一、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

二、出现死信的原因

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.

三、实战

（一）代码架构图

（二）消息被拒

生产者

public class Producer {public static final String NORMAL_EXCHANGE = "normal_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();for (int i = 0; i < 10; i++) {String message = "info" + i;channel.basicPublish(NORMAL_EXCHANGE, "zhangsan", null, message.getBytes());}}
}

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 {public static final String NORMAL_QUEUE = "normal_queue";public static final String NORMAL_EXCHANGE = "normal_exchange";public static final String DEAD_QUEUE = "dead_queue";public static final String DEAD_EXCHANGE = "dead_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();// 声明普通和死信交换机channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);// 声明死信队列channel.queueDeclare(DEAD_QUEUE, false, false, false, null);// 死信的绑定channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");Map<String, Object> arguments = new HashMap<>();// 普通队列设置对应的交换机arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);// 设置死信队列的RouteKeyarguments.put("x-dead-letter-routing-key", "lisi");// 设置队列最大长度arguments.put("x-max-length", 6);// 声明普通队列channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);// 普通的绑定channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zhangsan");DeliverCallback deliverCallback = (consumerTag, message) -> {String msg = new String(message.getBody());if (msg.equals("info5")) {System.out.println("Consumer01接收到消息" + message + "并拒绝签收该消息");channel.basicReject(message.getEnvelope().getDeliveryTag(), false);} else {System.out.println("consumer01接收到消息：" + msg);channel.basicAck(message.getEnvelope().getDeliveryTag(), false);}};channel.basicConsume(NORMAL_QUEUE, false, deliverCallback, consumerTag -> {});}
}

C2 消费者代码不变

启动消费者 1 然后再启动消费者 2

（三）消息TTL过期

生产者代码

public class Producer {public static final String NORMAL_EXCHANGE = "normal_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();AMQP.BasicProperties properties = new AMQP.BasicProperties().builder().expiration("10000").build();for (int i = 0; i < 10; i++) {String message = "info" + i;channel.basicPublish(NORMAL_EXCHANGE, "zhangsan", properties, message.getBytes());}}
}

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 {public static final String NORMAL_QUEUE = "normal_queue";public static final String NORMAL_EXCHANGE = "normal_exchange";public static final String DEAD_QUEUE = "dead_queue";public static final String DEAD_EXCHANGE = "dead_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();// 声明普通和死信交换机channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);// 声明死信队列channel.queueDeclare(DEAD_QUEUE, false, false, false, null);// 死信的绑定channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");Map<String, Object> arguments = new HashMap<>();// 普通队列设置对应的交换机arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);// 设置死信队列的RouteKeyarguments.put("x-dead-letter-routing-key", "lisi");// 声明普通队列channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);// 普通的绑定channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zhangsan");DeliverCallback deliverCallback = (consumerTag, message) -> {String msg = new String(message.getBody());System.out.println("consumer01接收到消息：" + msg);};channel.basicConsume(NORMAL_QUEUE, false, 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 {public static final String DEAD_QUEUE = "dead_queue";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();System.out.println("等待接收死信队列消息.....");DeliverCallback deliverCallback = (consumerTag, message) -> {System.out.println("Consumer02 接收死信队列的消息：" + new String(message.getBody()));};channel.basicConsume(DEAD_QUEUE, true, deliverCallback, consumerTag -> {});}
}

（四）队列达到最大长度

我们在声明普通队列时添加一个参数x-max-length即可

生产者

public class Producer {public static final String NORMAL_EXCHANGE = "normal_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();for (int i = 0; i < 10; i++) {String message = "info" + i;channel.basicPublish(NORMAL_EXCHANGE, "zhangsan", null, message.getBytes());}}
}

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)，这里设置了队列最多容纳6条消息，此时由于生产者发送了10条消息，所以有4条会进入死信队列。

public class Consumer01 {public static final String NORMAL_QUEUE = "normal_queue";public static final String NORMAL_EXCHANGE = "normal_exchange";public static final String DEAD_QUEUE = "dead_queue";public static final String DEAD_EXCHANGE = "dead_exchange";public static void main(String[] args) throws IOException {Channel channel = RabbitMqUtils.getChannel();// 声明普通和死信交换机channel.exchangeDeclare(NORMAL_EXCHANGE, BuiltinExchangeType.DIRECT);channel.exchangeDeclare(DEAD_EXCHANGE, BuiltinExchangeType.DIRECT);// 声明死信队列channel.queueDeclare(DEAD_QUEUE, false, false, false, null);// 死信的绑定channel.queueBind(DEAD_QUEUE, DEAD_EXCHANGE, "lisi");Map<String, Object> arguments = new HashMap<>();// 普通队列设置对应的交换机arguments.put("x-dead-letter-exchange", DEAD_EXCHANGE);// 设置死信队列的RouteKeyarguments.put("x-dead-letter-routing-key", "lisi");// 设置队列最大长度arguments.put("x-max-length", 6);// 声明普通队列channel.queueDeclare(NORMAL_QUEUE, false, false, false, arguments);// 普通的绑定channel.queueBind(NORMAL_QUEUE, NORMAL_EXCHANGE, "zhangsan");DeliverCallback deliverCallback = (consumerTag, message) -> {String msg = new String(message.getBody());System.out.println("consumer01接收到消息：" + msg);};channel.basicConsume(NORMAL_QUEUE, false, deliverCallback, consumerTag -> {});}
}