O'Reilly - Java Message Service

Java Message Service
listening, or even if nobody is listening. For example, consider a publisher that broadcasts
stock quotes. If any particular subscriber is not currently connected and misses out on a
great quote, the publisher is not concerned. Likewise, our Wholesaler class didn't care
whether there were any subscribers when it sent price quotes: if a Retailer missed a great
price, that wasn't the Wholesaler's problem. In contrast, a point-to-point session is likely to
be intended for a one-on-one conversation with a specific application at the other end. In
this scenario, every message really matters.
The range and variety of the data that the messages represent can be a factor as well. Using
pub/sub, messages are dispatched to the consumers based on filtering that is provided
through the use of specific topics. Even when messaging is being used to establish a oneon-one
conversation with another known application, it can be advantageous to use
pub/sub with multiple topics to segregate different kinds of messages. Each kind of
message can be dealt with separately through its own unique consumer and onMessage( )
handler.
Point-to-point is more convenient when you want one receiver to process any given
message once-and-only-once. This is perhaps the most critical difference between the two
models: point-to-point guarantees that only one consumer processes a given message. This
is extremely important when messages need to be processed separately but in tandem,
balancing the load of message processing across many JMS clients. Another advantage is
that the point-to-point model provides a QueueBrowser that allows the JMS client to peek
ahead on the queue to see messages waiting to be consumed. Pub/sub does not include a
browsing feature. We'll talk more about the QueueBrowser later in this chapter.
5.2 The QWholesaler and QRetailer
Let's rethink our wholesaler/retailer scenario in terms of the distinction between the two
message models. The pub/sub model is well suited for sending price quotes, since that is
naturally a one-to-many broadcast. However, when the retailer responds with a "buy"
order, it is more appropriate to use a point-to-point queue. In the real world, retailers
naturally deal with many wholesalers, and you would only send a purchase order to the
wholesaler that offered the quote.
From the user's perspective, the QWholesaler and QRetailer examples that we'll develop
now are functionally equivalent to the Wholesaler and Retailer examples introduced in
Chapter 4. The difference lies in the use of the point-to-point queue for responses to price
quotes. If you wish to see these classes in action, start your JMS provider and execute the
following commands, each in a separate command window:
java chap5.B2B.Q Wholesaler localhost username password
java chap5.B2B.Q Retailer localhost username password
5.2.1 The QRetailer Class
Here is the listing for the QRetailer class in its entirety. Later, we will examine this class
in detail:
import java.util.StringTokenizer;
import java.util.Properties;
import javax.naming.InitialContext;
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