（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。
/**
*
*/
package selector;

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;

/**
* @author Wang Sheng(Josh)
*
* Consumer only receive the message which interested.
*
*/
public class JMSSelectorTest {

public static void main(String[] args) throws Exception {
ActiveMQConnectionFactory factory = new ActiveMQConnectionFactory();
Connection connection = factory.createConnection();

connection.start();

Queue queue = new ActiveMQQueue("JoshSelectorQueue");
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);

// the consumer only receive the message which properties is receiver equals A
MessageConsumer consumerA = session.createConsumer(queue, "receiver = 'A'");
consumerA.setMessageListener(new MessageListener() {

@Override
public void onMessage(Message message) {
try {
System.out.println("Consumer A get " + ((TextMessage)message).getText());
} catch (JMSException e) {
e.printStackTrace();
}

}
});

MessageConsumer consumerB = session.createConsumer(queue, "receiver = 'B'");
consumerB.setMessageListener(new MessageListener() {

@Override
public void onMessage(Message message) {
try {
System.out.println("Consumer B get " + ((TextMessage)message).getText());
} catch (JMSException e) {
e.printStackTrace();
}

}
});

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);
}

}

}