140 lines
6.5 KiB
HTML
140 lines
6.5 KiB
HTML
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<html>
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<head>
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<title>ActiveMQ Artemis JMS QueueRequestor Example</title>
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<link rel="stylesheet" type="text/css" href="../common/common.css" />
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<link rel="stylesheet" type="text/css" href="../common/prettify.css" />
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<script type="text/javascript" src="../common/prettify.js"></script>
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</head>
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<body onload="prettyPrint()">
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<h1>JMS QueueRequestor Example</h1>
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<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 Artemis.</p>
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<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.
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However, there are cases where it is necessary to have a synchronous behavior: the code sending a message requires a reply for this message
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before continuing its execution.<br />
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A QueueRequestor facilitates this use case by providing a simple request/reply abstraction on top of JMS.</p>
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<p>The example consists in two classes:</p>
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<dl>
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<dt><code>TextReverserService</code></dt>
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<dd>A JMS MessageListener which consumes text messages and sends replies containing the reversed text</dd>
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<dt><code>QueueRequestorExample</code></dt>
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<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>
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</dl>
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<h2>Example step-by-step</h2>
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<p><i>To run the example, simply type <code>mvn verify -Pexample</code> from this directory</i></p>
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<ol>
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<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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<pre class="prettyprint">
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<code>InitialContext initialContext = getContext();</code>
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</pre>
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<li>We look up the JMS queue from JNDI</li>
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<pre class="prettyprint">
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<code>Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");</code>
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</pre>
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<li>We look up JMS queue connection factory from JNDI</li>
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<pre class="prettyprint">
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<code>QueueConnectionFactory cf = (QueueConnectionFactory)initialContext.lookup("/ConnectionFactory");</code>
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</pre>
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<li>We create the TextReverserService which will consume messages from the queue</li>
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<pre class="prettyprint">
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<code> TextReverserService reverserService = new TextReverserService(cf, queue);</code>
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</pre>
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<li>We Create a JMS queue connection</li>
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<pre class="prettyprint">
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<code>connection = cf.createQueueConnection();</code>
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</pre>
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<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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<pre class="prettyprint">
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<code>connection.start();</code>
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</pre>
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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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<pre class="prettyprint">
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<code>QueueSession session = connection.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);</code>
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</pre>
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<li>We create a JMS QueueRequestor using the session and the queue</li>
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<pre class="prettyprint">
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<code>QueueRequestor queueRequestor = new QueueRequestor(session, queue);</code>
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</pre>
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<li>We create a JMS text message to send as a request</li>
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<pre class="prettyprint">
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<code>TextMessage request = session.createTextMessage("Hello, World!");</code>
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</pre>
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<li><p>We use the queue requestor to send the request and block until a reply is received.<br />
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Using the queue requestor simplify request/reply use case by abstracting boilerplate JMS code
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(creating a temporary queue, a consumer and a producer, setting the JMS ReplyTo header on the request,
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sending the request with the producer, consuming the message from the consumer).
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All this code is replaced by a single call to <code>QueueRequestor.request()</code> method.</p>
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</li>
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<pre class="prettyprint">
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<code>TextMessage reply = (TextMessage)queueRequestor.request(request);</code>
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</pre>
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<li>The reply's text contains the reverse of the request's text</li>
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<pre class="prettyprint">
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<code>System.out.println("Send request: " + request.getText());
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System.out.println("Received reply:" + reply.getText());</code>
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</pre>
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<li>We close the queue requestor to release all the JMS resources it created to provide request/reply mechanism</li>
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<pre class="prettyprint">
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<code>queueRequestor.close()</code>
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</pre>
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<li>We do the same for the text reverser service</li>
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<pre class="prettyprint">
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<code>reverserService.close()</code>
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</pre>
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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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<pre class="prettyprint">
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<code>finally
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{
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if (initialContext != null)
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{
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initialContext.close();
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}
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if (connection != null)
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{
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connection.close();
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}
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}</code>
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</pre>
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</ol>
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</body>
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</html>
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