Why does `GlobalScope launch Dispatchers Main ` throw a `Fat kotlinlang #announcements

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Title

Barend

11/02/2018, 9:40 AM

Why does

GlobalScope.launch(Dispatchers.Main)

throw a

Fatal Exception: android.os.NetworkOnMainThreadException

and

GlobalScope.async

doesn't?

Jean Mainguy

11/02/2018, 9:42 AM

How about

CoroutineScope(Dispatchers.Main).launch

? See here: https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#0

gildor

11/02/2018, 9:47 AM

See this explanation https://stackoverflow.com/a/46226519/420412

Barend

11/02/2018, 9:58 AM

What does "You MUST NOT forget about the coroutine you’ve started with async." mean?

gildor

11/02/2018, 9:59 AM

It means exactly this

Barend

11/02/2018, 9:59 AM

Specifically "forget".

gildor

11/02/2018, 9:59 AM

if nobody never call

await()

you will never know about this error

it actually explained in comments to this answer

Can you elaborate on “You MUST NOT forget about the coroutine you’ve started with async”? Are there gotchas that one wouldn’t expect for example?

An uncaught exception inside the async code is stored inside the resulting Deferred and is not delivered anywhere else, it will get silently dropped unless processed.

If you forget the result of async than it will finish and will be garbage collected. However, if it crashes due to some bug in your code, you’ll never learn about that. That is why

Barend

11/02/2018, 10:00 AM

Do they mean like this?

val addJob = GlobalScope.async {
    val hostAddress: String
    val ip = InetAddress.getByName(hostName)

    hostAddress = ip.hostAddress

    addIPToMap(hostAddress)
}

runBlocking {
    addJob.join()
}

gildor

11/02/2018, 10:01 AM

yes, this is an example when you never handle error and do not know that your code is probably broken

Barend

11/02/2018, 10:07 AM

How could I make it better?

gildor

11/02/2018, 10:09 AM

If you don’t need result, use

launch

if you need result from async, use

.await()

instaed of join, to handle result or errors

Barend

11/02/2018, 10:13 AM

😒imple_smile: I see

gildor

11/02/2018, 10:16 AM

And in case of

launch

you should wrap code to try/catch or

runCatching

and handle result of operation, otherwise in case of error you application will crash

Returning to your original problem,

NetworkOnMainThreadException

happens there because you do network requesst (lookup ip address) on the main thread, to avoid this use

<http://Dispatchers.IO|Dispatchers.IO>

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