在SpringBoot的RabbitMQ配置中残腌，有如下一項：
spring.rabbitmq.template.mandatory = true
官方注釋：
Enable mandatory messages. If a mandatory message cannot be routed to a queue by the server, it will return an unroutable message with a Return method.
大意為：開啟強制消息投遞（mandatory為設(shè)置為true）枯夜，但消息未被路由至任何一個queue，則回退一條消息到RabbitTemplate.ReturnCallback中的returnedMessage方法：
public interface ReturnCallback {
/**
* Returned message callback.
* @param message the returned message.
* @param replyCode the reply code.
* @param replyText the reply text.
* @param exchange the exchange.
* @param routingKey the routing key.
*/
void returnedMessage(Message message, int replyCode, String replyText,
String exchange, String routingKey);
}
瀏覽RabbitAutoConfiguration發(fā)現(xiàn)如下代碼：
private boolean determineMandatoryFlag() {
Boolean mandatory = this.properties.getTemplate().getMandatory();
return (mandatory != null ? mandatory : this.properties.isPublisherReturns());
}
如果設(shè)置了mandatory參數(shù)，則直接取值疏尿；如若mandatory參數(shù)為空榆鼠，則取之于否起開了消息回退
