Enterprise JavaBeans Distilled
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：
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();
}
}
待续。。。。。。。。
(作者其它文章：http://www.csdn.net/develop/author/netauthor/worldheart/ )
谢谢！！
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