当发布消息时，有一个mandatory参数，参见源方法：
void basicPublish(String exchange, String routingKey, boolean mandatory, BasicProperties props, byte[] body)mandatory为true时，表示如果消息没有被正确路由，消息将退回消息的生产者，生产者需要添加监听器：
channel.addReturnListener((replyCode, replyText, exchange, routingKey, properties, body) -> System.out.println(String.format("消息返回监听: %s", new String(body))));我们知道在rabbitmq中，消息生产者其实并不关心消息队列，只需将消息发布至exchange，并指定routingKey路由队列，当routingKey配置不正确时，消息无法路由到指定队列时，生产者将会通过监听收到broker退回的消息。
public class Producer { private final static String QUEUE_NAME = "hello"; public static void main(String[] argv) throws Exception { ConnectionFactory factory = new ConnectionFactory(); factory.setHost("localhost"); try (Connection connection = factory.newConnection(); Channel channel = connection.createChannel()) { channel.queueDeclare(QUEUE_NAME, true, false, false, null); String message = "Hello World!"; channel.basicPublish("", "",true, null, message.getBytes(StandardCharsets.UTF_8)); channel.addReturnListener((replyCode, replyText, exchange, routingKey, properties, body) -> System.out.println(String.format("消息返回监听: %s", new String(body)))); System.out.println(" [x] Sent '" + message + "'"); } Scanner scanner = new Scanner(System.in); while(scanner.hasNext()){ System.out.println(scanner.next()); } } }如上定义了队列hello，在消息发布时未指定routingKey，生产者将收到被退回的消息，当mandatory参数为false时，未被正确路由的消息将会直接被丢弃。
