深入掌握JMS（九）：Selector

前面的例子中创建一个消息消费者使用的是:
                      sesssion.createConsumer(destination)
              另外，还提供了另一种方式：
                sesssion.createConsumer(destination, selector)
            这里selector是一个字符串，用来过滤消息。也就是说，这种方式可以创建一个可以只接收特定消息的一个消费者。Selector的格式是类似于SQL-92的一种语法。可以用来比较消息头信息和属性。

              下面的例子中，创建两个消费者，共同监听同一个Queue，但是它们的Selector不同，然后创建一个消息生产者，来发送多个消息。

            import javax.jms.Connection;
            import javax.jms.JMSException;
            import javax.jms.Message;
            import javax.jms.MessageConsumer;
            import javax.jms.MessageListener;
            import javax.jms.MessageProducer;
            import javax.jms.Queue;
            import javax.jms.Session;
            import javax.jms.TextMessage;

            import org.apache.activemq.ActiveMQConnectionFactory;
            import org.apache.activemq.command.ActiveMQQueue;

            public class JMSSelectorTest {

                public static void main(String[] args) throws Exception {
                    ActiveMQConnectionFactory factory = new
            ActiveMQConnectionFactory("vm://localhost");
                    Connection connection = factory.createConnection();
                    connection.start();
                    Queue queue = new ActiveMQQueue("testQueue");
                    Session session = connection.createSession(false,
            Session.AUTO_ACKNOWLEDGE);
                    MessageConsumer comsumerA = session.createConsumer(queue,
            "receiver = 'A'");
                    comsumerA.setMessageListener(new MessageListener(){
                        public void onMessage(Message m) {
                            try {
                                System.out.println("ConsumerA get " +
            ((TextMessage) m).getText());
                            } catch (JMSException e1) { }
                        }
                    });
                    MessageConsumer comsumerB = session.createConsumer(queue,
            "receiver = 'B'");
                    comsumerB.setMessageListener(new MessageListener(){
                        public void onMessage(Message m) {
                            try {
                                System.out.println("ConsumerB get " +
            ((TextMessage) m).getText());
                            } catch (JMSException e) { }
                        }
                    });
                    MessageProducer producer = session.createProducer(queue);
                    for(int i=0; i<10; i++) {
                        String receiver = (i%3 == 0 ? "A" : "B");
                        TextMessage message =
            session.createTextMessage("Message" + i + ", receiver:" + receiver);
                        message.setStringProperty("receiver", receiver);
                        producer.send(message );
                    }
                }
            }

            结果如下：
            ConsumerA get Message0, receiver:A
            ConsumerB get Message1, receiver:B
            ConsumerB get Message2, receiver:B
            ConsumerA get Message3, receiver:A
            ConsumerB get Message4, receiver:B
            ConsumerB get Message5, receiver:B
            ConsumerA get Message6, receiver:A
            ConsumerB get Message7, receiver:B
            ConsumerB get Message8, receiver:B
            ConsumerA get Message9, receiver:A

            可以看出，消息消费者只会取走它自己感兴趣的消息。