应用程序停止时不要停止服务

Question

实际上我正在开发一个Android应用程序，应该每分钟从服务器获取通知，所以我使用WakefulBroadcastReceiver, IntentService, AlarmManager这是我的代码：

    public class SampleAlarmReceiver extends WakefulBroadcastReceiver {
    private static final long PERIOD = 3000;
    private AlarmManager alarmMgr;
    private PendingIntent alarmIntent;

    @Override
    public void onReceive(Context context, Intent intent) {  
        Intent service = new Intent(context, SampleSchedulingService.class);

        startWakefulService(context, service);
    }

    public void setAlarm(Context context) {
        alarmMgr = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
        Intent intent = new Intent(context, SampleAlarmReceiver.class);
        alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);

        alarmMgr.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime()+60000, PERIOD, alarmIntent);
        Log.i("SampleAlarmReceiver ", "SampleAlarmReceiver " + new Date().toString());
        ComponentName receiver = new ComponentName(context, SampleBootReceiver.class);
        PackageManager pm = context.getPackageManager();

        pm.setComponentEnabledSetting(receiver,
                PackageManager.COMPONENT_ENABLED_STATE_ENABLED,
                PackageManager.DONT_KILL_APP);           
    }

    public void cancelAlarm(Context context) {
        if (alarmMgr!= null) {
            alarmMgr.cancel(alarmIntent);
        }

        ComponentName receiver = new ComponentName(context, SampleBootReceiver.class);
        PackageManager pm = context.getPackageManager();

        pm.setComponentEnabledSetting(receiver,
                PackageManager.COMPONENT_ENABLED_STATE_DISABLED,
                PackageManager.DONT_KILL_APP);
    }
}

这是我的服务代码：

    public class SampleSchedulingService extends IntentService {
    public SampleSchedulingService() {
        super("SchedulingService");
    }

    public static final String TAG = "Scheduling Demo";
    public static final int NOTIFICATION_ID = 1; 
    public static final String SEARCH_STRING = "doodle";
    public static final String URL = "http://www.google.com";
    private NotificationManager mNotificationManager;
    NotificationCompat.Builder builder;

    @Override
    protected void onHandleIntent(Intent intent) {
        String urlString = URL;

        String result ="";

        // Try to connect to the Google homepage and download content.
        try {
            result = loadFromNetwork(urlString);
        } catch (IOException e) {
            Log.i(TAG, getString(R.string.connection_error));
        }

        if (result.indexOf(SEARCH_STRING) != -1) {
            sendNotification(getString(R.string.doodle_found));
            Log.i(TAG, "Found doodle!!");
        } else {
            sendNotification(getString(R.string.no_doodle));
            Log.i(TAG, "No doodle found. :-(");
        }
        // Release the wake lock provided by the BroadcastReceiver.
        SampleAlarmReceiver.completeWakefulIntent(intent);
        // END_INCLUDE(service_onhandle)
    }

    // Post a notification indicating whether a doodle was found.
    private void sendNotification(String msg) {
        mNotificationManager = (NotificationManager)
               this.getSystemService(Context.NOTIFICATION_SERVICE);

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

        NotificationCompat.Builder mBuilder =
                new NotificationCompat.Builder(this)
        .setSmallIcon(R.drawable.ic_launcher)
        .setContentTitle(getString(R.string.doodle_alert))
        .setStyle(new NotificationCompat.BigTextStyle()
        .bigText(msg))
        .setContentText(msg);

        mBuilder.setContentIntent(contentIntent);
        mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
    }

    private String loadFromNetwork(String urlString) throws IOException {
        InputStream stream = null;
        String str ="";

        try {
            stream = downloadUrl(urlString);
            str = readIt(stream);
        } finally {
            if (stream != null) {
                stream.close();
            }      
        }
        return str;
    }

    private InputStream downloadUrl(String urlString) throws IOException {

        URL url = new URL(urlString);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(10000 /* milliseconds */);
        conn.setConnectTimeout(15000 /* milliseconds */);
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        // Start the query
        conn.connect();
        InputStream stream = conn.getInputStream();
        return stream;
    }

    private String readIt(InputStream stream) throws IOException {

        StringBuilder builder = new StringBuilder();
        BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
        for(String line = reader.readLine(); line != null; line = reader.readLine()) 
            builder.append(line);
        reader.close();
        return builder.toString();
    }
}

我的问题是：当我退出应用程序时服务停止了，如何修复它让服务永远活着。 （如facebook通知）