ActiveMQ之selector的用法

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。