How to fix Kotlin JobCancellationException?

Question

I got a crash because of Kotlin JobCancellationException.

The following is the detail about the crash :

kotlinx.coroutines.JobCancellationException: Job was cancelled; job=SupervisorJobImpl{Cancelling}@131dbe3

All I know is the SupervisorJobImpl is for ViewModelScope, and it will be called method cancel when ViewModel lifecycle is over.

I was so confused about the Exception because Kotlin coroutines will just ignore the Exception, but it was thrown and cause the App crash. If it has stack I can just figure out, but it doesn't, just tell me that the job was cancelled.

I spent about more than 3 days on the exception but just have no idea.

I saw the video : KotlinConf 2019: Coroutines! Gotta catch 'em all! by Florina Muntenescu & Manuel Vivo, I found if the scope is canceled, and if you call await on a Deferred, it will throw the Exception, but I found no await on the canceled scope.

So can someone just show me some code which perhaps causes the same exception and make the App crash? Thx, there.

Are you cancelling anything manually? Are you interacting with cancelled jobs or scope? It's hard to say what's the problem here. — Nicolas, May 04 '20 at 12:27
I show all the **ViewModel#aunch** method, find nowhere call cancel manually, just ViewModel call internally. — ParadiseHell, May 04 '20 at 12:36
I hope someone can some code which can cause JobCancellationException thrown cause crash. — ParadiseHell, May 04 '20 at 13:12
To get the full stack trace, [you need to enable coroutines debug mode](https://stackoverflow.com/q/68197547/590790). — Steven Jeuris, Jul 01 '21 at 12:53

score 15 · Answer 1 · answered May 06 '20 at 03:03

15

Finally, I found what causes the Exception and the issue address is flowing:

kotlin.coroutines.channels.awaitClose: JobCancellationException

Actually, awaitClose will not throw the JobCancellationException, because awaitClose is a cancellable suspended function. The offer method will throw JobCancellationException if the Job was canceled because offer is not a cancellable suspended function.

By the way, callbackFlow is an experimental API, so it may cause some bug, so when we use it, we need to be careful. Because it will not always ignore JobCancellationException when Job was canceled, and I don't think it's friendly to developers.

Now I have found 2 situations that will cause JobCancellationException so we need to try catch the exception.

async await, when we call the await method we need to try catch. And you can find and example in the Video.
callbackFlow offer, when we call the offer method we need to try catch. And you can find an example in the issue above.

answered May 06 '20 at 03:03

ParadiseHell

521
1
5
11

Hi, similar thing is happening with me. I wrapped my job = viewModelScope.launch(Dispatchers.IO) { } inside try-catch. Still exception not going in catch block. How did you fix the issue? – Shrikant Jan 20 '21 at 07:44
1

You should not wrap `launch` inside a try-catch, you need to try-catch the code inside `launch`. – ParadiseHell Jan 21 '21 at 08:14
@Shrikant You need to find which code causes the exception and wrap it with try-catch. – ParadiseHell Jan 21 '21 at 08:15
Take a look at this link : https://stackoverflow.com/questions/58734114/how-to-spot-where-job-was-cancelled-exception-comes-from-when-all-your-corouti – ParadiseHell Jan 21 '21 at 08:18

gauravmehra · Answer 2 · 2021-03-25T11:47:05.743

8

I know I am late but you can just check Job Status before offering objects. Like this

if(isActive) offer(Resource.success(response))

isActive is Coroutine Scope

edited Mar 25 '21 at 11:47

answered Mar 25 '21 at 11:41

gauravmehra

149
2
11

2

Yep, thanks. try catch is not the best way, this one is better. – ParadiseHell Mar 29 '21 at 02:43
Yep. This makes the most sense. If the job is no longer active just don't call offer. – Victor Ude May 16 '21 at 05:16

score 4 · Answer 3 · answered Dec 18 '20 at 21:49

4

I just saw the same problem. The issue was caused by the activity being finished manually before the job managed to complete.

answered Dec 18 '20 at 21:49

K.Cvetanovska

73
1
7

how do you find otu what job still needs completing? – Jono Sep 14 '21 at 10:33

CoolMind · Answer 4 · 2023-01-20T11:35:39.687

I closed a ViewModel and a dialog and then started a Job. It led to this exception and HTTP-request cancellation: HTTP FAILED: java.io.IOException: Canceled.
```
close()
modelScope.launch {
    val response = withContext(Dispatchers.IO) {
        ...
    }
    response?.let { ... }
}
```

I simply moved close() to the end.

When you call one coroutine from another, parent coroutine will cancel nested, if finish quicker. For instance:

viewModelScope.launch {
    // Launch a request.
    ...
    // Now launch a nested coroutine.
    viewModelScope.launch { // (1)
        ... // Can be cancelled. (2)
    }
}

You can remove viewModelScope.launch { in (1) and rewrite a nested block as suspended. If you want a parallel work, use async/await or this:

viewModelScope.launch {
    ...
}
// A parallel coroutine.
viewModelScope.launch {
    ...
}

There is a case when everything works well, but once in 100 clicks a coroutine cancels itself (the same "HTTP FAILED: java.io.IOException: Canceled"). Nothing helped, until I changed lifecycleScope.launch to lifecycleScope.launchWhenResumed.

score 0 · Answer 5 · answered May 27 '21 at 02:24

I have the same bug as you. So I try to write simple flow extensions like below:

fun <P> Flow<DataResult<P>>.safeStart(start: suspend FlowCollector<DataResult<P>>.() -> Unit)
        : Flow<DataResult<P>> =  onStart {
        if (!currentCoroutineContext().isActive) return@onStart
        start()
    }

fun <P> Flow<DataResult<P>>.safeCatch(onCatch: suspend FlowCollector<DataResult<P>>.(cause: Throwable) -> Unit)
        : Flow<DataResult<P>> = catch {
        if (!currentCoroutineContext().isActive) return@catch
        onCatch(it)
}

suspend inline fun <P> Flow<DataResult<P>>.safeCollect(crossinline onCollect: suspend (value: DataResult<P>) -> Unit)
        : Unit = collect {
    if (!currentCoroutineContext().isActive) return@collect
    onCollect(it)
}

score 0 · Answer 6 · answered Oct 27 '21 at 14:24

Had the same crash when I used callbackFlow with offer with coroutines version 1.3.5. Now instead of offer I use trySend and it's fixed.

Note: trySend method is available (and offer is deprecated) when you update coroutines version to:

org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.0

score -1 · Answer 7 · answered Mar 24 '21 at 12:07

class MyApp: Application() {

    override fun onCreate() {
        super.onCreate()
        System.setProperty(DEBUG_PROPERTY_NAME, DEBUG_PROPERTY_VALUE_ON)
       
    }

in this way you will be able to pinpoint the main cause of the issue an deal with it

How to fix Kotlin JobCancellationException?

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