Question

我正在和Mqtt paho司机挣扎......

我正在使用IMqttDeliveryToken在收到我的发布时从服务器获取Acknowledge。

为了将它与实际的发布消息进行比较，我在MqttMessage上设置了一个ID，以便从IMqttDeliveryToken中检索它...但它不起作用... IMqttDeliveryToken.getMessageId（）返回一个不正确的ID当我尝试在IMqttDeliveryToken.getMessage（）之后获取ID时，当QoS与0不同时，它返回一个NPE。

阅读Javadoc后，我读到这是通常的行为：

在邮件发送之前，将返回正在传递的邮件。邮件一旦传递，将返回null。

这引出了另一个问题...在Broker发送确认后，是否真的调用了deliveryComplete（）方法？

这是我的代码：

client.setCallback(new MqttCallback() {
    @Override
    public void connectionLost(Throwable thrwbl) { }

    @Override
    public void messageArrived(String string, MqttMessage mm) throws Exception { }

    @Override
    public void deliveryComplete(IMqttDeliveryToken token) {
        try {
            System.out.println("Message ID from getMessageId() method : " + token.getMessageId());
            MqttMessage message = token.getMessage();
            System.out.println("Message ID from getMessage() method : " + message.getId());
        } catch (MqttException ex) {
            System.out.println(ex);
        } catch (Exception ex) {
            System.out.println(ex);
        }
    }
});

MqttMessage message = new MqttMessage();
message.setId(76);
message.setPayload("pouet".getBytes());
message.setQos(0);

client.publish("TEST", message);

QoS为0：

Message ID from getMessageId() method : 1
Message ID from getMessage() method : 76

QoS为1：

Message ID from getMessageId() method : 1
java.lang.NullPointerException

Answer 1

如git MqttMessage.java

中所述

/**
 * This is only to be used internally to provide the MQTT id of a message
 * received from the server.  Has no effect when publishing messages.
 * @param messageId
 */
public void setId(int messageId) {
    this.messageId = messageId;
}

发布邮件时没有使用此选项。现在要理解为什么来自getMessageId（）方法的消息ID：1发生了以下内容。

public IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback) throws MqttException,
            MqttPersistenceException {
        final String methodName = "publish";
        //@TRACE 111=< topic={0} message={1}userContext={1} callback={2}
        log.fine(CLASS_NAME,methodName,"111", new Object[] {topic, userContext, callback});

        //Checks if a topic is valid when publishing a message.
        MqttTopic.validate(topic, false/*wildcards NOT allowed*/);

        MqttDeliveryToken token = new MqttDeliveryToken(getClientId());
        token.setActionCallback(callback);
        token.setUserContext(userContext);
        token.setMessage(message);
        token.internalTok.setTopics(new String[] {topic});

        MqttPublish pubMsg = new MqttPublish(topic, message);
        comms.sendNoWait(pubMsg, token);

        //@TRACE 112=<
        log.fine(CLASS_NAME,methodName,"112");

        return token;
    }

MqttDeliveryToken在此处未设置消息ID。发布时创建MqttPublish实例，该实例在内部将多级扩展为MqttWireMessage.java，默认情况下该值设置为0。

public MqttWireMessage(byte type) {
        this.type = type;
        // Use zero as the default message ID.  Can't use -1, as that is serialized
        // as 65535, which would be a valid ID.
        this.msgId = 0;
    }

当在ClientState.java中为mqtt发布调用Final Send时，MqttWireMessage实例（内部MqttPublish从上面的代码发送）转发了msgId = 0，因为if条件为true且getNextMessageId（）为调用哪个返回1（因为它是第一个消息，否则它将返回后续值，具体取决于最后的消息ID）并设置为令牌，您在代码中使用deliveryComplete（）进行跟踪。

public void send(MqttWireMessage message, MqttToken token) throws MqttException {
        final String methodName = "send";
        if (message.isMessageIdRequired() && (message.getMessageId() == 0)) {
            message.setMessageId(getNextMessageId());
        }
        if (token != null ) {
            try {
                token.internalTok.setMessageID(message.getMessageId());
            } catch (Exception e) {
            }
        }

        /////......
    }

Answer 2

回答你的下一个问题：在经纪人发送确认后，是否确实调用了deliveryComplete（）方法---是!!

从这一小段代码中调用已完成的交付。

private void handleActionComplete(MqttToken token)
            throws MqttException {
        final String methodName = "handleActionComplete";
        synchronized (token) {
            // @TRACE 705=callback and notify for key={0}
            log.fine(CLASS_NAME, methodName, "705", new Object[] { token.internalTok.getKey() });
            if (token.isComplete()) {
                // Finish by doing any post processing such as delete 
                // from persistent store but only do so if the action
                // is complete
                clientState.notifyComplete(token);
            }

            // Unblock any waiters and if pending complete now set completed
            token.internalTok.notifyComplete();

            if (!token.internalTok.isNotified()) {
                // If a callback is registered and delivery has finished 
                // call delivery complete callback. 
                if ( mqttCallback != null 
                    && token instanceof MqttDeliveryToken 
                    && token.isComplete()) {
                        mqttCallback.deliveryComplete((MqttDeliveryToken) token);
                }
                // Now call async action completion callbacks
                fireActionEvent(token);
            }

            // Set notified so we don't tell the user again about this action.
            if ( token.isComplete() ){
               if ( token instanceof MqttDeliveryToken || token.getActionCallback() instanceof IMqttActionListener ) {
                    token.internalTok.setNotified(true);
                }
            }



        }
    }

即一旦收到确认，即完成notifyComplete，设置了标志，则调用deliveryComplete方法。

如何判断ACK对应于MQTT上的哪个发布消息？

2 个答案: