java - How does Android handle background threads when leaving an Activity?

Question

I need my Android app to save it's state to disk when its activity is put in the background or killed. It's been suggested that I start a thread when onPause() is called and perform any expensive I/O procedures there (see Saving/loading document state quickly and robustly for image editor).

In what situations will the OS kill the thread and how commonly do these situations occur?

I assume it will be like how Activities are dealt with where the OS can arbitrary decide to kill the thread but will mostly only do this when resources are extremely limited. It would be nice to find some specific documentation of this though.

From playing around, with some test code, a background thread started in onPause() will run indefinitely in the background on my device (I tried loading lots of apps and couldn't get it to be killed).

For my specific app, I'm writing a bitmap editor where I'm using the Command pattern and the Memento pattern to allow undo and redo of edits. I'd like the user to be able to undo/redo their edits even e.g. the user gets a phone call and the activity is killed when it is put in the background. The best solution I can think of is to use a background thread to constantly save my command and memento objects to disk during application use and to finish up saving any objects that are left in a background thread if onPause is called. In the worse case, if the thread is killed I'll only lose some edits.

Answer 1

In what situations will the OS kill the thread and how commonly do these situations occur?

The OS will not kill the thread, unless it is killing the process -- Android does not do anything with threads you create yourself. If you are the foreground process, you will not be killed. The odds of Android killing the process within a few seconds of you losing the foreground (after onPause()) are miniscule. The documentation on process lifetime -- what there is of it -- can be found here.