一起养成写作习惯！这是我参与「掘金日新计划 · 4 月更文挑战」的第15天，点击查看活动详情。

六、死信队列

6.1 死信的概念

==死信，顾名思义就是无法被消费的消息==字面意思可以这样理解，一般来说，producer 将消息投递到 broker 或者直接到queue 里了，consumer 从 queue 取出消息 进行消费，但某些时候由于特定的原因导致 queue 中的某些消息无法被消费，这样的消息如果没有后续的处理，就变成了死信，有死信自然就有了死信队列。

应用场景：为了保证订单业务的消息数据不丢失，需要使用到 RabbitMQ 的死信队列机制，当消息消费发生异常时，将消息投入死信队列中。还有比如说：用户在商城下单成功并点击去支付后在指定时间未支付时自动失效

6.2 死信的来源

1. 消息 TTL 过期

   TTL是Time To Live的缩写, 也就是生存时间

2. 队列达到最大长度

   队列满了，无法再添加数据到 mq 中

3. 消息被拒绝

   (basic.reject 或 basic.nack) 并且 requeue=false.

6.3 死信实战

6.3.1 代码架构图

6.3.2 消息TTL过期

生产者代码

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.AMQP;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.impl.AMQBasicProperties;

import java.nio.charset.StandardCharsets;
import java.util.HashMap;
import java.util.Map;

/**
 * 死信队列生产者
 */
public class Producer {
    //交换机的名字
    public static final String NORMAL_EXCHANGE = "normal_exchange";

    //发送消息
    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
        //死信消息，设置TTL时间
        AMQP.BasicProperties properties =
                new AMQP.BasicProperties()
                        .builder().expiration("10000").build();
        for (int i = 0; i < 11; i++) {
            String message = "info" + i;
            channel.basicPublish(NORMAL_EXCHANGE, "zs", properties,message.getBytes(StandardCharsets.UTF_8));
            System.out.println("生产者发送消息："+message);
        }
    }
}

消费者 C1 代码：

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.DeliverCallback;
import java.util.HashMap;
import java.util.Map;


public class Consumer01 {

    public static final String NORMAL_EXCHANGE = "normal_exchange";
    public static final String DEAD_EXCHANGE = "dead_exchange";
    public static final String NORMAL_QUEUE = "normal_queue";
    public static final String DEAD_QUEUE = "dead_queue";

    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
//        声明死信和普通交换机 类型为direct
        channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);
        channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);

//        声明普通队列
        Map<String, Object> arguments = new HashMap<>();
        //过期时间
//        正常队列设置死信交换机
        arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);
//        设置死信routingkey
        arguments.put("x-dead-letter-routing-key", "lisi");

//        设置队列长度
//        arguments.put("x-max-length", 6);

        channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);

        
        
//        声明死信队列
        channel.queueDeclare(DEAD_QUEUE, false, false, false, null);

        //绑定普通交换机与普通队列
        channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zs");
        //绑定死信交换机与死信队列
        channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");
        System.out.println("等待接受消息..........");

        DeliverCallback deliverCallback = (consumerTag, message) -> {
            String msg = new String(message.getBody());
            System.out.println("Consumer01 接收到消息"+message);
        };
        //开启手动应答
        channel.basicConsume(NORMAL_QUEUE, true, deliverCallback, consumerTag -> {
        });
    }
}

启动 C1 ，之后关闭消费者，模拟其接收不到消息。再启动 Producer

消费者 C2 代码：

以上步骤完成后，启动 C2 消费者，它消费死信队列里面的消息

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.DeliverCallback;

import java.util.HashMap;
import java.util.Map;

/**
 * 死信队列
 * 消费者2
 */
public class Consumer02 {
    public static final String NORMAL_QUEUE = "dead_queue";

    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
        System.out.println("等待接受消息..........");
        DeliverCallback deliverCallback = (consumerTag, message) -> {
            System.out.println("Consumer02接受的消息是：" + new String(message.getBody()));
        };
        channel.basicConsume(NORMAL_QUEUE, true, deliverCallback, consumerTag -> {
        });
    }
}

6.3.3 队列达到最大长度

消息生产者代码去掉 TTL 属性

AMQP.BasicProperties properties = new AMQP.BasicProperties() .builder().expiration("10000").build();

消息生产者代码去掉 TTL 属性

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.AMQP;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.impl.AMQBasicProperties;

import java.nio.charset.StandardCharsets;
import java.util.HashMap;
import java.util.Map;

/**
 * 死信队列生产者
 */
public class Producer {
    //交换机的名字
    public static final String NORMAL_EXCHANGE = "normal_exchange";

    //发送消息
    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
        for (int i = 0; i < 11; i++) {
            String message = "info" + i;
            channel.basicPublish(NORMAL_EXCHANGE, "zs", null,message.getBytes(StandardCharsets.UTF_8));
            System.out.println("生产者发送消息:"+message);
        }
    }
}

C1 消费者修改以下代码**(启动之后关闭该消费者 模拟其接收不到消息)**

//设置正常队列的长度限制，例如发10个，4个则为死信 params.put("x-max-length",6);

注意此时需要把原先队列删除 因为参数改变了

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.DeliverCallback;

import java.util.HashMap;
import java.util.Map;

public class Consumer01 {

    public static final String NORMAL_EXCHANGE = "normal_exchange";
    public static final String DEAD_EXCHANGE = "dead_exchange";
    public static final String NORMAL_QUEUE = "normal_queue";
    public static final String DEAD_QUEUE = "dead_queue";

    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
        //        声明死信和普通交换机 类型为direct
        channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);
        channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);

        //        声明普通队列
        Map<String, Object> arguments = new HashMap<>();

        //        正常队列设置死信交换机
        arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);

        //        设置死信routingkey
        arguments.put("x-dead-letter-routing-key", "lisi");

        //        设置队列长度
        arguments.put("x-max-length", 6);

        channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);
        /******************************************************************/
        //        声明死信队列
        channel.queueDeclare(DEAD_QUEUE, false, false, false, null);

        //绑定普通交换机与普通队列
        channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zs");
        //绑定死信交换机与死信队列
        channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");
        System.out.println("等待接受消息..........");

        DeliverCallback deliverCallback = (consumerTag, delivery) -> { String message = new String(delivery.getBody(), "UTF-8");
                                                                      System.out.println("Consumer01 接收到消息"+message);
                 
        channel.basicConsume(NORMAL_QUEUE, true, deliverCallback, consumerTag -> {
        });
    }
}

C2 消费者代码不变(启动 C2 消费者)

6.3.4 消息被拒

生产者，C2消费者代码同上生产者一致

C1 消费者代码(启动之后关闭该消费者 模拟其接收不到消息)

package com.caq.rabbitmq.deadqueue;

import com.caq.rabbitmq.utils.RabbitMqUtils;
import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.DeliverCallback;

import java.util.HashMap;
import java.util.Map;

/**
 * 死信队列
 * <p>
 * 消费者1
 */
public class Consumer01 {

    public static final String NORMAL_EXCHANGE = "normal_exchange";
    public static final String DEAD_EXCHANGE = "dead_exchange";
    public static final String NORMAL_QUEUE = "normal_queue";
    public static final String DEAD_QUEUE = "dead_queue";

    public static void main(String[] args) throws Exception {
        Channel channel = RabbitMqUtils.getChannel();
//        声明死信和普通交换机 类型为direct
        channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);
        channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);

//        声明普通队列
        Map<String, Object> arguments = new HashMap<>();
        //过期时间
//        正常队列设置死信交换机
        arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);
//        设置死信routingkey
        arguments.put("x-dead-letter-routing-key", "lisi");

//        设置队列长度
//        arguments.put("x-max-length", 6);

        channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);
        /******************************************************************/
//        声明死信队列
        channel.queueDeclare(DEAD_QUEUE, false, false, false, null);

        //绑定普通交换机与普通队列
        channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zs");
        //绑定死信交换机与死信队列
        channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");
        System.out.println("等待接受消息..........");

        DeliverCallback deliverCallback = (consumerTag, message) -> {
            String msg = new String(message.getBody());
            if (msg.equals("info5")) {
                System.out.println("消费者1接受的消息是" + msg + "：此消息是被c1拒绝的");
                channel.basicReject(message.getEnvelope().getDeliveryTag(),false);
            } else {
                System.out.println("Consumer01接受的消息是:" + msg);
            }

        };
        //开启手动应答
        channel.basicConsume(NORMAL_QUEUE, false, deliverCallback, consumerTag -> {
        });
    }
}