rabbitmq消息发布mandatory参数

channel.basicPublish("", "",true, null, message.getBytes(StandardCharsets.UTF_8));

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。