How to use MQTT in Java

MQTT is a lightweight IoT message transmission protocol based on the publish/subscribe model, which can achieve stable transmission on severely constrained hardware devices and low-bandwidth, high-latency networks. It occupies half of the Internet of Things protocols by virtue of its simplicity and ease of implementation, support for QoS, and small packets.

This article mainly introduces how to use MQTT in Java projects to realize functions such as connection between client and server, subscription, and sending and receiving messages.

import client library

The development environment for this article is:

• Build tool: Maven
• IDE: IntelliJ IDEA
• Java version: JDK 1.8.0

This article will use the Eclipse Paho Java Client as the client, which is the most widely used MQTT client library in the Java language.

Add the following dependencies to the project pom.xml file.

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

Create an MQTT connection

MQTT server

This article will use the free public MQTT server provided by EMQX, which is based on EMQX's MQTT cloud platform . The server access information is as follows:

• Broker: broker.emqx.io (Chinese users can use broker-cn.emqx.io )
• TCP Port: 1883
• SSL/TLS Port: 8883

normal TCP connection

Set the basic connection parameters of MQTT Broker. Username and password are optional parameters.

 String broker = "tcp://broker.emqx.io:1883";
// TLS/SSL
// String broker = "ssl://broker.emqx.io:8883";
String username = "emqx";
String password = "public";
String clientid = "publish_client";

Then create an MQTT client and connect.

 MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
MqttConnectOptions options = new MqttConnectOptions();
options.setUserName(username);
options.setPassword(password.toCharArray());
client.connect(options);

illustrate

• MqttClient: calls the client synchronously, using blocking methods to communicate.
• MqttClientPersistence: Represents a persistent data store used to store outbound and inbound information during transmission, enabling it to be delivered to the specified QoS.

• MqttConnectOptions: connection options, used to specify the parameters of the connection, some common methods are listed below.

  • setUserName: set username
  • setPassword: set password
  • setCleanSession: Set whether to clear the session
  • setKeepAliveInterval: set the heartbeat interval
  • setConnectionTimeout: set the connection timeout
  • setAutomaticReconnect: Set whether to automatically reconnect

TLS/SSL connection

If you want to use a self-signed certificate for TLS/SSL connections, add bcpkix-jdk15on to the pom.xml file.

 <!-- https://mvnrepository.com/artifact/org.bouncycastle/bcpkix-jdk15on -->
<dependency>
   <groupId>org.bouncycastle</groupId>
   <artifactId>bcpkix-jdk15on</artifactId>
   <version>1.70</version>
</dependency>

Then use the following code to create the SSLUtils.java file.

 package io.emqx.mqtt;

import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.openssl.PEMKeyPair;
import org.bouncycastle.openssl.PEMParser;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;

import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManagerFactory;
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FileReader;
import java.security.KeyPair;
import java.security.KeyStore;
import java.security.Security;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;

public class SSLUtils {
   public static SSLSocketFactory getSocketFactory(final String caCrtFile,
                                                   final String crtFile, final String keyFile, final String password)
           throws Exception {
       Security.addProvider(new BouncyCastleProvider());

       // load CA certificate
       X509Certificate caCert = null;

       FileInputStream fis = new FileInputStream(caCrtFile);
       BufferedInputStream bis = new BufferedInputStream(fis);
       CertificateFactory cf = CertificateFactory.getInstance("X.509");

       while (bis.available() > 0) {
           caCert = (X509Certificate) cf.generateCertificate(bis);
      }

       // load client certificate
       bis = new BufferedInputStream(new FileInputStream(crtFile));
       X509Certificate cert = null;
       while (bis.available() > 0) {
           cert = (X509Certificate) cf.generateCertificate(bis);
      }

       // load client private key
       PEMParser pemParser = new PEMParser(new FileReader(keyFile));
       Object object = pemParser.readObject();
       JcaPEMKeyConverter converter = new JcaPEMKeyConverter().setProvider("BC");
       KeyPair key = converter.getKeyPair((PEMKeyPair) object);
       pemParser.close();

       // CA certificate is used to authenticate server
       KeyStore caKs = KeyStore.getInstance(KeyStore.getDefaultType());
       caKs.load(null, null);
       caKs.setCertificateEntry("ca-certificate", caCert);
       TrustManagerFactory tmf = TrustManagerFactory.getInstance("X509");
       tmf.init(caKs);

       // client key and certificates are sent to server so it can authenticate
       KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
       ks.load(null, null);
       ks.setCertificateEntry("certificate", cert);
       ks.setKeyEntry("private-key", key.getPrivate(), password.toCharArray(),
               new java.security.cert.Certificate[]{cert});
       KeyManagerFactory kmf = KeyManagerFactory.getInstance(KeyManagerFactory
              .getDefaultAlgorithm());
       kmf.init(ks, password.toCharArray());

       // finally, create SSL socket factory
       SSLContext context = SSLContext.getInstance("TLSv1.2");
       context.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);

       return context.getSocketFactory();
  }
}

Refer to the following settings options .

 // 设置 SSL/TLS 连接地址
String broker = "ssl://broker.emqx.io:8883";
// 设置 socket factory
String caFilePath = "/cacert.pem";
String clientCrtFilePath = "/client.pem";
String clientKeyFilePath = "/client.key";
SSLSocketFactory socketFactory = getSocketFactory(caFilePath, clientCrtFilePath, clientKeyFilePath, "");
options.setSocketFactory(socketFactory);

Publish MQTT messages

Create a publish client class PublishSample which will publish a Hello MQTT message to the topic mqtt/test .

 package io.emqx.mqtt;

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class PublishSample {

   public static void main(String[] args) {

       String broker = "tcp://broker.emqx.io:1883";
       String topic = "mqtt/test";
       String username = "emqx";
       String password = "public";
       String clientid = "publish_client";
       String content = "Hello MQTT";
       int qos = 0;

       try {
           MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
           // 连接参数
           MqttConnectOptions options = new MqttConnectOptions();
           // 设置用户名和密码
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
      options.setKeepAliveInterval(60);
           // 连接
           client.connect(options);
           // 创建消息并设置 QoS
           MqttMessage message = new MqttMessage(content.getBytes());
           message.setQos(qos);
           // 发布消息
           client.publish(topic, message);
           System.out.println("Message published");
           System.out.println("topic: " + topic);
           System.out.println("message content: " + content);
           // 关闭连接
           client.disconnect();
           // 关闭客户端
           client.close();
      } catch (MqttException e) {
           throw new RuntimeException(e);
      }
  }
}

Subscribe to MQTT topics

Create a subscription client class SubscribeSample which will subscribe to the topic mqtt/test .

 package io.emqx.mqtt;

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

public class SubscribeSample {
   public static void main(String[] args) {
       String broker = "tcp://broker.emqx.io:1883";
       String topic = "mqtt/test";
       String username = "emqx";
       String password = "public";
       String clientid = "subscribe_client";
       int qos = 0;

       try {
           MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
           // 连接参数
           MqttConnectOptions options = new MqttConnectOptions();
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
      options.setKeepAliveInterval(60);
           // 设置回调
           client.setCallback(new MqttCallback() {

               public void connectionLost(Throwable cause) {
                   System.out.println("connectionLost: " + cause.getMessage());
              }

               public void messageArrived(String topic, MqttMessage message) {
                   System.out.println("topic: " + topic);
                   System.out.println("Qos: " + message.getQos());
                   System.out.println("message content: " + new String(message.getPayload()));

              }

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

          });
           client.connect(options);
           client.subscribe(topic, qos);
      } catch (Exception e) {
           e.printStackTrace();
      }
  }
}

MqttCallback description:

• connectionLost(Throwable cause): called when the connection is lost
• messageArrived(String topic, MqttMessage message): called when a message is received
• deliveryComplete(IMqttDeliveryToken token): Called when message delivery is complete

test

Next run SubscribeSample and subscribe to the mqtt/test topic. Then run PublishSample and publish the message to the mqtt/test topic. We will see that the publisher successfully publishes the message, and the subscriber receives the message.

So far, we have completed using Paho Java Client in Java to connect to the public MQTT server as an MQTT client, and implemented the connection between the test client and the MQTT server, message publishing and subscription.

Copyright statement: This article is original by EMQ, please indicate the source when reprinting.

Original link: https://www.emqx.com/zh/blog/how-to-use-mqtt-in-java