What am i missing here? handler is not invoked ``...

# coroutines

[Sam]

What am i missing here? handler is not invoked

fun main( args : Array<String> ) {

    val handler = CoroutineExceptionHandler { _, throwable -> println( "Handled $throwable" ) }

    runBlocking(handler) {

        launch {
            println( "Handler is ${coroutineContext[CoroutineExceptionHandler]}" )
            delay( 1000)
            println( "first task" )
            throw UnsupportedOperationException()
        }

    }

}

[Alexandre]

I would write it like that :

import kotlinx.coroutines.*

fun main() = runBlocking {

    val handler = CoroutineExceptionHandler { _, throwable -> println( "Handled $throwable" ) }

    val job = GlobalScope.launch(handler) {
        println( "Handler is $handler" )
            delay( 1000)
            println( "first task" )
            throw UnsupportedOperationException()
    }
}

[Sam]

yeah, handler seems to work only with GlobalScope.launch

[Sam]

and not with the scope from runBlocking

[Alexandre]

Yep 👌
You’ll have more infos here : https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/

[Sam]

so why even support handlers for launch coroutines?

[gildor]

Because this is how coroutines work. exception handler works, but you cannot prevent scope from fail, or you need supervisorScope

[Sam]

I was looking at the docs

CoroutineExceptionHandler is invoked only on exceptions which are not expected to be handled by the user, so registering it in async builder and the like of it has no effect.

[Sam]

I get why it is ignored for async

[Sam]

but based on that documentation, shouldn't the handler be invoked for launch just like GlobalScopelaunch

[Sam]

https://kotlinlang.org/docs/reference/coroutines/exception-handling.html#coroutineexceptionhandler