Eclipse Paho read data from MQTT Broker

Question

I'm using the Eclipse Paho and I can ping my device but I don't know why I'm not being able to read my sensor data using MQTT.

The following is my code, please have a look to check what I could have been doing wrong:

  public static void main(String[] args) {
    MqttClient client;
    MqttMessage msg;
    MemoryPersistence persistence;
    MqttConnectOptions conn;
    IMqttMessageListener listen;

    String broker = "tcp://url:1883";
    String str = "password";
    char[] accessKey = str.toCharArray();
    String appEUI = "userID";

    try {
        persistence = new MemoryPersistence();
        client = new MqttClient(broker, appEUI, persistence);
        conn = new MqttConnectOptions();
        conn.setCleanSession(true);
        conn.setPassword(accessKey);
        conn.setUserName(appEUI);
        client.connect(conn);
        //client.connect();

        if(client.isConnected()) {
            System.out.println("Connected..");
        }else {
            System.out.println("Unable to connect");
            System.exit(0);
        }

        msg = new MqttMessage();
        byte[] data = msg.getPayload();
        System.out.println(d);

    }catch(Exception x) {
        x.printStackTrace();
    }

}
Please help!

Answer 1

From what I could analyze, you've been trying to read data from the broker directly, but that's not how it works exactly. With the MQTT broker, you will just have to subscribe to a topic and receive data everytime a new message gets published to it.

Try setting the connection on an instance of the MqttCallback interface:

client.setCallback(new MqttCallback() {
    public void connectionLost(Throwable cause) {
    }

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

    public void deliveryComplete(IMqttDeliveryToken token) {
    }
});

Although, you don't need to create the desired topic, just tell the broker the topics you are interested in, like this:

client.subscribe("topic/foo")