MessageConsumer的消息选择器
MessageConsumer是一个由Session创建的对象，用来从Destination接收消息。
看一下Session创建MessageConsumer的构造方法有哪些：
public MessageConsumer createConsumer(Destination destination) throws JMSException；
public MessageConsumer createConsumer(Destination destination, String messageSelector)
throws JMSException；
public MessageConsumer createConsumer(Destination destination, String messageSelector, boolean noLocal)
throws JMSException；
public MessageConsumer createConsumer(Destination destination,
String messageSelector,
boolean noLocal,
MessageListener messageListener) throws JMSException ;
public TopicSubscriber createDurableSubscriber(Topic topic, String name) throws JMSException;
public TopicSubscriber createDurableSubscriber(Topic topic,
String name,
String messageSelector,
boolean noLocal) throws JMSException;
其中
messageSelector为消息选择器；（采用SQL92的语法）
public final string SELECTOR = "color = 'blue'";该选择器检查了传入消息的color属性，并检查这个属性的值是否等于blue，如果相等则消息被消费，否则消息被忽略。
noLocal标识默认为false，当设置为true时，限制消费者只能接收和自己相同的连接(Connection)所发的消息,此标识只适用于主题topic，不适用于队列queue。
name标识订阅主题所对应的订阅名称，持久订阅时需要设置此参数；
我们接下来看下消息选择器是怎么使用的？
先看下生产者代码：
package jeff.mq.p2p;
import javax.jms.Connection;
import javax.jms.ConnectionFactory;
import javax.jms.DeliveryMode;
import javax.jms.Destination;
import javax.jms.JMSException;
import javax.jms.MapMessage;
import javax.jms.MessageProducer;
import javax.jms.Session;
import org.apache.activemq.ActiveMQConnectionFactory;
/**
* @author jeffSheng
* 2018年7月3日
*/
public class Producer {
//1连接工厂
private ConnectionFactory connectionFactory;
//2连接对象
private Connection connection;
//3Session对象
private Session session;
//4生产者
private MessageProducer messageProducer;
public Producer(){
try {
this.connectionFactory =
new ActiveMQConnectionFactory(
"jeff",
"123456",
"tcp://localhost:61616");
this.connection = connectionFactory.createConnection();
this.connection.start();
this.session = connection.createSession(Boolean.FALSE, Session.AUTO_ACKNOWLEDGE);
this.messageProducer = session.createProducer(null);
} catch (JMSException e) {
e.printStackTrace();
}
}
public Session getSession(){
return this.session;
}
public static void main(String[] args) throws Exception {
Producer producer = new Producer();
producer.sender1();
}
public void sender1() throws Exception{
Destination destination = this.session.createQueue("first");
MapMessage msg1 = this.session.createMapMessage();
msg1.setString("name", "张三");
msg1.setString("age", "23");
msg1.setStringProperty("color", "blue");
msg1.setIntProperty("sal", 2200);
int id = 1;
msg1.setInt("id", id);
String receiver = id % 2 == 0 ? "A" : "B";
msg1.setStringProperty("receiver", receiver);
MapMessage msg2 = this.session.createMapMessage();
msg2.setString("name", "李四");
msg2.setString("age", "26");
msg2.setStringProperty("color", "red");
msg2.setIntProperty("sal", 1300);
id = 2;
msg1.setInt("id", id);
receiver = id % 2 == 0 ? "A" : "B";
msg2.setStringProperty("receiver", receiver);
MapMessage msg3 = this.session.createMapMessage();
msg3.setString("name", "王五");
msg3.setString("age", "28");
msg3.setStringProperty("color", "green");
msg3.setIntProperty("sal", 1500);
id = 3;
msg3.setInt("id", id);
receiver = id % 2 == 0 ? "A" : "B";
msg3.setStringProperty("receiver", receiver);
MapMessage msg4 = this.session.createMapMessage();
msg4.setString("name", "赵六");
msg4.setString("age", "30");
msg4.setStringProperty("color", "blue");
msg4.setIntProperty("sal", 1800);
id = 4;
msg4.setInt("id", id);
receiver = id % 2 == 0 ? "A" : "B";
msg4.setStringProperty("receiver", receiver);
this.messageProducer.send(destination,msg1,DeliveryMode.NON_PERSISTENT,2,1000*60*10);
this.messageProducer.send(destination,msg2,DeliveryMode.NON_PERSISTENT,3,1000*60*10);
this.messageProducer.send(destination,msg3,DeliveryMode.NON_PERSISTENT,6,1000*60*10);
this.messageProducer.send(destination,msg4,DeliveryMode.NON_PERSISTENT,9,1000*60*10);
if(connection!=null){
connection.close();
}
}
}创建了四个消息msg1,msg2,msg3,msg4,这四个消息的类型是MapMessage，存放的是id,name,color,age,sal,receiver,这几个属性信息，生产者将消息存入first队列中。
运行：
接下来我们看怎么使用消息选择器去有条件的获取消息，这个例子中我们使用的是OnMessage这种异步的监听消息方式，不再使用receive这种轮询方式。
Consumer代码：
我们定义了四个选择器：
public final String SELECTOR_0 = "age > 25";
public final String SELECTOR_1 = "color = 'blue'";
public final String SELECTOR_2 = "color = 'blue' and sal > 2000";
public final String SELECTOR_3 = "receiver = 'A'";我们先将第一个选择器SELECTOR_0这个条件age>25放进去：
this.messageConsumer = session.createConsumer(this.destination,SELECTOR_0);运行打印：理论上李四、王五、赵六都可以被消费，验证下，结果却没有任何消息被消费：
这是因为：虽然李四、王五、赵六满足age>25，但是age存放的语法是：
msg1.setString("age", "23");这种对于选择器是无效的，只有类似于以下这种指定了数据类型的才是可以的：
msg1.setStringProperty("color", "blue");
msg1.setIntProperty("sal", 2200);那么，我们试试SELECTOR_1?
public final String SELECTOR_1 = "color = 'blue'";理论上张三和赵六可以被打印出来：
果然！所以队列中其实还有李四和王五没有被消费！其实我们也可以指定满足李四和王五的条件去选择消费，这里就不试验了！
mysql消息持久化
activemq默认的持久化策略是kahadb：
<persistenceAdapter>
<kahaDB directory="${activemq.data}/kahadb"/>
</persistenceAdapter>在对性能要求不高的情况下，我们可以采用mysql数据库来存储activemq的消息：
<persistenceAdapter>
<!-- <kahaDB directory="${activemq.data}/kahadb"/> -->
<jdbcPersistenceAdapter dataSource="#mysql-ds"/>
</persistenceAdapter>
另外需要在broker节点的外定义id为mysql-ds的bean，如下：
<bean id="mysql-ds" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/hello_mq?relaxAutoCommit=true"/>
<property name="username" value="root"/>
<property name="password" value="123456"/>
<property name="maxActive" value="200"/>
<property name="poolPreparedStatements" value="true"/>
</bean>接下来，我们在本地数据库中新建立一个hello_mq的数据库，然后记得将mysql的jdbc驱动，加入activemq安装文件的lib目录下：
需要注意，生产者发送数据的时候要【持久化】；
重新启动activemq：
很简单！