Android Application class lifecyle documentation

问题

I'm trying to find the official documentation about the Android Application class lifecycle. Apparently, for what I found on StackOverflow here and here the Application class can be killed if the system needs memory. Even this tutorial says so.

But few things irritates me a bit about this:

• I can't find an official documentation telling me that yes, the Application class can be killed on low memory.
• I can't find any official diagram representing the Application lifecycle neither.
• I can't find any proper callback to use when the Application class is killed except onLowMemory(). Does it mean that I have to use this method to persist my data?
• If the Application class is killed on low memory pressure and the app comes to foreground again, how can I know in its onCreate() that the app has been recreated after a system kill? In an Activity I would test the savedInstanceState, but as far as I know there is nothing similar in the Application class.

Thank you for your enlightenments.

回答1:

I can't find an official documentation telling me that yes, the Application class can be killed on low memory.

Below are the references to where it's been stated:

• Application Fundamentals
• Processes and Threads
• Multitasking the Android Way

I can't find any official diagram representing the Application lifecycle neither.

This is a reasonable observation... Although the following is opinion-based, my best guess is that such a diagram would contradict the Android's multitasking "philosophy" as described in the last reference provided:

"A key to how Android handles applications in this way is that processes don't shut down cleanly. When the user leaves an application, its process is kept around in the background, allowing it to continue working (for example downloading web pages) if needed, and come immediately to the foreground if the user returns to it. If a device never runs out of memory, then Android will keep all of these processes around, truly leaving all applications "running" all of the time."

I can't find any proper callback to use when the Application class is killed excepted onLowMemory(). Does it mean that I have to use this method to persist my data?

Regarding onLowMemory() whose description is quite straightforward, are we talking about a background process or foreground UI?...

If none of application Activities is in foreground and OS is low on memory, it may kill the app so that none of the Application's or the app component's (Activity, Service) callbacks will be invoked. That said, (since you're dealing with Activities) I recommend to store all persistent data as per the documentation, in onPause().

If the Application class is killed on low memory pressure and the app comes to foreground again, how can I know in its onCreate() that the app has been recreated after a system kill?

You can't recognize it in Application's onCreate().

回答2:

As far as I know, you can't handle the application killed event. Here is a quote from the Application's onTerminate method:

This method is for use in emulated process environments. It will never be called on a production Android device, where processes are removed by simply killing them; no user code (including this callback) is executed when doing so.

The general idea is that you shouldn't care whether the application was killed or not. If it was, the OS will restart the app next time it is needed otherwise it will be resume (and you'll use the Activity / Fragment lifecycle events to achieve this).

What data do you need to store - is it possible to store it earlier (when it is received from web service and so on) instead of waiting for the last moment?

来源：https://stackoverflow.com/questions/35497787/android-application-class-lifecyle-documentation