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# Connect to the Deployment with Java SDK

In this tutorial, you will learn how to use Eclipse Paho Java Client (opens new window) to connect to EMQX Cloud deployment.

# Prerequisites

  1. The deployment has been created. You can view connection-related information under Deployment Overview. Please make sure that the deployment status is running. At the same time, you can use WebSocket to test the connection to the MQTT server.
  2. Set the user name and password in Authentication & ACL > Authentication for connection verification.

The project use Maven as the build tool.

# Create project

  1. Create a Maven project with Intellij IDEA, refer to Creating a Maven project (opens new window)
  2. Craet package io.emqx.mqtt under src/main/java

# Add dependency

Add the dependency definition to the pom.xml

<dependency>
    <groupId>org.eclipse.paho</groupId>
    <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
    <version>1.2.5</version>
</dependency>
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Run

mvn install
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# Connection

You can view the information of connection on Deployment Overview. Please note that the port is not 1883 or 8883 if your choose a Basic package, make sure you get the right port. And you must add the authentication on Authentication & ACL in advance.

# Connect Options

Create MqttSample.java, setup the broker, port, topic and authentication.

String topic = "test/topic";
String content = "Hello World";
int qos = 2;
String broker = "tcp://broker.emqx.io:1883";
String clientId = MqttClient.generateClientId();
// persistence
MemoryPersistence persistence = new MemoryPersistence();
// MQTT connect options
MqttConnectOptions connOpts = new MqttConnectOptions();
// authentication
connOpts.setUserName("emqx_user");
connOpts.setPassword("emqx_password".toCharArray());
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# Connect

MqttClient client = new MqttClient(broker, clientId, persistence);
// callback
client.setCallback(new SampleCallback());

System.out.println("Connecting to broker: " + broker);
client.connect(connOpts);
System.out.println("Connected to broker: " + broker);
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The content of SampleCallback.java as bellow：

package io.emqx.mqtt;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class SampleCallback implements MqttCallback {
    
    public void connectionLost(Throwable cause) {
        System.out.println("connection lost：" + cause.getMessage());
    }

    
    public void messageArrived(String topic, MqttMessage message) {
        System.out.println("Received message: \n  topic：" + topic + "\n  Qos：" + message.getQos() + "\n  payload：" + new String(message.getPayload()));
    }

    
    public void deliveryComplete(IMqttDeliveryToken token) {
        System.out.println("deliveryComplete");
    }


}
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# Subscribe

client.subscribe(topic, qos);
System.out.println("Subscribed to topic: " + topic);
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# Publish

MqttMessage message = new MqttMessage(content.getBytes());
message.setQos(qos);
client.publish(topic, message);
System.out.println("Message published");
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# Source code

MqttSample.java

package io.emqx.mqtt;

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


public class MqttSample {
    public static void main(String[] args) {
        String topic = "test/topic";
        String content = "Hello World";
        int qos = 2;
        String broker = "tcp://broker.emqx.io:1883";
        String clientId = MqttClient.generateClientId();
        // persistence
        MemoryPersistence persistence = new MemoryPersistence();
        // connect options
        MqttConnectOptions connOpts = new MqttConnectOptions();
        // authentication 
        connOpts.setUserName("emqx_user");
        connOpts.setPassword("emqx_password".toCharArray());
        
        try {
            MqttClient client = new MqttClient(broker, clientId, persistence);
            // callback
            client.setCallback(new SampleCallback());

            System.out.println("Connecting to broker: " + broker);
            client.connect(connOpts);
            System.out.println("Connected to broker: " + broker);

            client.subscribe(topic, qos);
            System.out.println("Subscribed to topic: " + topic);
            
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            client.publish(topic, message);
            System.out.println("Message published");
            client.disconnect();
            System.out.println("Disconnected");
            client.close();
            System.exit(0);
        } catch (MqttException me) {
            System.out.println("reason " + me.getReasonCode());
            System.out.println("msg " + me.getMessage());
            System.out.println("loc " + me.getLocalizedMessage());
            System.out.println("cause " + me.getCause());
            System.out.println("excep " + me);
            me.printStackTrace();
        }
    }
}
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SampleCallback.java

package io.emqx.mqtt;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class SampleCallback implements MqttCallback {
    
    public void connectionLost(Throwable cause) {
        System.out.println("connection lost：" + cause.getMessage());
    }

    public void messageArrived(String topic, MqttMessage message) {
        System.out.println("Received message: \n  topic：" + topic + "\n  Qos：" + message.getQos() + "\n  payload：" + new String(message.getPayload()));
    }

    public void deliveryComplete(IMqttDeliveryToken token) {
        System.out.println("deliveryComplete");
    }


}
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# Test

Run with the command:

mvn compile exec:java -Dexec.mainClass="io.emqx.mqtt.MqttSample"
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The console output:

Connecting to broker: tcp://broker.emqx.io:1883
Connected to broker: tcp://broker.emqx.io:1883
Subscribed to topic: test/topic
Message published
Received message: 
  topic：test/topic
  Qos：1
  payload：Hello World
deliveryComplete
Disconnected
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That's the whole process of connecting, subscribing, publishing, and receiving the message.

# More

The above shows you how to connect to EMQX Cloud using the paho.mqtt.java client library, You can see the source code on GitHub (opens new window). You can also find more examples of other language on GitHub (opens new window).

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