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Developing Message Driven beans in NetBeans Today, I would like to start develop application that uses MDB bean in application tier. I hope that everyone knows Message Driven beans. If not, I shortly describe it. MDB is an enetrprise bean that enables J2EE applications to process messages asynchronously. JMS API provides two different approach for messaging:
Point-to-point messaging: Each message is addressed to a specific queue, and receiving clients extract messages from the queues established to hold their messages. Queues retain all messages sent to them until the messages are consumed or until the messages expire.
Publish/Subscribe: Clients address messages to a topic, which functions somewhat like a bulletin board. Publishers and subscribers are generally anonymous and can dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a topic's multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to current subscribers.
Now, we know basic approach and I will describe other terminology. We need connection factory and destination for sending/receiving messages. A connection factory is the object a client uses to create a connection to a provider. A connection factory encapsulates a set of connection configuration parameters that has been defined by an administrator JMS server. A destination is the object a client uses to specify the target of messages it produces and the source of messages it consumes. In the PTP messaging domain, destinations are called queues. In the pub/sub messaging domain, destinations are called topics.
This application will send orders from J2SE client in application tier. Order's parameters are encapsulated in Order class. The class is shared between client and EJB module, therefore creation of library module with this class is helpfulness.

• Create new J2SE library project
• Add new Order class in this project. This class represents transfer object that encapsulates all info about order.

Sources of the library project are avalaible here.
Let's jump in EJB module:

• Create EJB project
• Add library project on classpath
• Add new MDB bean, select topic as destination type. I think topic is better for this use case since we can add other MDB that will listen and then process order, e.g. billing , accounting systems
• IDE generate durable subscription for Topic by default. I wrote that topic retains messages only as long as it takes to distribute them to current subscribers. It means when subscribers are inactive (MDB is disabled) all messages for them are lost. This issue resolves durable subscription which receive messages published while the subscriber is not active. Durable subscriptions offer the reliability of queues to the publish/subscribe message domain. You can check these options in ejb-jar.xml file.
• Implement onMessage method that is invoked when topic receives message:

        public void onMessage(Message aMessage) {
          try{
          if (aMessage instanceof ObjectMessage){
              // process message
              Order order = (Order)((ObjectMessage) aMessage).getObject();
              logger.log(Level.SEVERE,order.toString());
          }else{
              logger.log(Level.SEVERE,
                      "Get message that isn't supported, only ObjectMessage is supported");
          }
          }catch(JMSException ex){
              logger.log(Level.SEVERE, ex.getMessage());
          }
      }
      

  We support only Object messages in this MDB. JMS API defines many types of messages, e.g. Text, Map, Stream and Byte messages. Different between bytes and stream messages is that stream messages are a stream of primitive values in Java. BytesMessage allows data to be read using any type. Thus even if your payload contains a long value, you can invoke a method to read a short and it will return you something. It will not give you a semantically correct data but the call will succeed in reading the first two bytes of data. In the code above message is typed to ObjectMessage that Order object is obtained and then printed in log.
• Under Server Resources are destination and connection factory generated automatically. These resources are registered during module deployment.
• Deploy EJB project in Sun Application server. EJB project module sources are avalaible here.
I will describe JMS client in next post.

Posted by pblaha ( Nov 23 2005, 05:47:23 PM CET ) Permalink Comments [4]