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Identify the interfaces and methods a JMS message-driven bean must implement.
Posted on: April 18, 2011 at 12:00 AM
This page discusses - Identify the interfaces and methods a JMS message-driven bean must implement.

Identify the interfaces and methods a JMS message-driven bean must implement.

All message-driven beans MUST implement, directly or indirectly, the MessageDrivenBean interface. The class MUST be defined as public and it cannot be defined as final nor abstract. The MessageDrivenBean interface defines TWO methods all message-driven beans must implement:

  1. The setMessageDrivenContext(...) method is called by the bean's container to associate a message-driven bean instance with its context maintained by the container. Typically a message-driven bean instance retains its message-driven context as part of its state. The container calls this method at the beginning of the bean's life cycle.

  2. The ejbRemove() notification signals that the instance is in the process of being removed by the container. In the ejbRemove() method, the instance releases the resources that it is holding (possibly allocated in the ejbCreate() method).

The Bean Provider CANNOT assume that the Container will always invoke the ejbRemove() method on a message-driven bean instance. The following scenarios result in ejbRemove() NOT being called on an instance:

  • A crash of the EJB Container.

  • A system exception thrown from the instance's method to the Container.

If the message-driven bean instance allocates resources in the ejbCreate() method and/or in the onMessage(...) method, and releases normally the resources in the ejbRemove() method, these resources will NOT be automatically released in the above scenarios. The application using the message-driven bean should provide some clean up mechanism to periodically clean up the unreleased resources.

The MessageDrivenBean extends the EnterpriseBean interface:

package javax.ejb;

public interface MessageDrivenBean extends javax.ejb.EnterpriseBean {
	public void setMessageDrivenContext(MessageDrivenContext context)
		throws EJBException;
	public void ejbRemove() throws EJBException;
}

public interface EnterpriseBean extends java.io.Serializable {
}

					

All message-driven beans MUST implement, directly or indirectly, the javax.jms.MessageListener interface.

The onMessage(...) method is called by the bean's CONTAINER when a message has arrived for the bean to service. The onMessage(...) method contains the business logic that handles the processing of the message. The onMessage(...) method has a single argument, the incoming message.

ONLY message-driven beans can asynchronously receive messages. Session and entity beans are NOT PERMITTED to be JMS MessageListeners.

package javax.jms;

public interface MessageListener {
	public void onMessage(Message message);
}
					

The container creates an instance of a message-driven bean in THREE steps. First, the container calls the bean class' newInstance() method to create a new message-driven bean instance. Second, the container calls the setMessageDrivenContext(...) method to pass the context object to the instance. Third, the container calls the instance's ejbCreate() method.

Each message-driven bean class MUST have one ejbCreate() method, with NO ARGUMENTS.

EVERY message-driven bean class MUST implement following FOUR methods:

  1. public void setMessageDrivenContext(MessageDrivenContext mdc)

    The container normally calls this method exactly once, after instantiation, to pass in the associated MessageDrivenContext.

  2. public void ejbCreate()

    Called after the setMessageDrivenContext(...) method. This is a good time to access or obtain any resources that will be used for the life of the bean.

  3. public void onMessage(Message msg)

    Called by the container when a message has arrived on the bean's associated Queue or Topic. Your code should check for the expected message types, using the instanceof operator, because there is nothing to stop a client from sending any of the available message types. Other than those that deal with the Message object, no JMS methods need be invoked by the bean; it just cracks or parses the received message and performs any associated operations.

  4. public void ejbRemove()

    Called when the container intends to terminate the bean. All resources should be released at this time.

public class ReservationProcessorBean
	implements javax.ejb.MessageDrivenBean, 
			javax.jms.MessageListener  {
   
	MessageDrivenContext ejbContext;
	Context jndiContext;
   
	public void setMessageDrivenContext (MessageDrivenContext mdc)  {
		ejbContext = mdc;
		try {
			jndiContext = new InitialContext ();
		} catch(NamingException ne) {
			throw new EJBException (ne);
		}
	}
   
	public void ejbCreate () {}
   
	public void onMessage (Message message) {
		try  {         
			MapMessage reservationMsg = (MapMessage)message;
         
			Integer customerPk = (Integer)
				reservationMsg.getObject ("CustomerID");
			Integer cruisePk =   (Integer)
				reservationMsg.getObject ("CruiseID");
			....
         
			// can access other EJBs
			ReservationHomeLocal resHome = (ReservationHomeLocal)
			jndiContext.lookup ("java:comp/env/ejb/ReservationHomeLocal");
         
			ReservationLocal reservation =
			resHome.create (customer, cruise, cabin, price, new Date ());
			....         
         
		} catch(Exception e) {
			// can throw only runtime (unchecked) exceptions
			throw new EJBException (e);
		}
	}
	
	...
   
	public void ejbRemove () {
		try {
			jndiContext.close ();
			ejbContext = null;
		} catch(NamingException ignored) { }
	}
}
					
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