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<document> <header> <product>resin-ee</product> <title>JMS Listener with EJB message bean</title> <description> <p>Introduces receiving a message using an EJB message bean.</p> </description> <type>tutorial</type> <tutorial-startpage>send</tutorial-startpage> </header><body><localtoc/><s1 title="Files in this tutorial"><deftable><tr> <th>File</th> <th>Description</th></tr><tr> <td><viewfile-link file="WEB-INF/web.xml"/></td> <td>Configures the Queue, MessageSender, MessageListener.</td></tr><tr> <td><viewfile-link file="WEB-INF/classes/example/MyListener.java"/></td> <td>The message listener.</td></tr><tr> <td><viewfile-link file="WEB-INF/classes/example/MessageServlet.java"/></td> <td>The message servlet</td></tr></deftable></s1><s1 title="Overview"><p>Messaging decouples a sending task from a receiving/processing task,allowing for batch processing or load balancing of tasks to another machine.The original task can respond immediately to the user without waiting forthe task complete. The servlet creates a messageand sends it to a queue. The servlet immediately completes and when thebatch process is ready, it processes the message.</p><p>Messaging is therefore comprised of three main components:</p><ul><li>A <var>Producer</var> creates messages and sends them toa <var>Queue</var>. The Producer could be something like a Servlet.</li><li>A <var>Queue</var> stores the messages from the Produces and providesthem to a Consumer when ready. The Queue is implemented by themessaging provider.</li><li>A <var>Consumer</var> processes messages as they becomeavailable in the Queue. The <var>Consumer</var> is typically a beanimplementing the <code>MessageListener</code> interface.</li></ul></s1><s1 title="Producer (MessageServlet)"><p>In this example, the Producer is a Servlet which sends a simple message.To simplify the example, we'll use the <code>BlockingQueue</code> API insteadof the JMS producers. The functionality is the same without the extrahousekeeping of the JMS API.</p><p>The servlet uses <a href="../../doc/resin-ioc.xtp">Resin-IoC</a>dependency injection to get the <code>BlockingQueue</code>. Thedependency injection cleans the code by decoupling it from the implementation.For example, the actual <code>BlockingQueue</code> could be facade aroundanother JMS implementation's queue.</p><example title="Example: MessageServlet">import javax.webbeans.In;import java.util.concurrent.BlockingQueue;public class MessageServlet extends GenericServlet { @In BlockingQueue _sender; ... String message = "sample message"; _sender.put(message);}</example><p>In this configuration, the <code>BlockingQueue</code> is a<code>java.util.concurrent.BlockingQueue</code>. It's also possibleto use the full JMS <code>MessageProducer</code> which is more verbose.</p><p>The <code>put</code> method completes as soon asthe message is stored in the queue. Later, when a thread is available, theQueue will send the message to the Consumer.</p></s1><s1 title="Consumer (MyListener)"><p>The Queue delivers messages to the Consumer at the consumer's rate.When the Consumer finishes processing a message the Queue will deliver the nextavailable message.</p><p>The Consumer implements <code>javax.jms.MessageListener</code>, andwill therefore be identical code in any application server. TheConsumer might even be on a different server or use a differentapplication server.</p><p>In this example, the Consumer just logs the message, but morerealistic applications might use Java Persistence to store resultsin a database. The listener instance has full access to Resin-IoCcapabilities, including dependency injection, standard aspects like@TransactionAttribute, interceptors, and WebBeans event processing.</p><example title="Example: MyListener.java implementation">package example;import java.util.logging.Logger;import java.util.logging.Level;import javax.jms.Message;import javax.jms.TextMessage;import javax.jms.MessageListener;public class MyListener implements MessageListener { private static final Logger log = Logger.getLogger(MyListener.class.getName()); public void onMessage(Message message) { try { ObjectMessage objMessage = (ObjectMessage) message; log.info("received: " + objMessage.getValue()); _lastMessage = textMessage.getValue(); } catch (Exception e) { log.log(Level.WARNING, e.toString(), e); } }}</example></s1><s1 title="Configuration"><p>Since Resin is an inversion of control container (IoC), it can configure the JMS resources in the standard Resin configuration file.The <a href="../../doc/resin-ioc.xtp">Resin IoC</a> documentation givesmore information.</p><p>The configuration is responsible for three things:</p><ul> <li>Configuring the Queue</li> <li>Configuring the MessageListener</li></ul><p>The <code>Queue</code> is configured directly in a <jms-queue>tag, together with any configuration setters. This example uses alocal file store.</p><p>Because the listener and sender need a reference to the queue,the jms-queue stores it in the "my_queue" variable.</p><example title="Example: Queue configuration in WEB-INF/resin-web.xml"><web-app xmlns="http://caucho.com/ns/resin"> <jms-queue name="my_queue" uri="file:path=WEB-INF/db"/></web-app></example><p>JMS also needs a configured ConnectionFactory, so the senderand listener can create JMS connections.</p><example title="Example: ConnectionFactory in resin-web.xml"><web-app xmlns="http://caucho.com/ns/resin"> <jms-connection-factory uri="resin:"/></web-app></example><p>The MessageListener is configured as an EJB message bean.Resin's EJB support instantiates the listenersand receives messages from the queue.</p><example title="Example: message bean configuration resin-web.xml"><web-app xmlns="http://caucho.com/ns/resin"> <ejb-message-bean class="example.MyListener"> <destination>${my_queue}</destination> </ejb-message-bean></web-app></example></s1></body></document>⌨️ 快捷键说明
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