Develop Reference

Android service without lock considered useless?

According to Android docs:
A Service is an application component representing either an application's desire to perform a longer-running operation while not interacting with the user or to supply functionality for other applications to use.
But my service get killed very quickly - around 10 seconds - when the user leaves the app or the screen gets turn off. It seems according to Android Kotlin Foreground Service stops after some time that you need some sort of mechanism to prevent the phone from getting in doze mode.
So, what is the purpose of having a service without a wake lock? And why does the documentation never mention something remotely related to wake lock when dealing with services?

You can have a Service that is doing work for an Activity. If the user navigates away from the app or dims the screen or ignores his phone long enough for it to be clear that he isn't looking at it, what is the point of the Service continuing to run? The user isn't looking at the Activity, so processing in the Service is not important enough to keep the phone from sleeping.
On the other hand, if you have a Service that is performing important work and you need to keep the phone from going to sleep, then you obviously need to get some kind of wake lock or use some other method to keep the phone on.
These are 2 different scenarios with different requirements.

Android: Running background service without need to run application. Is it possible?

I am creating an application that should show some Screen on incoming message.
I think I need some background job/service that's running permanently while phone is on. That job will handle for the incoming message and run the Application with some parameters, so based on these parameters the application will show corresponding Screen.
Is it possible to reach the goal this way? Or is there any other ways?
(I am creating the app using react-native, so if there's react-native solution, it would be even better, but native Java-Android solutions are welcome too)

This approach is very bad to battery life because it doesn't allow CPU to sleep. There is no guarantee that system will keep your background service alive. For deeper understanding I can suggest you to learn about services and doze mode.
Consider using Push notifications or at least job scheduling mecanism.

Does service.startForeground imply wakelock?

Question
I am wondering if we need to aquire the WakeLock or if the service.startForeground() command does that anyway for us? I didn't find anything in the documentation.
Goal
We want to remove unnesessary stuff which might slow down the service start as we want to start capturing the sensor data as soon as possible and as the service might be restarted frequently.
Context
We're developing an Android library to capture sensor data with ~ 200 Hz for up to a couple of hours (research environment). Right now we aquire a WakeLock and start the capturing service as ForegroundService to make sure our capturing isn't stopped when the device is not used.

To answer my own question (if no one else finds more details on this):
In my understanding "foreground" just describes that the user would notice if the app was killed e.g. when browsing a large page. For this reason the system is asked to try to avoid killing the service.
Just because a foreground service is running probably not implies
that the CPU can't go into deep sleep mode.
In order to collect sensor data all the time from user-defined start to stop we still need to aquire the WakeLock ourselves.

No, wakelock can be used to make sure your foreground services run when device goes to sleep
If you must use partial wake locks, follow these recommendations:
Make sure some portion of your app remains in the foreground. For example, if you need to run a service, start a foreground service instead. This visually indicates to the user that your app is still running.
https://developer.android.com/topic/performance/vitals/wakelock#best_practices

Constant Android Service

I've been looking through many questions about services, but I couldn't find one that suited me.
I need a service that both starts on BOOT_COMPLETED (not bound to an Activity) and runs ALL the time (therefore I can't user AlarmReceiver). I know it might drain my battery but so far I don't care. It is just for research purposes.
I have a service that monitors sensor's data. What I managed to do so far was: either start the service as a regular Activity, but it runs only for +-20s and it is stopped (I think the SO cuts it down to release its memory); or start a service that runs in foreground. It worked to keep the process running, however the class that actually runs the service somehow was not started, besides an annoying notification which is required.
The code I refered as the one that runs the service in foreground was taken from here:
Implement startForeground method in Android
I mean, how does an app like WhatsApp run constantly? Is it running in foreground? Because looking at Settings it seems the service is very stable, and it does not show any permanent notification, since it is not possible for a foreground service run without one.
( How to startForeground() without showing notification? )
Any advice?

You can use a WakeLock. But please remember, with great power comes great responsibility (to release them again and not over-use them).
But for now, just acquire a hefty WakeLock and only release it until you are done. This should keep your device's screen and CPU awake and allow you to do whatever it is you want to do.

Phonegap app on Android platform - keep app running

I have a Phonegap app that functions as a communication service for a specific group of people. Using the local notification plugin found on the phonegap-plugins GitHub page, I have implemented notifications into the app, so that whenever someone receives a new message, a notification will appear if the app is in the background.
After about an hour, though, no more notifications will occur, and it appears the process was killed. When I go back into the app, it starts completely over instead of resuming from where I last left off. I'm assuming that after a certain amount of time, Android stopped running the app in the background.
Does anyone know how to keep the app running in the background until it is told by the user to stop, and prevent Android from killing the process?

As CommonsWare suggests, you could write a dummy service to keep your app alive, but also as he rightly suggests, if you're going to go to the effort of writing a native dummy service, you may as well write the actual service natively and have done with it.
As a bit of a quick and dirty solution, you could maybe use a partial wakelock (see here) to keep the CPU running your app while it's in the background.
I successfully used this approach to keep my Phonegap-based walk navigation apps alive in the background so they can continue to receive and process position updates.
In your case, staying alive to receive notifications isn't exactly what a partial wakelock was intended for and so I'm unsure whether android will kill your app anyway after a while since it's not doing anything (unlike mine which is constantly receiving and processing position updates) but it might do the job without needing to write a service, so may be worth a try.
Have a look at my answer to this question which contains my code for an updated version of the PowerManagement plugin for Android. I updated the plugin for use with Cordova 2.8.0 but also extended it to be able to acquire a partial wakelock.