Android: Activity.onDestroy() is not called when Dalvik kills this Activity

Question

I'm confused about Activity.onDestroy(). I need to free some resources when my Activity is destroyed, but it seems like onDestroy() is called just when i press "Back" key, but not when my Activity is killed by Dalvik. I tested it just by adding log:

Log.v("my_tag", "onDestroy() called");

and the same in onCreate() method too:

Log.v("my_tag", "onCreate() called");

Then i start my Activity, and i see in logs: onCreate() called. I press "Back" key and then start Activity again, then i see:

onDestroy() called
onCreate() called 

Then i press "Home" key and go to my Activity again, logs does not change. All right here.

Then i press "Home" key again and start some really "heavy" applications. In logs is nothing about onDestroy(), but when i start my Activity again, i see in logs: onCreate() called! So, onDestroy() was not called, but my Activity was killed. What's wrong?

Answer 1

From the onDestroy() documentation:

[..] 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.

In Android, you won't get a guaranteed onDestroy() call.
If you want to release some resources, you should do that in onPause() instead.