Android destroying activities, killing processes

Question

First please have a look at this:

onPause() Called when the system is about to start resuming a previous activity. This is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, etc. Implementations of this method must be very quick because the next activity will not be resumed until this method returns. Followed by either onResume() if the activity returns back to the front, or onStop() if it becomes invisible to the user.

onStop() Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this one. This may happen either because a new activity is being started, an existing one is being brought in front of this one, or this one is being destroyed. Followed by either onRestart() if this activity is coming back to interact with the user, or onDestroy() if this activity is going away.

So, when you press "HOME" button on your device, your current foreground activity is put onto onPause() then onStop(), the other 4 should remain onStop()

According to Google's Documents:

• If an activity in the foreground of the screen (at the top of the stack), it is active or running.
• If an activity has lost focus but is still visible (that is, a new non-full-sized or transparent activity has focus on top of your activity), it is paused. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.
• If an activity is completely obscured by another activity, it is stopped. It still retains all state and member information, however, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.
• If an activity is paused or stopped, the system can drop the activity from memory by either asking it to finish, or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

And, for the process lifecycle:

Process Lifecycle 3. A background activity (an activity that is not visible to the user and has been paused) is no longer critical, so the system may safely kill its process to reclaim memory for other foreground or visible processes. If its process needs to be killed, when the user navigates back to the activity (making it visible on the screen again), its onCreate(Bundle) method will be called with the savedInstanceState it had previously supplied in onSaveInstanceState(Bundle) so that it can restart itself in the same state as the user last left it.

All the quotes above are come from: Android Developers Reference: Activity

It is confirmed that the system can destroy non-acitve activities and recycle memories when you launched some memory consuming applications. And you can implement like: isFinishing() in your activity and then using "kill" button in DDMS to detect which of your activities is being dropped by system. But I guess the system will destroy the oldest one first. However it is no point to keep other activities when the "Launch Activity" has been recycled.

UPDATE

Here's some opinions I found from here:

Stopped state

When an activity is not visible, but still in memory, we say it’s in a stopped state. Stopped activity could be brought back to the front to become a Running activity again. Or, it could be destroyed and removed from memory.

The system keeps activities around in a stopped state because it is likely that the user will still want to get back to those activities some time soon, and restarting a stopped activity is far cheaper than starting an activity from scratch. That is because we already have all the objects loaded in memory and simply have to bring it all up to the foreground.

Stopped activities can be removed from memory at any point.

Can system destroy only one or some of my activities to recover memory?

Yes. Android kills activities which are running in the background when there is a need for memory. Killing one or all might depend on some conditions. For an instance paused or stopped can make android kill an activity or a process itself. Here under Activity Lifecycle you can get the below points. I recommend you to go through that page completely. It will definitely clear your doubts.

If an activity has lost focus but is still visible (that is, a new non-full-sized or transparent activity has focus on top of your activity), it is paused. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.

If an activity is completely obscured by another activity, it is stopped. It still retains all state and member information, however, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.

If an activity is paused or stopped, the system can drop the activity from memory by either asking it to finish, or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

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Will system kill the whole process of my application? Will all activities be nicely destroyed?

Activity pertains to an individual whereas process pertains to group of activities. Look at the third point above again it kills the process as mentioned.

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What will happen when I get back to my application when it was totally killed?

Its similar to restart . Again the third point will give you some answers like When it is displayed again to the user, it must be completely restarted and restored to its previous state

Get some more information about memory related stuffs here.

Edit:
All activities in an application runs in a single process. So when a process is killed all the activities no matter 5 or 10 will be killed i.e., restarted. Restart will cause your application to start from a beginning no saved states.