org.eclipse.paho.client.mqttv3 示例代码

以下是一个简单的使用 Eclipse Paho MQTT Java 客户端库的示例代码：

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

public class MqttClientExample {

    public static void main(String[] args) {
        String broker = "tcp://mqtt.eclipseprojects.io:1883";
        String clientId = "JavaMqttClient";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient mqttClient = new MqttClient(broker, clientId, persistence);

            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker: " + broker);
            mqttClient.connect(connOpts);
            System.out.println("Connected");

            String topic = "test/topic";
            int qos = 1;

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

            String message = "Hello, MQTT!";
            MqttMessage mqttMessage = new MqttMessage(message.getBytes());
            mqttMessage.setQos(qos);

            mqttClient.publish(topic, mqttMessage);
            System.out.println("Message published");

            mqttClient.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException e) {
            System.out.println("Error: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

这个例子创建一个 MQTT 客户端连接到公共 MQTT 代理（broker），然后订阅一个主题并发布一条消息。在这个例子中，我们使用了 Eclipse Paho MQTT Java 客户端库的 `MqttClient` 类。您需要在构造函数中指定代理的 URL、客户端 ID 和持久化方式。然后，您需要创建一个 `MqttConnectOptions` 对象来设置连接选项，例如“清除会话”（clean session）选项。在这个例子中，我们订阅了一个主题并发布了一条消息。我们还调用了 `disconnect()` 方法来断开连接。