Can Android kill the activity without killing the entire process while the app is in the background?

Question

I'm wondering if Android system is able to kill the activity without the entire application process while the app is minimized. From Android documentation we know that onDestroy is only called when the activity is about to be destroyed and the systems guarantees to call this method whenever it is about to kill the activity, it will not be called only in case the entire application process is killed.

So, imagine such a situation - you send the app to the background(minimize) and after some time the os starts to run low on memory and decides to kill the activity, but since the app is currently suspended and cannot execute code it is not able to call its onDestroy method althought it is guaranteed that it will be called before every activity destruction.

So, this kind of reasoning gives me a thought that while the app is in the background os is only able to kill the entire process but not some specific activities. Is my reasoning correct, or did I miss something?

Answer 1

That's true: while the app is in the background os is only able to kill the entire process but not some specific activities.

Answer 2

Your reasoning is correct.

If the user navigates away from the activity/application (e.g. by pressing the home button) then the activity is said to be in the "Stopped" state. (States being "None-existent", "Stopped", "Paused" and "Resumed"). If android get low in memory and needs to kill some processes it will target those processes whose activities are in the "Stopped" state and it kill the whole process (not the activity). Furthermore, it will not be polite when doing so and therefore, will not call the activity's onDestroy() method.

Edit following comments about the confusion of saved state on process death:

If the activiy's process is killed, the system temporarily saves a set of settings outside the activity and using these settings, it recreates the activity the next time it is launched.

For example, just before moving to the "Stopped" state the system calls onSaveInstanceState(Bundle) on an activity that is not "finished" and saves this Bundle outside the activity. The system also does remember that it killed activity's process while it was not finished. Using these two along with other settings (saved outside the activity), the system recreates the activity.

If, However, the activity is finished (e.g. user presses the back button, swipes away the activity's card from the overview window, Activity.finish() is called explicitly, etc), onSaveInstanceState() is not called and the system doesn't save any settings to recreate the activity next time it's launched. It simply creates a fresh one.

This is good news, why? Becuase if it wasn't the case, the developer would have had to stash key state properties manually outside activities and reinstate them when activities are relaunched (that would've been a nightmare)

Answer 3

Since there's been much confusion on this issue, in large part due to the confusing state of the official docs in the past, here's what the docs say at present:

The system never kills an activity directly to free up memory. Instead, it kills the process in which the activity runs, destroying not only the activity but everything else running in the process, as well.

This as well as real world observation yields that the answer is no.

https://developer.android.com/guide/components/activities/activity-lifecycle#asem