我的問題是，當我想響應到達消息時，我沒有從經紀人那里獲得遞送令牌，但消息到達了經紀人。我可以在日志中看到它。所以我的問題是如何通過發布對收到的消息進行響應？這是我的方式，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());            }        });    }