问题描述： 在安卓Oreo/Pie版本上，使用Paho MQTT库无法在后台启动活动。

解决方法： 在安卓Oreo版本及以上，后台限制了活动的启动方式，需要使用前台服务（Foreground Service）来启动活动。以下是一个使用Paho MQTT库在后台启动活动的示例代码：

import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.IBinder;
import android.support.v4.app.NotificationCompat;

import org.eclipse.paho.android.service.MqttAndroidClient;
import org.eclipse.paho.client.mqttv3.IMqttActionListener;
import org.eclipse.paho.client.mqttv3.IMqttToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class MQTTService extends Service {

    private static final String CHANNEL_ID = "MQTTServiceChannel";
    private MqttAndroidClient mqttClient;

    @Override
    public void onCreate() {
        super.onCreate();
        startForeground(1, createNotification());
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if (intent.getAction().equals("START")) {
            String clientId = "your_client_id";
            String serverUri = "tcp://your_mqtt_server:1883";
            String topic = "your_topic";

            mqttClient = new MqttAndroidClient(this, serverUri, clientId);
            mqttClient.setCallback(new MqttCallback() {
                @Override
                public void connectionLost(Throwable cause) {
                    // 连接丢失时的处理
                }

                @Override
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    // 消息到达时的处理
                }

                @Override
                public void deliveryComplete(IMqttDeliveryToken token) {
                    // 消息发送完成后的处理
                }
            });

            MqttConnectOptions connectOptions = new MqttConnectOptions();
            connectOptions.setCleanSession(true);

            try {
                IMqttToken token = mqttClient.connect(connectOptions);
                token.setActionCallback(new IMqttActionListener() {
                    @Override
                    public void onSuccess(IMqttToken asyncActionToken) {
                        // 连接成功时的处理
                        subscribeToTopic();
                    }

                    @Override
                    public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                        // 连接失败时的处理
                    }
                });
            } catch (MqttException e) {
                e.printStackTrace();
            }
        } else if (intent.getAction().equals("STOP")) {
            stopSelf();
        }

        return START_STICKY;
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    private void subscribeToTopic() {
        String topic = "your_topic";
        int qos = 1;
        try {
            IMqttToken token = mqttClient.subscribe(topic, qos);
            token.setActionCallback(new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    // 订阅成功时的处理
                }

                @Override
                public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                    // 订阅失败时的处理
                }
            });
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }

    private Notification createNotification() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, "MQTT Service", NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager notificationManager = getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(channel);
        }

        Intent notificationIntent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);

        return new NotificationCompat.Builder(this, CHANNEL_ID)
                .setContentTitle("MQTT Service")
                .setContentText("Running in background")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentIntent(pendingIntent)
                .build();
    }
}

在你的活动或广播接收器中，使用以下代码启动和停止MQTT服务：

// 启动MQTT服务
Intent startIntent = new Intent(this, MQTTService.class);
startIntent.setAction("