Does Kotlin's launch start a coroutine in the main or a background thread?

Question

I'm trying to run a task in the background on Android, and I was wondering whether I need to specify GlobalScope.launch(Dispatchers.IO) {... } or if a simple GlobalScope.launch {... } is enough. My worry is whether the second form launches the coroutine in the main or a background/IO thread ?

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According to Android documentation ,

launch doesn't take a Dispatchers.IO parameter. When you don't pass a Dispatcher to launch, any coroutines launched from viewModelScope run in the main thread .

According to Kotlin documentation ,

The default dispatcher that is used when coroutines are launched in GlobalScope is represented by Dispatchers.Default and uses a shared background pool of threads , so launch(Dispatchers.Default) {... } uses the same dispatcher as GlobalScope.launch {... } .

I know coroutines were experimental until recently, and Android-Kotlin vs pure-Kotlin development are different, but these statements seem contradictory to me.

Answer 1

GlobalScope has EmptyCoroutineContext which implies Dispatchers.Default will be used when directly launching within it.

Example demonstrating the behavior: https://pl.kotl.in/cLy3UfuZO

My worry is whether the second form launches the coroutine in the main or a background/IO thread?

It'll launch it into CommonPool under Dispatchers.Default which shares max thread same as number of cores in your CPU, for instance if your CPU has 6 cores then there will be a max limit of 6 threads. Dispatchers.IO however allows upto borrowing 64 threads from the CommonPool. Dispatchers.Main is single threaded.

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Story of viewModelScope is different, that scope contains Dispatchers.Main as its default dispatcher to launch in. You can create a scope like this CoroutineScope(Dispatchers.Main) so that every launch without specifying dispatcher will launch in the Main similar to the viewModelScope .