How do I use message-driven bean (MDB)?, part 2

How do I use message-driven bean (MDB)?, part 2

How do I use message-driven bean (MDB)?

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Continued from part 1.

A message-driven bean (MDB) is an enterprise bean that allows enterprise applications to handle messages asynchronously. An instance of an MDB is instantiated by an EJB container to act as a message listener. Messages received by an MDB may be sent by any JMS-compatible message transmitter, such as a JMS-enabled publish/subscribe topic.

Unlike session and entity beans, a client program does not direct invocations at an MDB. Instead, the MDB receives invocations from the message source through the standard callback method, onMessage(), in the same manner as a JMS message listener.

A Message Driven Bean must implement two interfaces:

  1. javax.jms.MessageListener - This interface defines the onMessage callback method. When a message is put on the queue/topic, the onMessage method of the message-driven bean is called by the EJB container and passed the actual message.
  2. javax.ejb.MessageDrivenBean - This is the EJB interface that contains the EJB lifecycle methods:
    • ejbCreate() - called by the EJB container when the message-driven bean is created
    • ejbRemove() - called by the EJB container when the message-driven bean is destroyed or removed from the EJB pool
    • setMessageDrivenContext(MessageDrivenContext context) - called prior to ejbCreate and passed the message-driven context by the EJB container. The context has runtime information such as transaction data.

A message-driven bean must declare deployment information about itself in a deployment-descriptor file named ejb-jar-xml. The EJB container handles the duties of subscribing the bean to the topic or connecting it to the queue based on information placed in the deployment descriptor.

The ejb-jar.xml file contains:

  • The fully-qualified class name of the message-driven bean
  • A name for the message-driven bean
  • The destination type of the bean
  • Transaction attributes
  • Security information

    The following is an example of a typical ejb-jar.xml file:

    <ejb-jar>
   <enterprise-beans>
      <message-driven>
         <ejb-name>MyMDB</ejb-name>
         <ejb-class>com.jeffhanson.ejb.MyMDB</ejb-class>
         <transaction-type>Container</transaction-type>
         <message-driven-destination>
            <destination-type>javax.jms.Topic</destination-type>
         </message-driven-destination>
         <security-identity>
            <run-as-specified-identity>
               <role-name>system</role-name>
            </run-as-specified-identity>
         </security-identity>
      </message-driven>
   </enterprise-beans>
</ejb-jar>
    Message-driven beans are not located by client classes and client classes do not directly invoke methods on them. All access to a message-driven bean is through a JMS topic or queue which directs messages at the message-driven bean through the EJB container. The EJB container ultimately passes the JMS message to the message-driven bean through the bean's onMessage method. All message-driven beans must implement the javax.ejb.MessageDrivenBean and javax.jms.MessageListener interfaces, as the following example illustrates:

    public class MyMDB
   implements MessageDrivenBean, MessageListener
{
    private transient MessageDrivenContext ctx = null;

    public MyMDB()
    {
    }

    public void setMessageDrivenContext(MessageDrivenContext ctx)
    {
        this.ctx = ctx;
    }

    public void ejbCreate()
    {
    }

    public void onMessage(Message message)
    {
       System.out.println("MDB Message received: "
                          + message.toString());
    }

    public void ejbRemove()
    {
    }
}

    This was first published in October 2004

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