原文地址：http://blog.csdn.net/pashanhuxp/article/details/47154361
AlarmManager Api文档
Note: Beginning in API 19, the trigger time
passed to this (
set) method
is treated as inexact: the alarm will not be delivered before this time, but
may be deferred and delivered some time later. The OS will use this policy in order to "batch" alarms together across the entire system, minimizing the number of times the device needs to "wake up" and minimizing battery use. In general, alarms scheduled in the near future will not be deferred as long as alarms scheduled far in the future. With the new batching policy, delivery ordering guarantees are not as strong as they were previously. If the application sets multiple alarms, it is possible that these alarms' actual delivery ordering may not match the order of their requested delivery times. If your application has strong ordering requirements there are other APIs that you can use
to get the necessary behavior;
see setWindow(int, long, long, PendingIntent) and setExact(int, long, PendingIntent). Applications whose targetSdkVersion is before API 19 will continue to get the previous alarm behavior: all of their scheduled alarms will be treated as exact.
意思就是说，自Api19开始，set方法设置的闹钟可能会发生deferred(延迟)，因为OS基于尽可能减少设备唤醒和电池损耗考虑，所以OS不推荐设置过多的闹钟。如果实在需要闹钟准时响应，可以采用setExact方法。
【AlarmManager定时任务延迟执行】修改成如下解决：
int apiLevel = Build.VERSION.SDK_INT;
PendingIntent pendingIntent = setPendingIntent(mContext, note);
long triggerAtMillis = getTriggerTime(note);
AlarmManager alarm = (AlarmManager) mContext.getSystemService(Context.ALARM_SERVICE);
if (apiLevel >= 19) {
Log.d(TAG, "alarm.set Exact!");
alarm.setExact(AlarmManager.RTC_WAKEUP, triggerAtMillis, pendingIntent);
} else {
Log.d(TAG, "alarm.set only");
alarm.set(AlarmManager.RTC_WAKEUP, triggerAtMillis, pendingIntent);
}
