AlarmManager setExact with WakefulBroadcastReceiver sometimes not exact

Question

Using Android 19+

setExact in conjuction with WakefulBroadcastReceiver sometimes does not fire on time (can be a few seconds or so late). I mean most it of the time it does. probably 49 times out of 50 its correct.

I'm not sure if its just because the system is busy at the time and it can't handle the workload or what

Here is how I set the alarm:

  AlarmManager alarmMgr = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
  Intent intent = new Intent(AlarmReceiver.INTENT_FILTER);
  PendingIntent alarmIntent = PendingIntent.getBroadcast(context, MyApplication.ALARM_REQUEST_CODE, intent,  PendingIntent.FLAG_UPDATE_CURRENT);
  alarmMgr.setExact(AlarmManager.RTC_WAKEUP, timeToWakeUp, alarmIntent);

Here is my receiver code:

public class AlarmReceiver extends WakefulBroadcastReceiver {

public static final String INTENT_FILTER = "myfilter";

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

}

}

And in the WakefulService

public class MyWakefulService extends IntentService {

....

@Override
protected void onHandleIntent(Intent intent) {

....

Answer 1

For Marshmallow era(?), we need some ugly codes like below... :( And "delayInMillis" param should be more than 15 minutes on the API 23. If not, system ignore the minutes less than 15 minutes.

private void registerExactAlarm(PendingIntent sender, long delayInMillis) {
    final int SDK_INT = Build.VERSION.SDK_INT;
    AlarmManager am = (AlarmManager) mContext.getSystemService(Context.ALARM_SERVICE);
    long timeInMillis = (System.currentTimeMillis() + delayInMillis) / 1000 * 1000;     //> example

    if (SDK_INT < Build.VERSION_CODES.KITKAT) {
        am.set(AlarmManager.RTC_WAKEUP, timeInMillis, sender);
    }
    else if (Build.VERSION_CODES.KITKAT <= SDK_INT  && SDK_INT < Build.VERSION_CODES.M) {
        am.setExact(AlarmManager.RTC_WAKEUP, timeInMillis, sender);
    }
    else if (SDK_INT >= Build.VERSION_CODES.M) {
        am.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, timeInMillis, sender);
    }
}

Answer 2

This behaviour is added in API 19:

Beginning with API 19 (KITKAT) alarm delivery is inexact: the OS will shift alarms in order to minimize wakeups and battery use. There are new APIs to support applications which need strict delivery guarantees; see setWindow(int, long, long, PendingIntent) and setExact(int, long, PendingIntent). Applications whose targetSdkVersion is earlier than API 19 will continue to see the previous behavior in which all alarms are delivered exactly when requested.

from AlarmManager.

Important: setExact() still does not have to be exact, as the docs state:

The alarm will be delivered as nearly as possible to the requested trigger time.