MQTT java 心跳检测

要实现 MQTT Java 客户端的心跳检测，你可以使用 Eclipse Paho 提供的 MQTT Java 客户端库。以下是一个简单的示例代码：

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

public class MqttHeartbeat {
    public static void main(String[] args) throws MqttException {
        String broker = "tcp://mqtt.example.com:1883";
        String clientId = "heartbeatClient";
        String topic = "heartbeat";

        MqttClient client = new MqttClient(broker, clientId);
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(true);
        client.connect(options);

        // 定义心跳消息
        MqttMessage heartbeatMessage = new MqttMessage("heartbeat".getBytes());
        heartbeatMessage.setQos(0);
        heartbeatMessage.setRetained(false);

        // 设置心跳定时器
        int interval = 5000;  // 心跳间隔，单位为毫秒
        MqttPingSender pingSender = new MqttPingSender(client, topic, heartbeatMessage, interval);
        client.setPingSender(pingSender);

        // 订阅心跳回复消息
        client.subscribe(topic);

        // 发布心跳消息
        client.publish(topic, heartbeatMessage);

        // 等待心跳回复消息
        client.setCallback(new MqttCallback() {
            @Override
            public void messageArrived(String topic, MqttMessage message) throws Exception {
                System.out.println("Received heartbeat response: " + message.toString());
            }

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

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
            }
        });

        // 保持程序运行
        try {
            Thread.sleep(Long.MAX_VALUE);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 断开连接
        client.disconnect();
    }
}

class MqttPingSender implements MqttPingSender {
    private MqttClient client;
    private String topic;
    private MqttMessage message;
    private int interval;
    private MqttTopic mqttTopic;

    public MqttPingSender(MqttClient client, String topic, MqttMessage message, int interval) {
        this.client = client;
        this.topic = topic;
        this.message = message;
        this.interval = interval;
        this.mqttTopic = client.getTopic(topic);
    }

    @Override
    public void init() throws MqttException {
    }

    @Override
    public void schedule(long delayInMilliseconds) throws MqttException {
        try {
            Thread.sleep(delayInMilliseconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void start() throws MqttException {
        schedule(interval);
        sendHeartbeat();
    }

    @Override
    public void stop() throws MqttException {
    }

    private void sendHeartbeat() throws MqttException {
        mqttTopic.publish(message);
        schedule(interval);
    }
}

在上述示例中，我们使用 Eclipse Paho 提供的 `MqttClient` 类来创建 MQTT 客户端，并设置连接选项。然后，我们定义了心跳消息，并通过设置心跳定时器来定期发布心跳消息。同时，我们还订阅了心跳回复消息，并在回调函数中处理接收到的回复消息。

你可以根据自己的需求修改示例代码中的 broker、clientId、topic、心跳间隔等参数来适应你的实际情况。