Exception thrown by async is not caught when parent job is cancelled.

Sample code for the scenario.

import kotlinx.coroutines.*
import java.io.IOException
import java.lang.Exception

val contextJob = Job()

fun main() {
    GlobalScope.launch(contextJob) {
        try {
            apiRequest().await()
        } catch(e: Exception) {
            // Try to handle the exception gracefully
            println("Exception caught")
        }
    }.apply {
        // But parent job is canceled after a while
        Thread.sleep(300)
        contextJob.cancel()
    }

    Thread.sleep(3000L)
}

fun apiRequest() = GlobalScope.async(Dispatchers.IO) {
        // Do some network request
        Thread.sleep(1000)
        // An IOException is thrown
        throw IOException()
        "API Result"
    }

We have a contextJob in activity as the parent job of all coroutines which is cancelled in onDestroy(), as recommended in official guide.

The problem is that when the contextJob is cancelled while the async job is executing, the exception thrown by async will not be caught by the try…catch… block , and thus crashes our app.

I’m aware that CoroutineExceptionHandler can catch the exception, but then it brings a huge mental load during development. I would rather to not use contextJob at all.

When is the recommended way of dealing with this situation?

[qwwdfsad]

Then recommended solution is to wrap them into coroutineScope {} or supervisorScope {}`. We probably should update documentation in that place.

E.g.

suspend fun concurrentNetworkRequests() = coroutineScope {
  val first = firstRequest()
  val second = secondRequest()
  combineResults(first.await(), second.await())
}

[yihuaqi]

@yihuaqi I have an exact same issue how do u solve that problem ?

We use add an CoroutineExceptionHandler for every coroutine we start to avoid crash for now.