6. 死信队列6.1. 死信的概念先从概念解释上搞清楚这个定义，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

6.1. 死信的概念

先从概念解释上搞清楚这个定义，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

消息 TTL 过期

队列达到最大长度(队列满了，无法再添加数据到 mq 中) 
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

public class Producer {
    private static final String NORMAL_EXCHANGE = "normal_exchange";
    public static void main(String[] argv) throws Exception {
        try (Channel channel = RabbitMqUtils.getChannel())
        { channel.exchangeDeclare(NORMAL_EXCHANGE,
                BuiltinExchangeType.DIRECT);
//设置消息的 TTL 时间
            AMQP.BasicProperties properties =
                    new
                            AMQP.BasicProperties().builder().expiration("10000").build();
//该信息是用作演示队列个数限制
            for (int i = 1; i <11 ; i++) {
                String message="info"+i;
                channel.basicPublish(NORMAL_EXCHANGE,
                        message.getBytes());
                System.out.println("生产者发送消息:"+message);
            }
        }
    }
}

消费者 C1 代码(启动之后关闭该消费者模拟其接收不到消息)

public class Consumer01 {
    //普通交换机名称
    private static final String NORMAL_EXCHANGE = "normal_exchange";
    //死信交换机名称
    private static final String DEAD_EXCHANGE = "dead_exchange";
    public static void main(String[] argv) throws Exception {
        Channel channel = RabbitUtils.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);
//声明死信队列
        String deadQueue = "dead-queue";
        channel.queueDeclare(deadQueue, false, false, false, null);
//死信队列绑定死信交换机与 routingkey
        channel.queueBind(deadQueue, DEAD_EXCHANGE, "lisi");
//正常队列绑定死信队列信息
        Map<String, Object> params = new HashMap<>();
//正常队列设置死信交换机 参数 key 是固定值
        params.put("x-dead-letter-exchange", DEAD_EXCHANGE);
//正常队列设置死信 routing-key 参数 key 是固定值
        params.put("x-dead-letter-routing-key", "lisi");
        String normalQueue = "normal-queue";
        channel.queueDeclare(normalQueue, false, false, false, params);
        channel.queueBind(normalQueue, NORMAL_EXCHANGE, "zhangsan");
        System.out.println("等待接收消息........... ");
        DeliverCallback deliverCallback = (consumerTag, delivery) ->
        {String message = new String(delivery.getBody(), "UTF-8");
            System.out.println("Consumer01 接收到消息"+message);
        };
        channel.basicConsume(normalQueue, true, deliverCallback, consumerTag -> {
        });
    }
}

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)

public class Consumer02 {
    private static final String DEAD_EXCHANGE = "dead_exchange";
    public static void main(String[] argv) throws Exception {
        Channel channel = RabbitUtils.getChannel();
        channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);
        String deadQueue = "dead-queue";
        channel.queueDeclare(deadQueue, false, false, false, null);
        channel.queueBind(deadQueue, DEAD_EXCHANGE, "lisi");
        System.out.println("等待接收死信队列消息........... ");
        DeliverCallback deliverCallback = (consumerTag, delivery) ->
        {String message = new String(delivery.getBody(), "UTF-8");
            System.out.println("Consumer02 接收死信队列的消息" + message);
        };
        channel.basicConsume(deadQueue, 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

public class Producer {
    private static final String NORMAL_EXCHANGE = "normal_exchange";
    public static void main(String[] argv) throws Exception {
        try (Channel channel = RabbitMqUtils.getChannel())
        { channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);
//该信息是用作演示队列个数限制
            for (int i = 1; i <11 ; i++) {
                String message="info"+i;
                channel.basicPublish(NORMAL_EXCHANGE,"zhangsan",null, message.getBytes());
                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) 注意此时需要把原先队列删除因为参数改变了
C2 消费者代码不变(启动 C2 消费者)

6.3.4. 消息被拒

1.消息生产者代码同上生产者一致

2.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)

public class Consumer01 {
    //普通交换机名称
    private static final String NORMAL_EXCHANGE = "normal_exchange";
    //死信交换机名称
    private static final String DEAD_EXCHANGE = "dead_exchange";
    public static void main(String[] argv) throws Exception {
        Channel channel = RabbitUtils.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);
//声明死信队列
        String deadQueue = "dead-queue";
        channel.queueDeclare(deadQueue, false, false, false, null);
//死信队列绑定死信交换机与 routingkey
        channel.queueBind(deadQueue, DEAD_EXCHANGE, "lisi");
        //正常队列绑定死信队列信息
        Map<String, Object> params = new HashMap<>();
//正常队列设置死信交换机 参数 key 是固定值
        params.put("x-dead-letter-exchange", DEAD_EXCHANGE);
//正常队列设置死信 routing-key 参数 key 是固定值
        params.put("x-dead-letter-routing-key", "lisi");
        String normalQueue = "normal-queue";
        channel.queueDeclare(normalQueue, false, false, false, params);
        channel.queueBind(normalQueue, NORMAL_EXCHANGE, "zhangsan");
        System.out.println("等待接收消息........... ");
        DeliverCallback deliverCallback = (consumerTag, delivery) ->
        {String message = new String(delivery.getBody(), "UTF-8");
            if(message.equals("info5")){
                System.out.println("Consumer01 接收到消息" + message + "并拒绝签收该消息");
//requeue 设置为 false 代表拒绝重新入队 该队列如果配置了死信交换机将发送到死信队列中
                channel.basicReject(delivery.getEnvelope().getDeliveryTag(), false);
            }else {
                System.out.println("Consumer01 接收到消息"+message);
                channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
            }
        };
        boolean autoAck = false;
        channel.basicConsume(normalQueue, autoAck, deliverCallback, consumerTag -> {
        });
    }

3.C2 消费者代码不变

启动消费者 1 然后再启动消费者 2