何为死信队列？

死信队列实际上就是，当我们的业务队列处理失败(比如抛异常并且达到了retry的上限)，就会将消息重新投递到另一个Exchange(Dead Letter Exchanges)，该Exchange再根据routingKey重定向到另一个队列，在这个队列重新处理该消息。

来自一个队列的消息可以被当做‘死信’，即被重新发布到另外一个“exchange”去，这样的情况有：

消息被拒绝 (basic.reject or basic.nack) 且带 requeue=false不重新入队参数或达到的retry重新入队的上限次数
消息的TTL(Time To Live)-存活时间已经过期
队列长度限制被超越（队列满，queue的"x-max-length"参数）

Dead letter exchanges (DLXs) are normal exchanges.

For any given queue, a DLX can be defined by clients using the queue's arguments, or in the server using policies.

经过上面的认知，可知应用场景：重要的业务队列如果失败，就需要重新将消息用另一种业务逻辑处理；如果是正常的业务逻辑故意让消息中不合法的值失败，就不需要死信；具体场景具体分析。

SpringBoot配置文件

设置重试次数、间隔和投递到死信队列

spring.application.name=spring-boot-rabbitmq
spring.rabbitmq.host=localhost
spring.rabbitmq.port=5672
spring.rabbitmq.username=nut
spring.rabbitmq.password=nut

# 允许消息消费失败的重试
spring.rabbitmq.listener.simple.retry.enabled=true
# 消息最多消费次数3次
spring.rabbitmq.listener.simple.retry.max-attempts=3
# 消息多次消费的间隔1秒
spring.rabbitmq.listener.simple.retry.initial-interval=1000
#  设置为false，会丢弃消息或者重新发布到死信队列
spring.rabbitmq.listener.simple.default-requeue-rejected=false


server.port=5678

初始化和绑定重定向队列配置类

import org.springframework.amqp.core.*;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

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

/**
 * 死信队列的配置
 */
@Configuration
public class RabbitDeadLetterConfig {

    public static final String DEAD_LETTER_EXCHANGE = "TDL_EXCHANGE";
    public static final String DEAD_LETTER_TEST_ROUTING_KEY = "TDL_KEY";
    public static final String DEAD_LETTER_REDIRECT_ROUTING_KEY = "TKEY_R";
    public static final String DEAD_LETTER_QUEUE = "TDL_QUEUE";
    public static final String REDIRECT_QUEUE = "TREDIRECT_QUEUE";

    /**
     * 死信队列跟交换机类型没有关系 不一定为directExchange  不影响该类型交换机的特性.
     */
    @Bean("deadLetterExchange")
    public Exchange deadLetterExchange() {
        return ExchangeBuilder.directExchange(DEAD_LETTER_EXCHANGE).durable(true).build();
    }

    @Bean("deadLetterQueue")
    public Queue deadLetterQueue() {
        Map<String, Object> args = new HashMap<>(2);
//       x-dead-letter-exchange    声明  死信队列Exchange
        args.put("x-dead-letter-exchange", DEAD_LETTER_EXCHANGE);
//       x-dead-letter-routing-key    声明 死信队列抛出异常重定向队列的routingKey(TKEY_R)
        args.put("x-dead-letter-routing-key", DEAD_LETTER_REDIRECT_ROUTING_KEY);
        return QueueBuilder.durable(DEAD_LETTER_QUEUE).withArguments(args).build();
    }

    @Bean("redirectQueue")
    public Queue redirectQueue() {
        return QueueBuilder.durable(REDIRECT_QUEUE).build();
    }

    /**
     * 死信队列绑定到死信交换器上.
     *
     * @return the binding
     */
    @Bean
    public Binding deadLetterBinding() {
        return new Binding(DEAD_LETTER_QUEUE, Binding.DestinationType.QUEUE, DEAD_LETTER_EXCHANGE, DEAD_LETTER_TEST_ROUTING_KEY, null);

    }

    /**
     * 将重定向队列通过routingKey(TKEY_R)绑定到死信队列的Exchange上
     *
     * @return the binding
     */
    @Bean
    public Binding redirectBinding() {
        return new Binding(REDIRECT_QUEUE, Binding.DestinationType.QUEUE, DEAD_LETTER_EXCHANGE, DEAD_LETTER_REDIRECT_ROUTING_KEY, null);
    }
}

生产者向业务队列发送消息

这里为了方便测试没有往业务队列发送消息，直接往死信Exchange里投递消息。

import lombok.extern.slf4j.Slf4j;
import org.springframework.amqp.rabbit.core.RabbitTemplate;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import xy.study.rabbitmq.conf.RabbitDeadLetterConfig;

@Slf4j
@Component
public class DeadLetterSender {

    @Autowired
    private RabbitTemplate rabbitTemplate;

    public void send(int number) {
        log.warn("DeadLetterSender : {}", number);
        // 这里的Exchange可以是业务的Exchange，为了方便测试这里直接往死信Exchange里投递消息
        rabbitTemplate.convertAndSend(
                RabbitDeadLetterConfig.DEAD_LETTER_EXCHANGE,
                RabbitDeadLetterConfig.DEAD_LETTER_TEST_ROUTING_KEY,
                number);
    }
}

死信队列消费者

这里会抛异常

import lombok.extern.slf4j.Slf4j;
import org.springframework.amqp.rabbit.annotation.RabbitHandler;
import org.springframework.amqp.rabbit.annotation.RabbitListener;
import org.springframework.messaging.handler.annotation.Payload;
import org.springframework.stereotype.Component;
import xy.study.rabbitmq.conf.RabbitDeadLetterConfig;

@Slf4j
@Component
@RabbitListener(queues = RabbitDeadLetterConfig.DEAD_LETTER_QUEUE)
public class DeadLetterConsumer {

    /*@RabbitListener(bindings = @QueueBinding(
            value = @Queue(value = RabbitDeadLetterConfig.DEAD_LETTER_QUEUE, durable = "true"),
            exchange = @Exchange(value = RabbitDeadLetterConfig.DEAD_LETTER_EXCHANGE, type = ExchangeTypes.DIRECT),
            key = RabbitDeadLetterConfig.DEAD_LETTER_TEST_ROUTING_KEY)
    )*/
    @RabbitHandler
    public void testDeadLetterQueueAndThrowsException(@Payload Integer number){
        log.warn("DeadLetterConsumer :{}/0 ", number);
        int i = number / 0;
    }
}

重定向队列

队列"死信"后，会将消息投递到Dead Letter Exchanges，然后该Exchange会将消息投递到重定向队列。

此时，在重定向队列中，做对应的业务操作。

import lombok.extern.slf4j.Slf4j;
import org.springframework.amqp.rabbit.annotation.RabbitHandler;
import org.springframework.amqp.rabbit.annotation.RabbitListener;
import org.springframework.stereotype.Component;
import xy.study.rabbitmq.conf.RabbitDeadLetterConfig;

@RabbitListener(queues = RabbitDeadLetterConfig.REDIRECT_QUEUE)
@Component
@Slf4j
public class RedirectQueueConsumer {

    /**
     * 重定向队列和死信队列形参一致Integer number
     * @param number
     */
    @RabbitHandler
    public void fromDeadLetter(Integer number){
        log.warn("RedirectQueueConsumer : {}", number);
        // 对应的操作
        int i = number / 1;
    }
}

测试

先启动项目

然后利用测试类发送一条信息

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import xxx.DeadLetterSender;

@RunWith(SpringRunner.class)
@SpringBootTest
public class RabbitmqApplicationTests {


    @Autowired
    private DeadLetterSender deadLetterSender;


    @Test
    public void testSendDeadLetterQueue(){
        deadLetterSender.send(15);
    }

}