Android AlarmManager - RTC_WAKEUP vs ELAPSED_REALTIME_WAKEUP

Question

Can someone explain to me the difference between AlarmManager.RTC_WAKEUP and AlarmManager.ELAPSED_REALTIME_WAKEUP? I have read the documentation but still don't really understand the implication of using one over the other.

Example code:

    alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, 
                     scheduledAlarmTime, 
                     pendingIntent);

    alarmManager.set(AlarmManager.RTC_WAKEUP, 
                     scheduledAlarmTime, 
                     pendingIntent);

How different will the two lines of code execute? When will those two lines of code execute relative to each other?

I appreciate your help.

Answer 1

AlarmManager.ELAPSED_REALTIME_WAKEUP type is used to trigger the alarm since boot time:

alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, 600000, pendingIntent);

will actually make the alarm go off 10 min after the device boots.

There is a timer that starts running when the device boots up to measure the uptime of the device and this is the type that triggers your alarm according to the uptime of the device.

Whereas, AlarmManager.RTC_WAKEUP will trigger the alarm according to the time of the clock. For example if you do:

long thirtySecondsFromNow = System.currentTimeMillis() + 30 * 1000;
alarmManager.set(AlarmManager.RTC_WAKEUP, thirtySecondsFromNow , pendingIntent);

this, on the other hand, will trigger the alarm 30 seconds from now.

AlarmManager.ELAPSED_REALTIME_WAKEUP type is rarely used compared to AlarmManager.RTC_WAKEUP.