Example MQTT Messaging in Java

SimpleMqttClient.java

import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttTopic;

public class SimpleMqttClient implements MqttCallback {

  MqttClient myClient;
	MqttConnectOptions connOpt;

	static final String BROKER_URL = "tcp://q.m2m.io:1883";
	static final String M2MIO_DOMAIN = "<Insert m2m.io domain here>";
	static final String M2MIO_STUFF = "things";
	static final String M2MIO_THING = "<Unique device ID>";
	static final String M2MIO_USERNAME = "<m2m.io username>";
	static final String M2MIO_PASSWORD_MD5 = "<m2m.io password (MD5 sum of password)>";

	// the following two flags control whether this example is a publisher, a subscriber or both
	static final Boolean subscriber = true;
	static final Boolean publisher = true;

	/**
	 * 
	 * connectionLost
	 * This callback is invoked upon losing the MQTT connection.
	 * 
	 */
	@Override
	public void connectionLost(Throwable t) {
		System.out.println("Connection lost!");
		// code to reconnect to the broker would go here if desired
	}

	/**
	 * 
	 * deliveryComplete
	 * This callback is invoked when a message published by this client
	 * is successfully received by the broker.
	 * 
	 */
	@Override
	public void deliveryComplete(MqttDeliveryToken token) {
		//System.out.println("Pub complete" + new String(token.getMessage().getPayload()));
	}

	/**
	 * 
	 * messageArrived
	 * This callback is invoked when a message is received on a subscribed topic.
	 * 
	 */
	@Override
	public void messageArrived(MqttTopic topic, MqttMessage message) throws Exception {
		System.out.println("-------------------------------------------------");
		System.out.println("| Topic:" + topic.getName());
		System.out.println("| Message: " + new String(message.getPayload()));
		System.out.println("-------------------------------------------------");
	}

	/**
	 * 
	 * MAIN
	 * 
	 */
	public static void main(String[] args) {
		SimpleMqttClient smc = new SimpleMqttClient();
		smc.runClient();
	}
	
	/**
	 * 
	 * runClient
	 * The main functionality of this simple example.
	 * Create a MQTT client, connect to broker, pub/sub, disconnect.
	 * 
	 */
	public void runClient() {
		// setup MQTT Client
		String clientID = M2MIO_THING;
		connOpt = new MqttConnectOptions();
		
		connOpt.setCleanSession(true);
		connOpt.setKeepAliveInterval(30);
		connOpt.setUserName(M2MIO_USERNAME);
		connOpt.setPassword(M2MIO_PASSWORD_MD5.toCharArray());
		
		// Connect to Broker
		try {
			myClient = new MqttClient(BROKER_URL, clientID);
			myClient.setCallback(this);
			myClient.connect(connOpt);
		} catch (MqttException e) {
			e.printStackTrace();
			System.exit(-1);
		}
		
		System.out.println("Connected to " + BROKER_URL);

		// setup topic
		// topics on m2m.io are in the form <domain>/<stuff>/<thing>
		String myTopic = M2MIO_DOMAIN + "/" + M2MIO_STUFF + "/" + M2MIO_THING;
		MqttTopic topic = myClient.getTopic(myTopic);

		// subscribe to topic if subscriber
		if (subscriber) {
			try {
				int subQoS = 0;
				myClient.subscribe(myTopic, subQoS);
			} catch (Exception e) {
				e.printStackTrace();
			}
		}

		// publish messages if publisher
		if (publisher) {
			for (int i=1; i<=10; i++) {
		   		String pubMsg = "{\"pubmsg\":" + i + "}";
		   		int pubQoS = 0;
				MqttMessage message = new MqttMessage(pubMsg.getBytes());
		    	message.setQos(pubQoS);
		    	message.setRetained(false);

		    	// Publish the message
		    	System.out.println("Publishing to topic \"" + topic + "\" qos " + pubQoS);
		    	MqttDeliveryToken token = null;
		    	try {
		    		// publish message to broker
					token = topic.publish(message);
			    	// Wait until the message has been delivered to the broker
					token.waitForCompletion();
					Thread.sleep(100);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}			
		}
		
		// disconnect
		try {
			// wait to ensure subscribed messages are delivered
			if (subscriber) {
				Thread.sleep(5000);
			}
			myClient.disconnect();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

This is a very helpful example. I did have to change the first parameter type in the deliveryComplete method to IMqttDeliveryToken instead of MqttDeliveryToken and add the IMqttDeliveryToken import at the top to get this example to run against my mosquitto broker...I am using the nightly build of Mqtt so that could be the issue or something else that myself a java newbie overlooked ;)

Hi,
I am new to java,when running this code its showing error Unable to connect to server (32103).Please help.
Thanks

Hello there,
Is "myClient.connect(connOpt);" a blocking call or not ?

Hello,
How to send PUBACK when message arrived ?

Your example is working well but does get the Message when publisher sent the message to channel that subscriber subscribed that..

Thanks! To keep subscriber running "forever" just increase Thread.sleep(5000);

Hi, how to create publish and subscriber class and can any one help with that

directly created mqttClient. java but in that condition is if(publisher) and if(subscriber) this condition where it will check without providing classes

@m2mIO-gister; Hi gister need your help on this exapmle. I am not getting output from this class MqttApp.java and when I run publisher.class output displayed and when I run Subscriber no output. Please can you check below code and update me if possible.

public class MqttApp {

public static void main(String[] args) throws MqttException {

	if(args.length < 1) {
		throw new IllegalArgumentException("Must have either 'publisher' or 'subscriber' as argument");
	}
	
	switch (args[0]) {
	case "publisher":
		Publisher.main(args);
	case "subscriber":
		Subscriber.main(args);
	break;
	default:
		throw new IllegalArgumentException("What to pass args" + args[0]);
	
	}
	
}

}
and

public class SimpleMqttCallBack implements MqttCallback{

@Override
public void connectionLost(Throwable cause) {
	
	System.out.println("Connection to MQTT broker Lost!");
	
}

@Override
public void messageArrived(String topic, MqttMessage message) throws Exception {
	
	System.out.println("Message Received:\t"+ new String(message.getPayload()));
	
}

@Override
public void deliveryComplete(IMqttDeliveryToken token) {
			
} 

}

and

public class Publisher {

public static void main(String[] args) throws MqttException{
	// TODO Auto-generated method stub
	
	String messageString = "Hello World from Java";
	
	if(args.length == 2) {
		messageString = args[1];
	}
	
	System.out.println("== START PUBLISHER ==");
	
	MqttClient client = new MqttClient("tcp://localhost:1883", MqttClient.generateClientId());
	client.connect();
	//System.out.println(MqttClient.generateClientId());
	MqttMessage msg =  new MqttMessage();
	msg.setPayload(messageString.getBytes());
	client.publish("iot_data", msg);
	System.out.println("\tMessage '"+ messageString +"' to 'iot_data'");
	
	
}

}

and

package com.mqtt.mqttCall;

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;

public class Subscriber {

public static void main(String[] args) throws MqttException {
	
	System.out.println("== START SUBSCRIBE ==");
	
	MqttClient client = new MqttClient("tcp://localhost:1883", MqttClient.generateClientId());
	
	//System.out.println(MqttClient.generateClientId());
	
	client.setCallback(new SimpleMqttCallBack());
	
	client.connect();
	
	client.subscribe("iot_data");
	
}

}

Please help me with that code what is error in that?

Please reply for the example I am started learning MQTT from yesterday onwards I didn't understand anything. Please help me