Voorbeeld: Connect->Subscribe->Publish->Disconnect

MqttExample.java

package org.example;

import org.eclipse.paho.client.mqttv3.*;

public class MqttExample implements MqttCallback {
    private static final String TOPIC       = "l2mqtt/exampleTopic";
    private static final String CONTENT     = "Test message";
    private static final String BROKER      = "tcp://test.mosquitto.org:1883";
    private static final String CLIENT_ID   = "mqttExampleClient";
    private static final int QOS            = 0;
    
    public void testMQTT() {
        try {
            MqttAsyncClient mqttClient = new MqttAsyncClient(BROKER, CLIENT_ID, null);
            MqttConnectOptions mqttConnectOptions = generateMQTTConnectOptions();
            mqttClient.setCallback(this);

            System.out.printf("Connecting to broker: %s\n", BROKER);
            IMqttToken tok = mqttClient.connect(mqttConnectOptions);
            tok.waitForCompletion();
            System.out.println("Connected");

            System.out.printf("Subscribing to topic: %s\n", TOPIC);
            mqttClient.subscribe(TOPIC, QOS);

            System.out.printf("Publish message to topic: %s\n", TOPIC);
            mqttClient.publish(TOPIC, CONTENT.getBytes(), QOS, false);

            //Small delay to make sure the message gets caught before disconnecting.
            Thread.sleep(500);

            mqttClient.disconnect();
            System.out.println("Disconnected");

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

    private static MqttConnectOptions generateMQTTConnectOptions() {
        MqttConnectOptions mqttConnectOptions = new MqttConnectOptions();
        mqttConnectOptions.setCleanSession(true);
        mqttConnectOptions.setConnectionTimeout(30);
        mqttConnectOptions.setKeepAliveInterval(60);
        return mqttConnectOptions;
    }

    @Override
    public void connectionLost(Throwable cause) {
        System.out.println("Connection lost...");
    }

    @Override
    public void messageArrived(String topic, MqttMessage message) {
        try {
            String msg = new String(message.getPayload());
            System.out.printf("Received message from topic: %s\n", topic);
            System.out.printf("Message: %s\n", msg);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken token) {
        System.out.println("Published message!");
    }
}