The official doc about onDestroy method:
protected void onDestroy ()
Added in API level 1
Perform any final cleanup before an activity is destroyed. This can happen either because the activity is finishing
(someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save
space. You can distinguish between these two scenarios with the isFinishing() method.
Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in
a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here. This
method is usually implemented to free resources like threads that are associated with an activity, so that a destroyed
activity does not leave such things around while the rest of its application is still running. There are situations where
the system will simply kill the activity's hosting process without calling this method (or any others) in it, so it
should not be used to do things that are intended to remain around after the process goes away.
Derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown.