QoS1 vs QoS2 messages – difference in handling it with Paho MQTT client in Java

In reference to this article I think I know the difference between QoS1 and QoS2 messages, but I don’t know the difference in handling both of them as a Paho MQTT client.

Imagine I subscribe to the topic like this:

MqttClient subscriber = new MqttClient(SERVER_URI, SUBSCRIBER_ID);

And then I’m publishing messages like this:

publisher.publish(TOPIC, PAYLOAD, 1, false);

At this moment I’m using MqttCallback interface to handle messages that arrives to subscribers.

There is a method to override:

public void messageArrived(String topic, MqttMessage mqttMessage) {
    if(mqttMessage.isDuplicate()) {
        // is it really the duplicate message from my perspective?
    } else {...}

In the MqttMessage we can find a isDuplicate() method, but how can I be sure that the mqttMessage that returns true is not the first message that my subscriber received?

I am very interested in finding a solution that shows how to handle QoS1, but every answer which will clarify anything here will be appreciated.

Best regards from Cracow!

Source: java

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