What happens if I run an Android thread in the bac

Question

Android docs indicate that Oreo has new restrictions on background execution: https://developer.android.com/about/versions/oreo/background. This seems reasonable and they're clearly aiming to make their platform more like iOS and prevent apps running rampant in the background.

The thing that's unclear to me (in fact, not documented at all) is what can you do on a thread when the UI goes to the background. Specifically,

GIVEN I create a thread with

new Thread(() -> { 
     // Naughty thread doing something forever
}).start();

AND I send the app to the background

THEN ...what happens to that thread?

I've created very simple code to do this and my thread has been happily churning out to logcat for 10+ minutes with no issues.

Does anyone have any clear information on what restrictions there are on such threads? I would have thought that since Android restricts what a background service can do that it would also restrict what such threads can do.

Note that we have no plans to write an app which does anything like this. We just want to be able to write safe code which doesn't cause issues on newer versions of android. On iOS, if you go to the background then you get a period of grace to finish off whatever you're doing (and you can ask for more time) but eventually your thread will be suspended.

Answer 1

As @Sagar's answer mentioned, while the system may selectively terminate "background services", it does not selectively terminate individual "background threads".

But your question has more to it than just avoiding termination; you expect your thread to execute continuously. Other answers on this page don't mention WakeLocks, and there are some misleading comments in that regard, so I will address that here.

We are dealing with two separate issues: App termination, and device sleep.

App termination: Android terminates apps to free up resources -- preferring apps that are in the background (e.g., apps that are not on screen, and that don't contain a foreground service). When your app is terminated, all its threads are terminated too.

Device sleep: Android will pause your thread, as it pauses all threads, when the device goes to sleep. That's how so much of the battery savings are realized on mobile devices: The CPU goes into a low-power "standby" mode, and ceases executing instructions. Even though the device may be sleeping, your app (and its threads) continue to exist.

• Use a foreground service to stop your app from being terminated after it leaves the screen.
• Use a WakeLock (PARTIAL_WAKE_LOCK) to keep the CPU in a state where it can execute, even though the screen may be off.

Having a foreground service does not stop the device from going to sleep. Similarly, holding a WakeLock does not prevent your app from being terminated. Note that newer versions of Android can ignore WakeLocks in doze. See here for one approach to dealing with this.

Having a Foreground service does not guarantee your code will execute any faster than with a background service. Foreground services do not "terminate" doze.

If your logs continued to print out for 10 minutes, I would suggest it's probably because your device was plugged in. On most Android devices, the CPU stays awake when plugged in (even though the screen may be off).

Answer 2

Does anyone have any clear information on what restrictions there are on such threads? I would have thought that since Android restricts what a background service can do that it would also restrict what such threads can do.

There are no restrictions as such on how long such threads can run. As long as your app is running in background, you can continue to execute Thread in background

However, you need to consider how to gracefully terminate/release the Thread, since the Thread won't run endlessly. If OS needs to relinquish memory during memory crunch then the process of your app hosting this background Activity will be terminated, eventually destroying the thread. If not handled properly, this would lead to Thread/memory leaks.

Answer 3

As long as you start the thread when the App is in foreground, the thread might(I mean might) continue to run when the App goes to background.

BUT, the thread would be killed if the phone is running low on Memory and the System decides to reclaim memory. This is prone to happen on lower end devices much frequently. You need to have a look at Android GO.

Alternative : By all means, you can leverage JobScheduler or the new and improved WorkManager to achieve the same functionality which you might be wanting to achieve by running a thread. Since JobScheduler and WorkManager are managed by the Android System, you can be guaranteed that your functionality keeps working irrespective of the constrains on the System. Additionally, you can specify contains like Network/Battery when you are using JobScheduler/WorkManager. This is something you don't get when using a Thread.