一、死信的概念
死信，顾名思义就是无法被消费的消息。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，档消息消费发生异常时，将消息投入到死信队列中。还有比如说：用户在商城下单成功并点击支付后再指定时间未支付时自动失效。
二、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（不再重新入队）
三、死信实战
3.1 代码架构图
3.2 消息 TTL 过期
生产者
public class DeadLetterProducer {
private static String EXCHANGE_NAME = "normal_exchange";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
// 声明一个交换机
channel.exchangeDeclare(EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 设置消息 TTL 过期时间
AMQP.BasicProperties properties = new AMQP.BasicProperties().builder().expiration("10000").build();
String message = "info";
channel.basicPublish(EXCHANGE_NAME, "zhangsan", properties, message.getBytes());
System.out.println("消息发送完成：" + message);
}
}
消费者1
public class DeadLetterConsumer1 {
private static String NORMAL_EXCHANGE_NAME = "normal_exchange";
private static String NORMAL_QUEUE_NAME = "normal-queue";
private static String DEAD_EXCHANGE_NAME = "dead_exchange";
private static String DEAD_QUEUE_NAME = "dead-queue";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
// 声明一个死信队列
channel.queueDeclare(DEAD_QUEUE_NAME, false, false, false, null);
// 声明一个死信交换机
channel.exchangeDeclare(DEAD_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 死信队列与死信交换机绑定
channel.queueBind(DEAD_QUEUE_NAME, DEAD_EXCHANGE_NAME, "lisi");
// 正常队列与死信交换机的绑定关系
Map<String, Object> deadLetterParams = new HashMap<>(2);
deadLetterParams.put("x-dead-letter-exchange", DEAD_EXCHANGE_NAME);
deadLetterParams.put("x-dead-letter-routing-key","lisi");
// 声明一个正常队列
channel.queueDeclare(NORMAL_QUEUE_NAME, false, false, false, deadLetterParams);
// 声明一个正常交换机
channel.exchangeDeclare(NORMAL_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 把队列和交换机进行绑定
channel.queueBind(NORMAL_QUEUE_NAME, NORMAL_EXCHANGE_NAME, "zhangsan");
System.out.println("C1消费者启动等待消费消息：");
channel.basicConsume(NORMAL_QUEUE_NAME, true, (consumerTag, delivery) -> {
String receivedMessage = new String(delivery.getBody());
System.out.println("消费者接收到消息：" + receivedMessage);
},(consumerTag) -> {
System.out.println(consumerTag + "消费者取消消费消息");
});
}
}
消费者2
public class DeadLetterConsumer2 {
private static String NORMAL_EXCHANGE_NAME = "normal_exchange";
private static String DEAD_QUEUE_NAME = "dead-queue";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
channel.exchangeDeclare(NORMAL_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
System.out.println("C2消费者启动等待消费消息：");
channel.basicConsume(DEAD_QUEUE_NAME, true, (consumerTag, delivery) -> {
String receivedMessage = new String(delivery.getBody());
System.out.println("消费者接收到死信：" + receivedMessage);
},(consumerTag) -> {
System.out.println(consumerTag + "消费者取消消费消息");
});
}
}
先启动消费者1，将正常交换机、死信交换机、正常队列、死信队列创建出来，否则会报错。接着启动消费者2，然后在启动生产者，观察控制台。
消费者1启动后进入RabbitMQ系统后台，可以看到队列 normal-queue 的 features 一列多了两个信息。其中 DLX 表示死信交换机，DLK 表示死信交换机的路由键（RoutingKey）。
此时由于消费者1可以正常消费消息，所以在消费者2中，死信队列是接收不到消息的。控制台情况如下:
将消费者1和消费者2的服务停止，重新运行生产者，10s 后消息会被进入到死信队列
再来看下后台系统：
生产者未发送消息
生产者发送了 1 条消息，此时正常队列中有 1 条未消费消息
时间过去 10 秒，正常队列里面的消息由于没有被消费，消息进入死信队列。
3.3 队列达到最大长度
生产者
public class DeadLetterLengthProducer {
private static String EXCHANGE_NAME = "normal_exchange";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
// 声明一个交换机
channel.exchangeDeclare(EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 设置消息 TTL 过期时间
for (int i = 0; i < 10; i++) {
String message = "info" + i;
channel.basicPublish(EXCHANGE_NAME, "zhangsan", null, message.getBytes());
}
System.out.println("消息发送完成");
}
}
消费者1
public class DeadLetterLengthConsumer1 {
private static String NORMAL_EXCHANGE_NAME = "normal_exchange";
private static String NORMAL_QUEUE_NAME = "normal-queue";
private static String DEAD_EXCHANGE_NAME = "dead_exchange";
private static String DEAD_QUEUE_NAME = "dead-queue";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
// 声明一个死信队列
channel.queueDeclare(DEAD_QUEUE_NAME, false, false, false, null);
// 声明一个死信交换机
channel.exchangeDeclare(DEAD_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 死信队列与死信交换机绑定
channel.queueBind(DEAD_QUEUE_NAME, DEAD_EXCHANGE_NAME, "lisi");
// 正常队列与死信交换机的绑定关系
Map<String, Object> deadLetterParams = new HashMap<>(2);
deadLetterParams.put("x-dead-letter-exchange", DEAD_EXCHANGE_NAME);
deadLetterParams.put("x-dead-letter-routing-key","lisi");
deadLetterParams.put("x-max-length", 6);
// 声明一个正常队列
channel.queueDeclare(NORMAL_QUEUE_NAME, false, false, false, deadLetterParams);
// 声明一个正常交换机
channel.exchangeDeclare(NORMAL_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 把队列和交换机进行绑定
channel.queueBind(NORMAL_QUEUE_NAME, NORMAL_EXCHANGE_NAME, "zhangsan");
System.out.println("C1消费者启动等待消费消息：");
channel.basicConsume(NORMAL_QUEUE_NAME, true, (consumerTag, delivery) -> {
String receivedMessage = new String(delivery.getBody());
System.out.println("消费者接收到消息：" + receivedMessage);
},(consumerTag) -> {
System.out.println(consumerTag + "消费者取消消费消息");
});
}
}
消费者2
public class DeadLetterLengthConsumer2 {
private static String NORMAL_EXCHANGE_NAME = "normal_exchange";
private static String DEAD_QUEUE_NAME = "dead-queue";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
channel.exchangeDeclare(NORMAL_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
System.out.println("C2消费者启动等待消费消息：");
channel.basicConsume(DEAD_QUEUE_NAME, true, (consumerTag, delivery) -> {
String receivedMessage = new String(delivery.getBody());
System.out.println("消费者接收到死信：" + receivedMessage);
},(consumerTag) -> {
System.out.println(consumerTag + "消费者取消消费消息");
});
}
}
由于消费者1中修改了队列参数，所以启动前需要先将原先的队列删除，然后再启动消费者1，创建相关的队列及交换机。接着关闭消费者 1，启动生产者。打开后台系统：
普通队列中有 6 条消息未消费，超出队列长度的 4 条消息进入到了死信队列。
然后启动消费者1 和消费者2
3.4 消息被拒
生产者和消费者2 的代码不需要修改，修改消费者1 的代码，修改后的代码如下：
消费者2
public class DeadLetterRejectConsumer1 {
private static String NORMAL_EXCHANGE_NAME = "normal_exchange";
private static String NORMAL_QUEUE_NAME = "normal-queue";
private static String DEAD_EXCHANGE_NAME = "dead_exchange";
private static String DEAD_QUEUE_NAME = "dead-queue";
public static void main(String[] args) throws IOException, TimeoutException {
Channel channel = RabbitMqUtil.getChannel();
// 声明一个死信队列
channel.queueDeclare(DEAD_QUEUE_NAME, false, false, false, null);
// 声明一个死信交换机
channel.exchangeDeclare(DEAD_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 死信队列与死信交换机绑定
channel.queueBind(DEAD_QUEUE_NAME, DEAD_EXCHANGE_NAME, "lisi");
// 正常队列与死信交换机的绑定关系
Map<String, Object> deadLetterParams = new HashMap<>(2);
deadLetterParams.put("x-dead-letter-exchange", DEAD_EXCHANGE_NAME);
deadLetterParams.put("x-dead-letter-routing-key","lisi");
// 声明一个正常队列
channel.queueDeclare(NORMAL_QUEUE_NAME, false, false, false, deadLetterParams);
// 声明一个正常交换机
channel.exchangeDeclare(NORMAL_EXCHANGE_NAME, BuiltinExchangeType.DIRECT);
// 把队列和交换机进行绑定
channel.queueBind(NORMAL_QUEUE_NAME, NORMAL_EXCHANGE_NAME, "zhangsan");
System.out.println("C1消费者启动等待消费消息：");
channel.basicConsume(NORMAL_QUEUE_NAME, false, (consumerTag, delivery) -> {
String receivedMessage = new String(delivery.getBody());
if ("info5".equals(receivedMessage)) {
System.out.println("C1接收到消息：" + receivedMessage+"并且拒绝签收了");
// 禁止重新入队
channel.basicReject(delivery.getEnvelope().getDeliveryTag(), false);
} else {
System.out.println("消费者接收到消息：" + receivedMessage);
channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
}
},(consumerTag) -> {
System.out.println(consumerTag + "消费者取消消费消息");
});
}
}
将原先的队列删除，重新启动消费者2，接着启动生产者
最后启动消费者2
