Enterprise JavaBeans Distilled

<?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Enterprise JavaBeans Distilled<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

作者： Tnk Luo

第七次：

消息驱动Bean(续)

JMS 应用客户程序

To get a better idea of how JMS is used, we can create a Java application whose sole purpose is receiving and processing reservation messages. We will develop a very simple JMS client that simply prints a description of each ticket as it receives the messages. We'll assume that the TravelAgent EJB is using the TextMessage to send a description of the ticket to the JMS clients. The following code shows how the JMS application client might look:

为更好的理解如何使用JMS，可以开发一个Java应用，其唯一的用途在于接收和处理预定消息。在这里，将开发一个非常简单的JMS客户。当该客户接收到消息时，会打印出所每张票的描述信息。假定TravelAgent EJB使用TextMessage以发送票的描述给JMS客户。下列代码显示了JMS应用客户可能的样子：

import javax.jms.Message;

import javax.jms.TextMessage;

import javax.jms.TopicConnectionFactory;

import javax.jms.TopicConnection;

import javax.jms.TopicSession;

import javax.jms.Topic;

import javax.jms.Session;

import javax.jms.TopicSubscriber;

import javax.jms.JMSException;

import javax.naming.InitialContext;

public class JmsClient_1 implements javax.jms.MessageListener {

public static void main(String [] args) throws Exception {

if(args.length != 2)

throw new Exception("Wrong number of arguments");

new JmsClient_1(args[0], args[1]);

while(true){Thread.sleep(10000);}

}

public JmsClient_1(String factoryName, String topicName) throws Exception {

InitialContext jndiContext = getInitialContext();

TopicConnectionFactory factory = (TopicConnectionFactory)

jndiContext.lookup("TopicFactoryNameGoesHere");

Topic topic = (Topic)jndiContext.lookup("TopicNameGoesHere");

TopicConnection connect = factory.createTopicConnection();

TopicSession session =

connect.createTopicSession(false,Session.AUTO_ACKNOWLEDGE);

TopicSubscriber subscriber = session.createSubscriber(topic);

subscriber.setMessageListener(this);

connect.start();

}

public void onMessage(Message message) {

try {

TextMessage textMsg = (TextMessage)message;

String text = textMsg.getText();

System.out.println("/n RESERVATION RECIEVED:/n"+text);

} catch(JMSException jmsE) {

jmsE.printStackTrace();

}

public static InitialContext getInitialContext() {

// 创建具体产品厂商的JNDI上下文

}

JmsClient_1的构建器含有来自JNDI InitialContext的TopicConnectionFactory和Topic。这些对象是使用具体厂商产品的properties创建的，从而使得客户能够连接到TravelAgent EJB使用的同一JMS供应者。比如，WebLogic应用服务器中getInitialContext()方法的代码如下：（JNDI也允许将properties放置在jndi.properties文件中，该文件包含有用于InitialContext的property值，并能够在运行时被动态找到。本书中，显式的给出了properties值。）

public static InitialContext getInitialContext() {

Properties env = new Properties();

env.put(Context.SECURITY_PRINCIPAL, "guest");

env.put(Context.SECURITY_CREDENTIALS, "guest");

env.put(Context.INITIAL_CONTEXT_FACTORY,

"weblogic.jndi.WLInitialContextFactory");

env.put(Context.PROVIDER_URL, "t3://localhost:7001");

return new InitialContext(env);

}

一旦客户获得了TopicConnectionFactory和Topic，就可以采取和TravelAgent EJB相同的方式创建TopicConnection和TopicSession。它们的主要区别在于这里的TopicSession对象创建了TopicSubscriber对象，而不是TopicPublisher。其中，TopicSubscriber被明确设计成处理来自特定Topic的消息：

TopicSession session =

connect.createTopicSession(false,Session.AUTO_ACKNOWLEDGE);

TopicSubscriber subscriber = session.createSubscriber(topic);

subscriber.setMessageListener(this);

connect.start();

TopicSubscriber能直接接收消息，或者能够将消息的处理委派给接口javax.jms.MessageLister。在例子中，JmsClient_1实现了MessageListener接口，使得它可以处理消息。MessageListener对象实现了单一的方法，onMessage()，每当新消息发送到订阅者的topic中时，该方法都会被调用。在这里，每次TravelAgent EJB发送预定消息到topic中时，JMS客户中的onMessage()方法都会被调用，使得它能够接收到消息的拷贝并处理它：