MQTT（Message Queuing Telemetry Transport）是一种轻量级物联网消息传输协议，它基于发布/订阅模式，适用于硬件受限、网络带宽有限、延迟较高的环境。在本文中，我们将详细介绍如何在Java项目中使用MQTT，实现连接到MQTT服务器、订阅主题和发布消息的功能。

步骤1：引入客户端库

首先，我们需要添加Eclipse Paho Java Client库的依赖项到项目的pom.xml文件中：

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

步骤2：创建MQTT连接

我们将使用免费公共MQTT服务器提供的MQTT服务器，以下是服务器接入信息：

  • Broker: iot.mqtt.cn
  • TCP Port: 1883
  • SSL/TLS Port: 8883

普通TCP连接

设置MQTT Broker的基本连接参数，用户名和密码为非必选参数：

String broker = "tcp://iot.mqtt.cn:1883";
String username = "emqx";
String password = "public";
String clientid = "publish_client";

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

TLS/SSL连接

如果需要使用自签名证书进行TLS/SSL连接，需要添加bcpkix-jdk15on依赖项到pom.xml文件，并创建一个SSLUtils工具类。

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

然后，根据TLS/SSL连接设置选项：

// 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 = SSLUtils.getSocketFactory(caFilePath, clientCrtFilePath, clientKeyFilePath, "");
options.setSocketFactory(socketFactory);

步骤3：发布MQTT消息

创建一个发布客户端类PublishSample，该类将发布一条"Hello MQTT"消息到主题mqtt/test：

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());
           MqttConnectOptions options = new MqttConnectOptions();
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
           options.setKeepAliveInterval(60);
           client.connect(options);

           MqttMessage message = new MqttMessage(content.getBytes());
           message.setQos(qos);

           client.publish(topic, message);
           System.out.println("Message published");
           System.out.println("topic: " + topic);
           System.out.println("message content: " + content);

           client.disconnect();
           client.close();
      } catch (MqttException e) {
           throw new RuntimeException(e);
      }
  }
}

步骤4：订阅MQTT主题

创建一个订阅客户端类SubscribeSample，该类将订阅主题mqtt/test：

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());
           MqttConnectOptions options = new MqttConnectOptions();
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
           options.setKeepAliveInterval(60);

           client.setCallback(new 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 from 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("Delivery complete: " + token.isComplete());
              }
          });

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

步骤5：测试

现在，您可以运行SubscribeSample订阅mqtt/test主题。然后运行PublishSample发布消息到mqtt/test主题。您将看到发布端成功发布消息，同时订阅端接收到消息。

总结

通过这篇文章，我们详细学习了如何在Java中使用Paho Java Client来连接到公共MQTT服务器，并实现了测试客户端与MQTT服务器的连接、消息发布和订阅。希望这篇文章对您在Java中使用MQTT有所帮助。

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