死信队列: DLX，dead-letter-exchange

利用 dlx，当消息在一个队列中变成死信 (dead message) 之后，它能被重新 publish 到另一个 exchange，这个 exchange 就是 dlx

消息变成死信的原因有:

1.消息被拒绝 (basic.reject / basic.nack) 并且 reQueue=false

2.消息 TTL 过期

3.队列达到最大长度了

dlx 也是一个正常的 exchange，和一般的 exchange 没什么区别，它能在任何队列上被指定，实际上就是设置一个属性。

当这个队列中有死信时，rabbitMQ 就会自动的将这个消息重新发布到设置的 exchange 上去，进而被路由到另一个队列。

可以监听这个队列中消息做相应的处理，这个特性可以弥补 rabbitMQ3.0 以前支持的 immediate 参数功能。

死信队列的设置:

首先要设置死信队列的 exchange 和 queue，然后进行绑定:

  exchange: dlx.exchange

  queue: dlx.queue

  routingkey:  #

然后进行正常声明交换机、队列、绑定，只不过需要在队列加上一个参数即可: argument.put("x-dead-letter-exchange", "dlx.exchange");

这样消息在过期、reQueue、队列在达到最大长度时，消息就可以直接路由到死信队列

代码地址：    https://github.com/hmilyos/rabbitmqdemo.git  rabbitmq-api 项目下

public class Producer {

    private static final Logger log = LoggerFactory.getLogger(Producer.class);
    
    public static void main(String[] args) throws IOException, TimeoutException {
         ConnectionFactory connectionFactory = new ConnectionFactory();
            connectionFactory.setHost(RabbitMQCommon.RABBITMQ_HOST);
            connectionFactory.setPort(RabbitMQCommon.RABBITMQ_PORT);
            connectionFactory.setVirtualHost(RabbitMQCommon.RABBITMQ_DEFAULT_VIRTUAL_HOST);

            Connection connection = connectionFactory.newConnection();
            Channel channel = connection.createChannel();

            //注意，这只是普通的交换机和routingKey
            String exchange = "test_dlx_exchange";
            String routingKey = "dlx.save";

            for (int i = 0; i < 1; i++) {
                String msg = "Hello RabbitMQ DLX Message" + i;
                AMQP.BasicProperties properties = new AMQP.BasicProperties.Builder()
                        .deliveryMode(2)
                        .contentEncoding("UTF-8")
                        .expiration("10000")
                        .build();
                log.info("生产端发送：{}", msg);
                channel.basicPublish(exchange, routingKey, true, properties, msg.getBytes());
            }
    }
}

/**
 *   死信队列
 *
 */
public class Consumer {

    private static final Logger log = LoggerFactory.getLogger(Consumer.class);
    
    public static void main(String[] args) throws IOException, TimeoutException {
        //1 创建ConnectionFactory
        ConnectionFactory connectionFactory = new ConnectionFactory();
        connectionFactory.setHost(RabbitMQCommon.RABBITMQ_HOST);
        connectionFactory.setPort(RabbitMQCommon.RABBITMQ_PORT);
        connectionFactory.setVirtualHost(RabbitMQCommon.RABBITMQ_DEFAULT_VIRTUAL_HOST);
        //2 获取Connection
        Connection connection = connectionFactory.newConnection();
        //3 通过Connection创建一个新的Channel
        Channel channel = connection.createChannel();

        // 这就是一个普通的交换机 和 队列 以及路由
        String exchangeName = "test_dlx_exchange";
        String routingKey = "dlx.#";
        String queueName = "test_dlx_queue";
        channel.exchangeDeclare(exchangeName, "topic", true, false, null);

        Map<String, Object> agruments = new HashMap<String, Object>();
        agruments.put("x-dead-letter-exchange", "dlx.exchange");
        //这个agruments属性，要设置到声明队列上
        channel.queueDeclare(queueName, true, false, false, agruments);
        channel.queueBind(queueName, exchangeName, routingKey);

        //要进行死信队列的声明: dlx.exchange/queue都是由你自己命名的，只不过为了这里只是为了简洁明了而已
        channel.exchangeDeclare("dlx.exchange", "topic", true, false, null);
        channel.queueDeclare("dlx.queue", true, false, false, null);
        channel.queueBind("dlx.queue", "dlx.exchange", "#");

        channel.basicConsume(queueName, true, new MyConsumer(channel));
        log.info("消费端启动成功");
        
    }
}

public class MyConsumer extends DefaultConsumer {

    private static final Logger log = LoggerFactory.getLogger(MyConsumer.class);

    public MyConsumer(Channel channel) {
        super(channel);
    }

    @Override
    public void handleDelivery(String consumerTag, // 消费者标签
            Envelope envelope, AMQP.BasicProperties properties, byte[] body) throws IOException {
        log.info("----dlx--MyConsumer-----consume message----------");
        log.info("consumerTag: " + consumerTag);
        log.info("envelope: " + envelope);
        log.info("properties: " + properties);
        log.info("body: " + new String(body));
    }

}

先把消费端启动，去管控台查看 test_dlx_exchange 以及 test_dlx_queue 这两个普通的交换机、队列， 死信队列 dlx.queue 以及绑定的 dlx.exchange 是否创建成功

image

确认创建成功就关闭消费端，然后再启动生产端，这时候消息没被消费，一直在 test_dlx_queue 中，

image

image

自此，死信队列的简单用法介绍完毕。