java mqtt

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

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

public class MqttExample {
    public static void main(String[] args) {
        String broker = "tcp://broker.hivemq.com:1883";
        String clientId = "JavaSample";
        MemoryPersistence persistence = new MemoryPersistence();
        try {
            MqttClient client = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            client.connect(connOpts);
            System.out.println("Connected to broker: " + broker);

            // 订阅主题
            String topic = "test/topic";
            client.subscribe(topic);
            System.out.println("Subscribed to topic: " + topic);

            // 发布消息
            String content = "Hello MQTT!";
            int qos = 2;
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            client.publish(topic, message);
            System.out.println("Message published to topic: " + topic);

            // 断开连接
            client.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException me) {
            me.printStackTrace();
        }
    }
}