O'Reilly - Java Message Service

Java Message Service
JMSPriority Purpose: Routing
Messages may be assigned a priority by the message producer when they are delivered.
The message servers may use message's priority to order delivery of messages to
consumers; messages with a higher priority are delivered ahead of lower priority messages.
The message's priority is contained in the JMSPriority header, which is set automatically
by the JMS provider. The priority of messages can be declared by the JMS client using the
setPriority( ) method on the producer. The following code shows how this method is
used by both the p2p and pub/sub message models:
// p2p setting the message priority to 9
QueueSender queueSender = QueueSession.createSender(someQueue);
queueSender.setPriority(9);
//pub/sub setting the message priority to 9
TopicPublisher topicPublisher = TopicSession.createPublisher(someTopic);
topicPublisher.setPriority(9);
Once a priority is established on a producer (QueueSender or TopicPublisher), that priority
will be used for all messages delivered from that producer, unless it is explicitly
overridden. The priority of a specific message can be overridden during the send operation.
The following code shows how to override the priority of a message during the send
operation. In both cases, the priority is set to 3:
// p2p setting the priority on the send operation
QueueSender queueSender = QueueSession.createSender(someQueue);
queueSender.send(message,DeliveryMode.PERSISTENT, 3, 0);
// pub/sub setting the priority on the send operation
TopicPublisher topicPublisher = TopicSession.createPublisher(someTopic);
topicPublisher.publish(message,DeliveryMode.PERSISTENT, 3, 0);
There are two basic categories of message priorities: levels 0-4 are gradations of normal
priority; levels 5-9 are gradations of expedited priority. Message servers are not required to
enforce message ordering based on the JMSPriority header, but they should attempt to
deliver expedited messages before normal messages.
The JMSPriority header is set automatically when the message is delivered. It can be read
by JMS clients using the Message.getJMSPriority( ) method, but it is mostly used by
message servers when routing messages.
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