初识activemq（pub/sub模式）

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发布、订阅模式，想象生活中订阅报纸或者关注的公众号发布消息

注意：在pub/sub中，必须先启动sub，不然的话，在启动sub之前发布的消息是不能消费的，就像你今天订开始报纸，那今天之前的报纸你肯定是收不到了，一样的。

pub/sub和p2p模式基本一样，变化的只有Queue和topic

Subscriber

public class Subscriber {
    //用户名
    private static final String USERNAME = ActiveMQConnection.DEFAULT_USER;
    //密码
    private static final String PASSWORD = ActiveMQConnection.DEFAULT_PASSWORD;
    //url
    private static final String URL = ActiveMQConnection.DEFAULT_BROKER_URL;

    private static ConnectionFactory factory = null;
    private static Connection connection = null;
    private static Session session = null;

    public static void main(String[] args) throws JMSException {
        ActiveMQConnectionFactory factory = new ActiveMQConnectionFactory(USERNAME,PASSWORD,URL);
        connection = factory.createConnection();
        connection.start();
        session = connection.createSession(false,Session.AUTO_ACKNOWLEDGE);

        Destination topic = session.createTopic("topic1");

        MessageConsumer subscriber1 = session.createConsumer(topic);
        MessageConsumer subscriber2 = session.createConsumer(topic);

        subscriber1.setMessageListener(new Listener1());
        subscriber2.setMessageListener(new Listener1());


    }
}

Listener1

public class Listener1 implements MessageListener {

    @Override
    public void onMessage(Message message) {
        try {
            System.out.println("订阅者一收到的消息：" + ((TextMessage) message).getText());
        } catch (JMSException e) {
            e.printStackTrace();
        }
    }
}

Publisher

public class Publisher {
    //默认连接用户名
    private static final String USERNAME = ActiveMQConnection.DEFAULT_USER;
    //默认连接密码
    private static final String PASSWORD = ActiveMQConnection.DEFAULT_PASSWORD;
    //默认的连接地址
    private static final String BROKEURL = ActiveMQConnection.DEFAULT_BROKER_URL;
    //发送的消息数量
    private static final int SENNUM = 10;

    public static void main(String[] args){
        ConnectionFactory factory ; //连接工厂
        Connection connection = null ; //连接
        Session session ; //会话，接收或者发送消息的线程
        Destination destination; //消息的目的地
        MessageProducer messageProducer; //消息生产者
        //实例化连接工厂
        factory = new ActiveMQConnectionFactory(Publisher.USERNAME, Publisher.PASSWORD, Publisher.BROKEURL);
        //通过连接工厂获取connection
        try {
            connection = factory.createConnection();
            connection.start(); //启动连接
            //创建session
            session = connection.createSession(Boolean.TRUE, Session.AUTO_ACKNOWLEDGE);
            //创建消息队列

            //创建主题
            destination = session.createTopic("topic1");
            //创建消息发布者
            messageProducer = session.createProducer(destination);
            //发送消息
            sendMessage(session, messageProducer);
            session.commit();
        } catch (JMSException e) {
            e.printStackTrace();
        }finally{
            if (connection != null) {
                try {
                    connection.close();
                } catch (JMSException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    /**
     * 发送消息
     * @param session
     * @param mp
     * @throws JMSException
     */
    public static void sendMessage(Session session, MessageProducer mp) throws JMSException{
        for(int i = 0;i<Publisher.SENNUM;i++){
            TextMessage message = session.createTextMessage("ActiveMq 发布的消息" + i);
            System.out.println("发布消息：" + "ActiveMq 发布的消息" + i);
            mp.send(message);
        }
    }

}

需要注意的是，在这个例子中，Publisher发布了10条消息，但是通过activemq的控制台会发现，一共消费了20条消息，这是因为有两个消费者。就像两个人订了同一份报纸，那出版社一定会生产这种报纸两张，而不是一张。