mqtt java demo

一个简单的MQTT Java Demo可以通过使用Eclipse Paho客户端库来实现。首先，你需要下载并导入Paho库，然后在你的Java项目中使用以下代码创建一个MQTT客户端：

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

public class MqttDemo {

    public static void main(String[] args) {
        String broker = "tcp://iot.eclipse.org:1883";
        String clientId = "JavaClient";
        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);

            String topic = "topic/demo";
            int qos = 1;
            client.subscribe(topic, qos);
            
            String content = "Hello, MQTT!";
            client.publish(topic, content.getBytes(), qos, false);
            System.out.println("Message published");

            client.disconnect();
            System.out.println("Disconnected");

        } 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);
        }
    }
}

在这个Demo中，我们创建了一个MqttClient实例并连接到一个MQTT代理。然后，我们订阅了一个名为"topic/demo"的主题，并向该主题发布了一条消息"Hello, MQTT!"。最后，我们断开了与代理的连接。这个Demo演示了如何使用Java语言来实现MQTT客户端，并进行连接、订阅和发布消息的操作。希望这能对你有所帮助！