Java tutorial
/* * JBoss, Home of Professional Open Source * Copyright 2005, JBoss Inc., and individual contributors as indicated * by the @authors tag. See the copyright.txt in the distribution for a * full listing of individual contributors. * * This is free software; you can redistribute it and/or modify it * under the terms of the GNU Lesser General Public License as * published by the Free Software Foundation; either version 2.1 of * the License, or (at your option) any later version. * * This software is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public * License along with this software; if not, write to the Free * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA * 02110-1301 USA, or see the FSF site: http://www.fsf.org. */ package org.jbpm.ejb; import java.io.Serializable; import javax.annotation.Resource; import javax.ejb.EJB; import javax.ejb.MessageDrivenContext; import javax.jms.Connection; import javax.jms.ConnectionFactory; import javax.jms.Destination; import javax.jms.JMSException; import javax.jms.Message; import javax.jms.MessageListener; import javax.jms.MessageProducer; import javax.jms.ObjectMessage; import javax.jms.Session; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; import org.jbpm.command.Command; import org.jbpm.persistence.db.DbPersistenceService; import org.jbpm.persistence.db.StaleObjectLogConfigurer; /** * This message-driven bean listens for {@link ObjectMessage object messages} containing a * command instance. The received commands are executed by the {@link CommandServiceBean command * service} bean, using the local interface. * * The body of the message must be a Java object that implements the {@link Command} interface. * The message properties, if any, are ignored. * * <h3>Environment</h3> * * <p> * The environment entries and resources available for customization are summarized in the table * below. * </p> * * <table border="1"> * <tr> * <th>Name</th> * <th>Type</th> * <th>Description</th> * </tr> * <tr> * <td><code>ejb/LocalCommandService</code></td> * <td>EJB Reference</td> * <td>Link to the local {@linkplain CommandServiceBean session bean} that executes commands on * a separate jBPM context.</td> * </tr> * <tr> * <td><code>jms/JbpmConnectionFactory</code></td> * <td>Resource Manager Reference</td> * <td>Logical name of the factory that provides JMS connections for producing result messages. * Required for command messages that indicate a reply destination.</td> * </tr> * </table> * * @author Alejandro Guizar */ public class CommandListenerBean implements MessageListener { private static final long serialVersionUID = 1L; @Resource private MessageDrivenContext messageDrivenContext; @EJB(name = "ejb/LocalCommandService") private LocalCommandService commandService; @Resource(name = "jms/JbpmConnectionFactory", shareable = true) private ConnectionFactory jmsConnectionFactory; private static final Log log = LogFactory.getLog(CommandListenerBean.class); public void onMessage(Message message) { try { // extract command from message Command command = extractCommand(message); // a null return value means the message did not carry a valid command // warnings were logged already; just swallow the message and return if (command == null) return; // execute command via local command executor bean Object result; try { if (log.isDebugEnabled()) { log.debug("executing " + command); } result = commandService.execute(command); if (log.isTraceEnabled()) { log.trace(command + " completed successfully, committing"); } } catch (RuntimeException e) { // if this is a locking exception, keep it quiet if (DbPersistenceService.isLockingException(e)) { StaleObjectLogConfigurer.getStaleObjectExceptionsLog() .error(message + " failed to execute " + command, e); } else { log.error(message + " failed to execute " + command, e); } // MDBs are not supposed to throw exceptions messageDrivenContext.setRollbackOnly(); return; } // send a response back if a "reply to" destination is set Destination replyTo; if (jmsConnectionFactory != null && (replyTo = message.getJMSReplyTo()) != null && (result instanceof Serializable || result == null)) { sendResult((Serializable) result, replyTo, message.getJMSMessageID()); } } catch (JMSException e) { messageDrivenContext.setRollbackOnly(); log.error("failed to process message " + message, e); } } /** * Retrieves a {@link Command} instance from the given message, which is assumed to be an * {@link ObjectMessage}. * <p> * Subclasses may override this method to materialize the command in some other way. * </p> */ protected Command extractCommand(Message message) throws JMSException { if (message instanceof ObjectMessage) { ObjectMessage objectMessage = (ObjectMessage) message; Serializable object = objectMessage.getObject(); if (object instanceof Command) { return (Command) object; } else { log.warn(object + " is not a command"); } } else { log.warn(message + " is not an object message"); } return null; } private void sendResult(Serializable result, Destination destination, String correlationId) throws JMSException { if (log.isDebugEnabled()) log.debug("sending " + result + " to " + destination); Connection jmsConnection = jmsConnectionFactory.createConnection(); try { /* * if the connection supports xa, the session will be transacted, else the session will * auto acknowledge; in either case no explicit transaction control must be performed - * see ejb 2.1 - 17.3.5 */ Session jmsSession = jmsConnection.createSession(false, Session.AUTO_ACKNOWLEDGE); Message resultMessage = jmsSession.createObjectMessage(result); resultMessage.setJMSCorrelationID(correlationId); MessageProducer producer = jmsSession.createProducer(destination); producer.send(resultMessage); } finally { // there is no need to close the sessions and producers of a closed connection // http://download.oracle.com/javaee/1.4/api/javax/jms/Connection.html#close() try { jmsConnection.close(); } catch (JMSException e) { log.warn("failed to close jms connection", e); } } } }