Exception thrown by deferred.await() within a runBlocking treated as unhandled even after caught

Question

This code:

fun main() {
    runBlocking {
        try {
            val deferred = async { throw Exception() }
            deferred.await()
        } catch (e: E         


        

Answer 1

A normal CoroutineScope (which is created by runBlocking) immediately cancels all child coroutines when one of them throws an exception. This behaviour is documented here: https://kotlinlang.org/docs/reference/coroutines/exception-handling.html#cancellation-and-exceptions

You can use a supervisorScope to get the behaviour you want. If a child coroutine fails inside a supervisor scope, it will not immediately cancel the other children. The children will only be cancelled if the exception is unhandled.

For more info, see here: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/supervisor-scope.html

fun main() {
    runBlocking {
        supervisorScope {
            try {
                val deferred = async { throw Exception() }
                deferred.await()
            } catch (e: Exception) {
                println("Caught $e")
            }
        }
    }
    println("Completed")
}