MQTT simple example in java

Here, we have SimpleMqttDemo java class we publish for a topic that is "data" and subscribe for the same topic and get published message after subscribing on topic.

public class SimpleMqttDemo {

    private MqttConnectOptions setUpConnectionOptions(String username, String password) {
        MqttConnectOptions connOpts = new MqttConnectOptions();
        connOpts.setCleanSession(true);
        connOpts.setAutomaticReconnect(true);
        connOpts.getMqttVersion();
        connOpts.setUserName(username);
        connOpts.setPassword(password.toCharArray());
        return connOpts;
      }

    private MqttClient void mqttClientConnection() {
        MqttClient client = null;
        try {
            client = new MqttClient("tcp://localhost:1883", MqttClient.generateClientId());
            MqttConnectOptions connOpts = setUpConnectionOptions("shivani","shivani123")
            client.connect(connOpts);
                   System.out.println("********** Client connection established with MQTT broker **************");
        } catch (MqttException e) {
            System.out.println("client connection error"));
            e.printStackTrace();
        }
        return client;
    }

    private void publishMessage(MqttClient client) {
        try {
            MqttMessage message = new MqttMessage();
            message.setQos(2);
            String payload = "publishing test"
            message.setPayload(payload.getBytes());
            client.publish("data", message);
            System.out.println("\t Message send : '" + payload.toString() + "' to '" + "data");
            client.disconnect();
        } catch (MqttException e) {
            System.out.println("publishing error");
            e.printStackTrace();
        }
     }

    public void publish() {
    System.out.println(" Start Publisher ");
    MqttClient client = mqttClientConnection();
    publishMessage(client);
    System.out.println(" Stop Publisher ");

    }   

    public void subscribe() {

    System.out.println(" Start Subscriber (Broker) ");
    try {
    MqttClient client = mqttClientConnection();
    client.setCallback(new MqttCallBack(){

        @Overide
        public void connectionLost(Throwable throwable) {
            System.out.println("connection lost");
        }

        @Overide
        public void messageArrived(String s, MqttMessage mqttMessage) {
            System.out.println(" Message received : \t" + new String(mqttMessage.getPayload()));
        }

        @Overide
        public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {
            // Not used here
        }
    });

    } catch (MqttException e) {
    System.out.println("client connection error");
    e.printStackTrace();
    }

        if (client != null) {
            try {
                client.subscribe("data");
            } catch (MqttException e) {
                System.out.println("subscriber error");
                e.printStackTrace();
            }
        } else {
            System.out.println("topic error");
        }

    }

    public static void main(String[] args) {
    SimpleMqttDemo obj = new SimpleMqttDemo();
    obj.publish();
        obj.subscribe();
    }
 

The output will be: 

Start Publisher

********** Client connection established with MQTT broker **************

Message send: publishing test  to  data

Stop Publisher

Start Subscriber (Broker)

********** Client connection established with MQTT broker **************

Message received: publishing test

Thanks.