Java Eclipse Paho Implementation - Auto reconnect

Question

I'm trying to implement eclipse.paho in my project to connect Mqtt Broker (Both subscribing and publishing purpose). The problem is, when I using the subscribing feature (Implementing MqttCallback interface), I couldn't figure our how can I reconnect if the connection lost. MqttCallback interface has a connectionLost method, but it is useful for the debug what causes the connection lost. I searched but couldn't find a way to establish auto reconnect. Can you suggest a way or document about this problem?

Epirogeny

Answer 1

The best way to do this is to structure your connection logic so it lives in a method on it's own so it can be called again from the connectionLost callback in the MqttCallback instance.

The connectionLost method is passed a Throwable that will be the exception that triggered the disconnect so you can make decisions about the root cause and how this may effect when/how you reconnect.

The connection method should connect and subscribe to the topics you require.

Something like this:

public class PubSub {

  MqttClient client;
  String topics[] = ["foo/#", "bar"];
  MqttCallback callback = new MqttCallback() {
    public void connectionLost(Throwable t) {
      this.connect();
    }

    public void messageArrived(String topic, MqttMessage message) throws Exception {
      System.out.println("topic - " + topic + ": " + new String(message.getPayload()));
    }

    public void deliveryComplete(IMqttDeliveryToken token) {
    }
  };

  public static void main(String args[]) {
    PubSub foo = new PubSub();
  }

  public PubSub(){
    this.connect();
  }

  public void connect(){
    client = new MqttClient("mqtt://localhost", "pubsub-1");
    client.setCallback(callback);
    client.connect();
    client.subscribe(topics);
  }

}

Seligman

Ask a new question for a new problem – Seligman

I might be too late to this party... once the MqttCallback::connectionLost is called it will try to connect again. If the network is still down, that connect attempt will fail. So at that point the Mqtt part will stop trying (I assume). What is a good way to do the next part of when the network comes back up to call the connect? (BroadcastReceiver onReceive?) – Breannebrear

This question and answer are not Android related. Please ask a new question – Seligman

Answer 2

I'm using the paho client 1.2.0. With the MqttClient.setAutomaticReconnect(true) and interface MqttCallbackExtended API, and thanks to https://github.com/eclipse/paho.mqtt.java/issues/493, I could manage to reconnect automatically when the connection to broker is down.

See below the code.

//Use the MqttCallbackExtended to (re-)subscribe when method connectComplete is invoked
public class MyMqttClient implements MqttCallbackExtended {
    private static final Logger logger = LoggerFactory.getLogger(MqttClientTerni.class);
    private final int qos = 0;
    private String topic = "mytopic";
    private MqttClient client;

    public MyMqttClient() throws MqttException {
        String host = "tcp://localhost:1883";
        String clientId = "MQTT-Client";

        MqttConnectOptions conOpt = new MqttConnectOptions();
        conOpt.setCleanSession(true);
        //Pay attention here to automatic reconnect
    conOpt.setAutomaticReconnect(true);
        this.client = new org.eclipse.paho.client.mqttv3.MqttClient(host, clientId);
        this.client.setCallback(this);
        this.client.connect(conOpt);
    }

    /**
     * @see MqttCallback#connectionLost(Throwable)
     */
    public void connectionLost(Throwable cause) {
        logger.error("Connection lost because: " + cause);


    /**
     * @see MqttCallback#deliveryComplete(IMqttDeliveryToken)
     */
    public void deliveryComplete(IMqttDeliveryToken token) {
    }

    /**
     * @see MqttCallback#messageArrived(String, MqttMessage)
     */
    public void messageArrived(String topic, MqttMessage message) throws MqttException {
        logger.info(String.format("[%s] %s", topic, new String(message.getPayload())));
    }

    public static void main(String[] args) throws MqttException, URISyntaxException {
        MyMqttClient s = new MyMqttClient();
    }

    @Override
    public void connectComplete(boolean arg0, String arg1) {
        try {
      //Very important to resubcribe to the topic after the connection was (re-)estabslished. 
      //Otherwise you are reconnected but you don't get any message
        this.client.subscribe(this.topic, qos);
        } catch (MqttException e) {
            e.printStackTrace();
        }

    }
}

Pocketbook

Answer 3

To use auto reconnect, just set setAutomaticReconnect(true) on the MqttConnectOptions object.

MqttAndroidClient mqttClient = new MqttAndroidClient(context, mqttUrl, clientId);

MqttConnectOptions mqttConnectOptions = new MqttConnectOptions();
mqttConnectOptions.setAutomaticReconnect(true);

mqttClient.connect(mqttConnectOptions, null, mqttActionListener());

Underling

I tried this solution with auto reconnect. still getting connection errors. I Use mosquitto docker broker so it is easy to stop and start the broker. any other missing configs to make this work? – Zolner

This just doesn't work. The callback get's called once, throws an error and then paho just seem to die. – Zindman

@Zolner - Make sure your clientId is unique. I am trying now put a truncated timestamp at the end and clearing the clientId on disconnect. – Delao