O'Reilly - Java Message Service

Java Message Service
The receive( ) method with no parameters blocks indefinitely, until a message is
received. The receive(long timeout) method blocks until a message is received, or until
the timeout period expires, whichever comes first. The receive( ) method will return null
if the session is closed while the method is blocking. The receiveNoWait( ) method does
not block at all. It either returns a message if one is available, or it returns null, if there is
nothing currently pending to be delivered. Here is a slightly modified version of
Wholesaler.publishPriceQuotes() that makes use of the receive( ) method:
private void publishPriceQuotes(String dealDesc, String username,
String itemDesc, float oldPrice,
float newPrice){
...
System.out.println("\nInitiating Synchronous Request");
// Publish the message persistently
publisher.publish(
msg, //message
javax.jms.DeliveryMode.PERSISTENT, //publish persistently
javax.jms.Message.DEFAULT_PRIORITY,//priority
MESSAGE_LIFESPAN); //Time to Live
javax.jms.Message aMessage = subscriber.receive( );
System.out.println("\nRequest Sent, Reply Received!");
if (aMessage != null)
{
onMessage(aMessage);
}
...
}
In this example the subscriber, which subscribes to the "Buy Order" temporary topic, has
its receive( ) method called. The receive( ) method blocks until a message is published
by the Retailer to the "Buy Order" topic. The Wholesaler client becomes a synchronous
client waiting for the Retailer to respond. When the receive( ) method returns with a
message, the Wholesaler simply calls onMessage( ) directly to process the message.
Due to threading restrictions imposed on a JMS session object, it is impractical to have
both synchronous and asynchronous operations on a session. Hence the Wholesaler's
constructor does not make a call to setMessageListener(this). The onMessage( ) handler
will never get called automatically.
The recipient side of the conversation still looks the same as in our previous example. The
Retailer.autoBuy( ) method receives the message, gets the return address from the
JMSReplyTo header, and publishes a response using that topic.
It is erroneous for a session to be operated by more than one thread of control at any given
time. In our example, there appears to be only one thread of control: the main thread of the
application. However, when the onMessage( ) handler is invoked, it is being called by
another thread that is owned by the JMS provider. Due to the asynchronous nature of the
onMessage( ) callback, it could possibly be invoked while the main thread is blocking on a
synchronous receive( ) .
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