Synopsis

There are two types of delivery modes in JMS: persistent and nonpersistent. A persistent message should be delivered once-and-only-once, which means that a message is not lost if the JMS provider fails; it will be delivered after the server recovers. A nonpersistent message is delivered at-most-once, which means that it can be lost and never delivered if the JMS provider fails. In both persistent and nonpersistent delivery modes the message server should not send a message to the same consumer more than once, but it is possible; see the section on JMSRedelivered for more details.

Persistent messages are intended to survive system failures of the JMS provider (the message server). Persistent messages are written to disk as soon as the message server receives them from the JMS client. After the message is persisted to disk the message server can then attempt to deliver the message to its intended consumer. As the messaging server delivers the message to the consumers it keeps track of which consumers successfully receive the message. If the JMS provider fails while delivering the message, the message server will pick up where it left off following a recovery. Persistent messages are delivered once-and-only-once. The mechanics of this are covered in greater detail in Chapter 6.

Nonpersistent messages are not written to disk when they are received by the message server, so if the JMS provider fails, the message will be lost. In general nonpersistent messages perform better than persistent messages. They are delivered more quickly and require less system resources on the message server. However, nonpersistent messages should only be used when a loss of messages due to a JMS provider failures is not an issue. The chat example used in Chapter 2 is a good example of a system that doesn't require persistent delivery. It's not critical that every message be delivered to all consumers in a chat application. In most business systems, however, messages are delivered using the persistent mode, because it's important that they be successfully delivered.

The delivery mode can be set using the setDeliveryMode( ) method defined in both the TopicPublisher and QueueSender message producers. The javax.jms.DeliveryMode class defines the two constants used to declare the delivery mode: PERSISTENT and NON_PERSISTENT :

// Publish-and-subscribe
TopicPublisher topicPublisher = topicSession.createPublisher(topic);
topicPublisher.setDeliveryMode(DeliveryMode.NON_PERSISTENT);

// Point-to-point
QueueSender queueSender = queueSession.createSender(queue);
queueSender.setDeliverMode(DeliveryMode.PERSISTENT);

Once the delivery mode has been set on the message producer, it will be applied to all the messages delivered by that producer. The delivery mode can be changed at any time using the setDeliveryMode( ) method; the new mode will be applied to subsequent messages. The default delivery mode of a message producer is always PERSISTENT.

The delivery mode of a message producer can be overridden for an individual message during the send operation, which allows a message producer to deliver a mixture of persistent and nonpersistent messages to the same destination (topic or queue):

// Publish-and-subscribe
Message message = topicSession.createMessage(  );
topicPublisher.publish(message, DeliveryMode.PERSISTENT, 5, 0);

// Point-to-point
Message message = queueSession.createMessage(  );
queueSender.send(message, DeliveryMode.NON_PERSISTENT, 5, 0);

The JMSDeliveryMode can be obtained from the Message object using the getJMSDeliveryMode( ) method:

public void onMessage(Message message){
    try {
     if (message.getJMSDeliveryMode(  ) == DeliveryMode.PERSISTENT){
	      // Do something
     } else {
	      // Do something else
     }
    } catch (JMSException jmse){jmse.printStackTrace(  );}
}