将消息发布到代理后，MQTT 客户端未收到确认

Question

我的问题是，当我想响应到达的消息时，我没有从代理那里取回传递令牌，但消息已到达代理。我可以在日志中看到它。所以我的问题是如何通过发布来响应传入的消息？

这是我的做法，但它不起作用......

public void setMqttCallback() {

        mqttClient.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(final Throwable throwable) {
                System.out.println("Lost connection to Broker because of: " + throwable.getMessage());
            }

            @Override
            public void messageArrived(final String topic, final MqttMessage mqttMessage) throws Exception {
                System.out.println("Received on " + topic + ": " + new String(mqttMessage.getPayload()));

                topics = new Topics();

                //Answer to arriving messages (Logic)
                if (topic.equals(topics.getVehicleNavLandmarkInfo(Mqtt.VIN_ID))) {

                    landmarks = new Landmarks();

                    MqttMessage message = new MqttMessage(landmarks.getLandmarks().getBytes());
                    message.setQos(2);

                    System.out.println("Sending on " + topic + ": " + landmarks.getLandmarks());
                    
                    mqttClient.publish(topics.getBackendNavLandsmarks(Mqtt.VIN_ID), message);
                }

                if (topic.equals(topics.getVehicleNavDestination(Mqtt.VIN_ID))) {

                    routing = new Routing(49.0000, 8.0000, "A");

                    MqttMessage message = new MqttMessage(routing.getShortestPath().getBytes());
                    message.setQos(2);

                    System.out.println("Sending on " + topic + ": " + routing.getShortestPath());
                    
                    mqttClient.publish(topics.getBackendNavRoute(Mqtt.VIN_ID), message);
                }
            }

            @Override
            public void deliveryComplete(final IMqttDeliveryToken iMqttDeliveryToken) {
                System.out.println("Delivered Message: " + iMqttDeliveryToken.toString());
            }
        });
    }

Answer 1

messageArrived 方法被阻塞，因此您不能尝试使用相同的 mqttClient 连接发布消息。您需要使用另一个与代理有自己连接的线程。

如果是一次性情况，那么您可以启动一个新线程并在其中执行所有操作，但如果您的应用程序将不断向到达的消息发布消息，那么最好启动一个线程，连接到代理然后等待用于从 messageArrived 方法传递的信息。