SimpleAuth | Simplest way to Authenticate and make Rest API | Form library

package com.servicegateway.mqtt.client;

import java.util.UUID;

import com.hivemq.client.mqtt.MqttClient;
import com.hivemq.client.mqtt.mqtt3.Mqtt3AsyncClient;
import com.hivemq.client.mqtt.mqtt3.message.connect.connack.Mqtt3ConnAck;

public class MQTTTestClient {

    private static Mqtt3AsyncClient _client = null;
    public static void main(String[] args) {
        System.out.println("Starting");
        for(int i=0; i<2; i++) {
            multiClient();
        }

    }

    private static void multiClient() {
        UUID uuid = UUID.randomUUID();
        String clientID = "jdeon-" + uuid.toString();
        System.out.println("\tClient ID: " + clientID);

        // Instantiate the client object
        _client = MqttClient.builder()
                .useMqttVersion3()
                .identifier(clientID)
                .serverHost("localhost")
                .serverPort(1883)
    // Test server does not have SSL enabled
    //.sslWithDefaultConfig()       
                .buildAsync();

        // Connect to the MQTT Server
        System.out.println("Attempting to connect...");
        _client.connectWith()
            .simpleAuth()
                .username("my-user")                      // Not used by test server
                .password("my-password".getBytes())       // Not used by test server
                .applySimpleAuth()
            .send()
            .whenComplete((connAck, throwable) -> {
                if (throwable != null) {
                    System.out.println("Error occurred connecting to server: " + throwable.getMessage());
                    throwable.printStackTrace();
                } else {
                    handleConnectSuccess(connAck,clientID);
                    System.out.println("Done");
                }
            });

        System.out.println("Main is done, but client connection code is still running due to asynchronous call.");
    }

    // Handle a successful connection to the server
    private static void handleConnectSuccess(Mqtt3ConnAck connAck,String clientID) {
        System.out.println("\tSuccessfully connected to server.");

        // Display information returned in connAck variable
        System.out.println("\tconnAck -> " + connAck);

        // Subscribe to a topic            
       String topicName = "jdeon/testTopic "+clientID;          
        subscribeToTopic(topicName);

        // Publish a message to the topic
        String message = "Hello World! "+clientID;
        publishToTopic(topicName, message);

        // Disconnect from the MQTT server
        if (_client != null) {
            try {
                System.out.println("\tWait 300ms before disconnecting from server");

                Thread.sleep(30000L);
                System.out.println("moving to _client.disconnect() method at line :78 "+_client);
                _client.disconnect();

                System.out.println("\tDisconnected");
            } catch (InterruptedException e) {
                System.out.println("Error sleeping for 3000ms");
                e.printStackTrace();
            }
        }
    }

    // Subscribe to specified topic
    private static void subscribeToTopic(String topicName) {
        System.out.println("\tSubscribing to topic: " + topicName);

        _client.subscribeWith()
            .topicFilter(topicName)
            .callback(publish -> {
                // Process the received message
                System.out.println("\tReceived message: " + publish);
                String message = new String(publish.getPayloadAsBytes());
                System.out.println("\t\tMessage content received: " + message);
            })
            .send()
            .whenComplete((subAck, throwable) -> {
                if (throwable != null) {
                    System.out.println("Error occurred subscribing to topic (" + topicName + "): " + throwable.getMessage());
                    throwable.printStackTrace();

                } else {
                    System.out.println("\tSuccessfully subscribed to topic: " + topicName);
                }
            });

    }

    // Publish specified message to specified topic
    private static void publishToTopic(String topicName, String message) {
        System.out.println("\tPublishing message to topic: " + topicName+",   message: "+message);

        _client.publishWith()
            .topic(topicName)
            .payload(message.getBytes())
            .send()
            .whenComplete((publish, throwable) -> {
                if (throwable != null) {
                    System.out.println("Error occurred publishing message to topic (" + topicName + "): " + throwable.getMessage());
                    throwable.printStackTrace();
                } else {
                    System.out.println("\tSuccessfully published message to topic: " + topicName);
                    System.out.println("\t\tMessage content sent: " + message);
                }
            });
    }

}