java - Subscribe and Read MQTT Message Using PAHO

Question

I'm using paho to send and receive mqtt messages. So far it has been no problem to send the messages, I'm receiving them by using mosquitto.

Now I want to read the messages by using a java client and I noticed that there has been less documentation about receiving the messages.

I implemented the MqttCallback interface but I still couldn't figure out how to read a message of a topic I've subscribed to.

This is my sourcecode so far, I can read the messages using mosquitto_sub.

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class PahoDemo implements MqttCallback {
    MqttClient client;
    MqttClient subClient;

    public PahoDemo() {
    }

    public static void main(String[] args) {
        new PahoDemo().doDemo();
    }

    public void doDemo() {
        try {
            client = new MqttClient("tcp://192.168.118.11:1883", "Sending");
            subClient = new MqttClient("tcp://192.168.118.11:1883",
                    "Subscribing");
            client.connect();
            subClient.connect();
            subClient.subscribe("foo");
            MqttMessage message = new MqttMessage();
            message.setPayload("A single message from my computer fff"
                    .getBytes());
            client.publish("foo", message);
            client.disconnect();
            client.close();
            subClient.disconnect();
            subClient.close();
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void connectionLost(Throwable cause) {
        // TODO Auto-generated method stub

    }

    @Override
    public void messageArrived(String topic, MqttMessage message)
            throws Exception {
 System.out.println(message);       
    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken token) {
        // TODO Auto-generated method stub

    }

}

Answer 1

You are closing the client down before the broker has time to send the message back.

Also you don't need 2 instance of the client, you can send and receive with just one.

I've edited your code a little, it now will continue to run and receive messages until you kill it.

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class PahoDemo implements MqttCallback {

MqttClient client;

public PahoDemo() {
}

public static void main(String[] args) {
    new PahoDemo().doDemo();
}

public void doDemo() {
    try {
        client = new MqttClient("tcp://192.168.118.11:1883", "Sending");
        client.connect();
        client.setCallback(this);
        client.subscribe("foo");
        MqttMessage message = new MqttMessage();
        message.setPayload("A single message from my computer fff"
                .getBytes());
        client.publish("foo", message);
    } catch (MqttException e) {
        e.printStackTrace();
    }
}

@Override
public void connectionLost(Throwable cause) {
    // TODO Auto-generated method stub

}

@Override
public void messageArrived(String topic, MqttMessage message)
        throws Exception {
 System.out.println(message);   
}

@Override
public void deliveryComplete(IMqttDeliveryToken token) {
    // TODO Auto-generated method stub

}

}

EDIT: added the missing client.setCallback(this)