msg; try { String symbol = map.getString("Symbol"); String description = map.getString("Description"); System.out.println("MDB received Symbol : " + symbol + " " + description ); } catch(Exception ex) { ex.printStackTrace(); } } }

To ensure a single message is not duplicated across instances in the message-driven bean pool, the message-driven bean instances should use a message queue as the destination type. Listing 6.17 contains the deployment descriptor for the bean.

<enterprise-beans> <message-driven> <ejb-name>concurrentMDB</ejb-name> <ejb-class>concurrent.MessageBean</ejb-class> <transaction-type>Container</transaction-type> <message-driven-destination> <destination-type>javax.jms.Queue</destination-type> </message-driven-destination> </message-driven> </enterprise-beans>

Message-driven bean instance pools are created in a vendor-specific manner. Listing 6.18 shows how this is accomplished using the Weblogic application server. Notice the vendor XML creates a pool maximum size of five beans, with an initial size of two beans.

<weblogic-ejb-jar> <weblogic-enterprise-bean> Sets up the message<ejb-name>concurrentMDB</ejb-name> driven bean pool <message-driven-descriptor> <pool> <max-beans-in-free-pool>5</max-beans-in-free-pool> <initial-beans-in-free-pool>2</initial-beans-in-free-pool> </pool> <destination-jndi-name>BookJMSQueue</destination-jndi-name> </message-driven-descriptor> <jndi-name>concurrent.MBD</jndi-name> </weblogic-enterprise-bean> </weblogic-ejb-jar>

Using a bean pool is a quick and dirty way to achieve concurrent processing of messages. The pool, combined with a message queue, provides a way to process many messages at once without duplicating messages across instances. Creating an environment like this allows messages to start processing instead of waiting for previous messages to complete. You should use this type of processing only when

concurrent processing of messages will not cause problems in your business logic or invalid states in your data.

Message-driven beans that subscribe to a topic or receive messages from a queue should be able to handle messages of the wrong type (which should not invoke the message-driven business logic). Beans should just politely discard these messages when they are encountered. This is especially true for message-driven beans that exist in an environment with many different beans that watch a single source for incoming messages. However, it would be more efficient to avoid the execution time used for discarding messages and instead avoid receiving unwanted messages.

To selectively deliver messages to a message-driven bean, the bean should be deployed with a message selector. The bean source needs no changes in order to use the message selector. Listing 6.19 shows a simple message-driven bean that only wants messages that contain an attribute UserRole set to "BuyerRole". The bean prints out the role of the incoming message for verification.

public class MessageBean implements MessageDrivenBean, MessageListener { private MessageDrivenContext ctx; public void onMessage(Message msg) { MapMessage map=(MapMessage)msg; try { String role = map.getString( "UserRole" );

System.out.println("Received Message for Role: " + role); ProcessTheMessage( message ); } catch(Exception ex) { ex.printStackTrace(); } } }

In the XML descriptor for the bean, you describe the message selector that filters undesired messages for the message-driven bean. Listing 6.20 shows the partial XML descriptor that describes the simple EJB and its message selector.