O'Reilly - Java Message Service

Java Message Service
We can demonstrate durable subscriptions with the B2B example. If you still have the
Retailer application up and running, try simulating an abnormal shutdown by typing Ctrl-
C in the command window. Leave the Wholesaler running. In the command window for
the wholesaler application, type:
Surfboards, 500.00, 299.99
Hockey Sticks, 20.00, 9.99
Once the deals have been entered, restart the Retailer application:
java chap4.B2B.Retailer localhost username password
The first time you ran the Retailer application, a topic was registered as durable. When
you abnormally terminated the application, the subscription information was retained by
the JMS provider. When the Retailer application comes back up, the surfboards and
hockey sticks messages are received, processed, and responded to. Because the Retailer
had a durable subscription to the "Hot Deals" topic, the JMS server saved the messages
that arrived while the Retailer was down. The messages were then delivered when the
Retailer resubscribed to the topic.
Here's how we set up the durable subscription. A durable subscription is created by a
TopicSession object, the same as with a nondurable subscription. The Retailer class
obtains a durable subscription in its constructor:
public Retailer( String broker, String username, String password){
try {
...
hotDealsTopic = (Topic)jndi.lookup("Hot Deals");
javax.jms.TopicSubscriber subscriber =
session.createDurableSubscriber(hotDealsTopic,
"Hot Deals Subscription");
subscriber.setMessageListener(this);
connect.start( );
....
}
The createDurableSubscriber( ) method takes two parameters: a topic name, and a
subscription name. In our example we are using the String "Hot Deals Subscription" to
identify the subscription name. While topic names are specified as being supported as JMS
administered objects, subscription names are not. While not required by JMS, it is good
practice for a JMS provider to provide an administration tool that monitors active
subscription names, as illustrated in Figure 4.2.
A durable subscription's uniqueness is defined by the client ID and the subscription name.
In the event that the client disconnects without unsubscribing, a JMS provider will store
these messages until they can be delivered later. Upon reconnecting and resubscribing, the
JMS provider will match up the messages based on these two identifiers, and deliver them
to the subscriber.
You might think that the client ID and the topic would be enough for the provider to
uniquely identify a durable subscription. However, a client may have multiple
subscriptions on the same topic; for example, a client may want to use different message
selectors to sort the incoming messages. (Message selectors are discussed in detail in
62