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<h1>JMS QueueRequestor Example</h1>
<p>This example shows you how to use a <a href="http://java.sun.com/javaee/5/docs/api/javax/jms/QueueRequestor.html">QueueRequestor</a> with ActiveMQ.</p>
<p>JMS is mainly used to send messages asynchronously so that the producer of a message is not waiting for the result of the message consumption.
However, there are cases where it is necessary to have a synchronous behavior: the code sending a message requires a reply for this message
before continuing its execution.<br />
A QueueRequestor facilitates this use case by providing a simple request/reply abstraction on top of JMS.</p>
<p>The example consists in two classes:</p>
<dl>
<dt><code>TextReverserService</code></dt>
<dd>A JMS MessageListener which consumes text messages and sends replies containing the reversed text</dd>
<dt><code>QueueRequestorExample</code></dt>
<dd>A JMS Client which uses a QueueRequestor to send text requests to a queue and receive replies with the reversed text in a synchronous fashion</dd>
</dl>
<h2>Example step-by-step</h2>
<p><i>To run the example, simply type <code>mvn verify</code> from this directory</i></p>
<ol>
<li>First we need to get an initial context so we can look-up the JMS connection factory from JNDI. This initial context will get it's properties from the <code>client-jndi.properties</code> file in the directory <code>../common/config</code></li>
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<code>InitialContext initialContext = getContext();</code>
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<li>We look up the JMS queue from JNDI</li>
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<code>Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");</code>
</pre>
<li>We look up JMS queue connection factory from JNDI</li>
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<code>QueueConnectionFactory cf = (QueueConnectionFactory)initialContext.lookup("/ConnectionFactory");</code>
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<li>We create the TextReverserService which will consume messages from the queue</li>
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<code> TextReverserService reverserService = new TextReverserService(cf, queue);</code>
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<li>We Create a JMS queue connection</li>
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<code>connection = cf.createQueueConnection();</code>
</pre>
<li>We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started</li>
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<code>connection.start();</code>
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<li>We create a JMS queue session. The session is created as non transacted and will auto acknowledge messages (this is mandatory to use it to create a QueueRequestor)</li>
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<code>QueueSession session = connection.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);</code>
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<li>We create a JMS QueueRequestor using the session and the queue</li>
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<code>QueueRequestor queueRequestor = new QueueRequestor(session, queue);</code>
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<li>We create a JMS text message to send as a request</li>
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<code>TextMessage request = session.createTextMessage("Hello, World!");</code>
</pre>
<li><p>We use the queue requestor to send the request and block until a reply is received.<br />
Using the queue requestor simplify request/reply use case by abstracting boilerplate JMS code
(creating a temporary queue, a consumer and a producer, setting the JMS ReplyTo header on the request,
sending the request with the producer, consuming the message from the consumer).
All this code is replaced by a single call to <code>QueueRequestor.request()</code> method.</p>
</li>
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<code>TextMessage reply = (TextMessage)queueRequestor.request(request);</code>
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<li>The reply's text contains the reverse of the request's text</li>
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<code>System.out.println("Send request: " + request.getText());
System.out.println("Received reply:" + reply.getText());</code>
</pre>
<li>We close the queue requestor to release all the JMS resources it created to provide request/reply mechanism</li>
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<code>queueRequestor.close()</code>
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<li>We do the same for the text reverser service</li>
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<code>reverserService.close()</code>
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<li>And finally, <b>always</b> remember to close your JMS connections and resources after use, in a <code>finally</code> block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects</li>
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<code>finally
{
if (initialContext != null)
{
initialContext.close();
}
if (connection != null)
{
connection.close();
}
}</code>
</pre>
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