什么是死信队列

public class DirectConsumer {    @RabbitListener(bindings = {            @QueueBinding(value = @Queue(value = "javatrip",arguments =                     {@Argument(name="x-dead-letter-exchange",value = "deadExchange"),                     @Argument(name="x-dead-letter-routing-key",value = "deadKey")                    }),                    exchange = @Exchange(value="javatripDirect"),                    key = {"info","error","warning"}            )    })public void receive1(String message, @Headers Map<String,Object> headers, Channel channel)throws Exception{    System.out.println("消费者1"+message);}

public void publishMessage(String message){
    rabbitTemplate.setMandatory(true);    rabbitTemplate.convertAndSend("javatripDirect","info",message);}

spring:  rabbitmq:    listener:      simple:        acknowledge-mode: manual

Long deliverTag = (Long) headers.get(AmqpHeaders.DELIVERY_TAG);channel.basicNack(deliverTag,false,false);

@RabbitListener(bindings = {    @QueueBinding(        value = @Queue(value = "javatripDead"),        exchange = @Exchange(value = "deadExchange"),        key = "deadKey"    )})public void receive2(String message){    System.out.println("我是一条死信:"+message);}

@RabbitListener(bindings = {            @QueueBinding(value = @Queue(value = "javatrip",arguments =                    {@Argument(name="x-dead-letter-exchange",value = "deadExchange"),                     @Argument(name="x-dead-letter-routing-key",value = "deadKey"),                     @Argument(name = "x-message-ttl",value = "3000")                    }),                    exchange = @Exchange(value="javatripDirect"),                    key = {"info","error","warning"}            )    })public void receive1(String message, @Headers Map<String,Object> headers, Channel channel)throws Exception{    System.out.println("消费者1"+message);}

@RabbitListener(bindings = {            @QueueBinding(value = @Queue(value = "javatrip",arguments =                    {@Argument(name="x-dead-letter-exchange",value = "deadExchange"),                     @Argument(name="x-dead-letter-routing-key",value = "deadKey"),                     @Argument(name = "x-max-length",value = "3")                    }),                    exchange = @Exchange(value="javatripDirect"),                    key = {"info","error","warning"}            )    })public void receive1(String message, @Headers Map<String,Object> headers, Channel channel)throws Exception{    System.out.println("消费者1"+message);}

@Target({})@Retention(RetentionPolicy.RUNTIME)public @interface QueueBinding {
	/**	 * @return the queue.	 */	Queue value();
	/**	 * @return the exchange.	 */	Exchange exchange();
	/**	 * @return the routing key or pattern for the binding.	 * Multiple elements will result in multiple bindings.	 */	String[] key() default {};}

@Target({})@Retention(RetentionPolicy.RUNTIME)public @interface Queue {
	/**	 * @return the queue name or "" for a generated queue name (default).	 */	@AliasFor("name")	String value() default "";
	/**	 * @return the queue name or "" for a generated queue name (default).	 * @since 2.0	 */	@AliasFor("value")	String name() default "";
	/**	 * 是否持久化	 */	String durable() default "";
	/**	 * 是否独享、排外的.	 */	String exclusive() default "";
	/**	 * 是否自动删除；	 */	String autoDelete() default "";
	/**	 * 队列的其他属性参数	 * （1）x-message-ttl：消息的过期时间，单位：毫秒；         *（2）x-expires：队列过期时间，队列在多长时间未被访问将被删除，单位：毫秒；         *（3）x-max-length：队列最大长度，超过该最大值，则将从队列头部开始删除消息；         *（4）x-max-length-bytes：队列消息内容占用最大空间，受限于内存大小，超过该阈值则从队列头部开始删除消息；         *（5）x-overflow：设置队列溢出行为。这决定了当达到队列的最大长度时消息会发生什么。有效值是drop-head、reject-publish或reject-publish-dlx。仲裁队列类型仅支持drop-head；         *（6）x-dead-letter-exchange：死信交换器名称，过期或被删除（因队列长度超长或因空间超出阈值）的消息可指定发送到该交换器中；         *（7）x-dead-letter-routing-key：死信消息路由键，在消息发送到死信交换器时会使用该路由键，如果不设置，则使用消息的原来的路由键值         *（8）x-single-active-consumer：表示队列是否是单一活动消费者，true时，注册的消费组内只有一个消费者消费消息，其他被忽略，false时消息循环分发给所有消费者(默认false)         *（9）x-max-priority：队列要支持的最大优先级数;如果未设置，队列将不支持消息优先级；         *（10）x-queue-mode（Lazy mode）：将队列设置为延迟模式，在磁盘上保留尽可能多的消息，以减少RAM的使用;如果未设置，队列将保留内存缓存以尽可能快地传递消息；         *（11）x-queue-master-locator：在集群模式下设置镜像队列的主节点信息。	 */	Argument[] arguments() default {};}

@Target({})@Retention(RetentionPolicy.RUNTIME)public @interface Exchange {
	String TRUE = "true";
	String FALSE = "false";
	/**	 * @return the exchange name.	 */	@AliasFor("name")	String value() default "";
	/**	 * @return the exchange name.	 * @since 2.0	 */	@AliasFor("value")	String name() default "";
	/**	 * 交换机类型，默认DIRECT	 */	String type() default ExchangeTypes.DIRECT;
	/**	 * 是否持久化	 */	String durable() default TRUE;
	/**	 * 是否自动删除	 */	String autoDelete() default FALSE;
	/**	 * @return the arguments to apply when declaring this exchange.	 * @since 1.6	 */	Argument[] arguments() default {};}