Why Exception Handling with Kotlin Coroutines is hard and how to master it

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Why Exception Handling in Kotlin Coroutines is hard … … and how to master it! Lukas Lechner Freelance Developer & Online Instructor www.lukaslechner.com November 17, 2020

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Lukas Lechner Freelance Developer & Online Instructor www.lukaslechner.com @LukasLechnerDev

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Kotlin Coroutines: Write asynchronous and concurrent code in a sequential fashion by using conventional coding constructs.

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Exceptions in General

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fun main() { // some code throw Exception() } >_ Exception in thread "main" java.lang.Exception

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fun main() { // some code throw Exception() }

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fun main() { try { // some code throw Exception() } catch (exception: Exception) { println(“Handled exception") } } >_ Handled exception fun main() { try { // some code throw Exception() } catch (exception: Exception) { println(“Handled exception") } }

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fun main() { try { functionThatThrows() } catch (exception: Exception) { println(“Handled exception") } } fun functionThatThrows() { // some code throw Exception() } >_ Handled exception fun main() { try { functionThatThrows() } catch (exception: Exception) { println(“Handled exception") } } fun functionThatThrows() { // some code throw Exception() }

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Exceptions in Coroutines

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.launch { throw Exception() } Thread.sleep(100) } >_ Exception in thread "DefaultDispatcher-worker-1" java.lang.Exception

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.launch { try { throw Exception() } catch (exception: Exception) { println(“Handled $exception") } } Thread.sleep(100) } >_ Handled exception fun main() { val rootScope = CoroutineScope(Job()) rootScope.launch { try { throw Exception() } catch (exception: Exception) { println(“Handled $exception") } } Thread.sleep(100) }

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... rootScope.launch { try { suspendFunctionThatThrows() } catch (exception: Exception) { println(“Handled $exception") } } suspend fun suspendFunctionThatThrows() { throw Exception() } >_ Handled exception ... rootScope.launch { try { suspendFunctionThatThrows() } catch (exception: Exception) { println(“Handled $exception") } } suspend fun suspendFunctionThatThrows() { throw Exception() }

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... rootScope.launch { try { throw Exception() } catch (exception: Exception) { println("Handled $exception") } } }

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... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println("Handle $exception") } } >_ Exception in thread “DefaultDispatcher-worker-2" java.lang.Exception ... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } rootScope Job

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... val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Job Coroutine started with launch{} rootScope Job

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Job Child Coroutine started with launch{} ... val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Job Coroutine started with launch{} rootScope Job

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Uncaught Exception Handler Job Child Coroutine started with launch{} ... val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Job Coroutine started with launch{} rootScope Job

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Coroutine Exception Handler

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Every Coroutine and CoroutineScope has a CoroutineContext CoroutineContext Context Elements Job ExceptionHandler Name Dispatcher

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val ceh = CoroutineExceptionHandler { coroutineContext, exception -> println(“Handled $exception in CoroutineExceptionHandler") }

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val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Child Coroutine Root Coroutine rootScope

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Uncaught Exception Handler val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch(ceh) { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Child Coroutine Root Coroutine rootScope Coroutine Exception Handler val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job()) rootScope.launch { try { launch(ceh) { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } >_ Exception in thread “DefaultDispatcher- worker-2" java.lang.Exception

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val ceh = CoroutineExceptionHandler {…} val topLevelScope = CoroutineScope(Job()) topLevelScope.launch(ceh) { try { launch { throw Exception() } } catch (exception: Exception) { println("Handle $exception") } } val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job()) rootScope.launch(ceh) { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Child Coroutine Root Coroutine rootScope Coroutine Exception Handler

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val ceh = CoroutineExceptionHandler {…} val topLevelScope = CoroutineScope(Job()) topLevelScope.launch(ceh) { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job()) rootScope.launch(ceh) { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } } Child Coroutine Root Coroutine rootScope Coroutine Exception Handler >_ Handled exception in CoroutineExceptionHandler

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val ceh = CoroutineExceptionHandler {…} val rootScope = CoroutineScope(Job() + ceh) rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println("Handled $exception") } } Child Coroutine Root Coroutine rootScope Coroutine Exception Handler

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Root Coroutines have some special properties

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Coroutine Exception Handler VS try/catch

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Coroutine Exception Handler try/catch ➡ Global “catch all” behaviour ➡ Coroutines have already completed => no recovery possible ➡ e.g. log exception, show error message ➡ handle the exception directly in the Coroutine ➡ possibility to recover ➡ e.g. retry operation or perform other arbitrary operations

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There are 2 important aspects to consider when using try/catch !

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1. Keep the “Cancellation Mechanism” of Structured Concurrency in mind

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) } rootScope

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) } launch rootScope

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) } async launch rootScope async

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launch async async rootScope rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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launch async async rootScope rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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launch async async rootScope rootScope.launch { val result1 = async { try { networkRequest1() } catch (e: Exception) { // Log exception null } } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope launch async async rootScope.launch { val result1 = async { try { networkRequest1() } catch (e: Exception) { // Log exception null } } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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val ceh = CoroutineExceptionHandler {…} rootScope.launch { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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val ceh = CoroutineExceptionHandler {…} rootScope.launch(ceh) { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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val ceh = CoroutineExceptionHandler {…} rootScope.launch(ceh) { val result1 = async { networkRequest1() } val result2 = async { networkRequest2() } display(awaitAll(result1, result2)) }

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2. You might catch a CancellationException and therefor your coroutine continues to run

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { println("Starting network request") delay(1000) println("Coroutine still running ... ") } delay(500) job.cancel() throws CancellationException after 500ms •>_ Starting network request

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val job = launch { try { println("Starting network request") delay(1000) } catch (e: Exception) { // Log Exception println("Caught: $e") } println("Coroutine still running ... ") } delay(500) job.cancel() •>_ Starting network request • Caught: JobCancellationException • Coroutine still running ...

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val job = launch { try { println("Starting network request") delay(1000) } catch (e: HttpException) { // Log Exception println("Caught: $e") } println("Coroutine still running ... ") } delay(500) job.cancel()

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val job = launch { try { println("Starting network request") delay(1000) } catch (e: Exception) { if (e is CancellationException) { throw e } } println("Coroutine still running ... ") } delay(500) job.cancel()

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ExceptionHandling launch{} VS async{}

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launch{} rootScope

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launch{} rootScope Coroutine Exception Handler

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.async { throw Exception() } Thread.sleep(100) }

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.async { throw Exception() } Thread.sleep(100) }

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.async { throw Exception() } Thread.sleep(100) }

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.async { throw Exception() } Thread.sleep(100) } •>_ No Output

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async{} rootScope

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async{} rootScope Coroutine Exception Handler Deferred re-thrown when calling .await()

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fun main() { val rootScope = CoroutineScope(Job()) rootScope.async { throw Exception() } Thread.sleep(100) }

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... val deferredResult = rootScope.async { throw Exception() }

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... val deferredResult = rootScope.async { throw Exception() }

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... val deferredResult = rootScope.async { throw Exception() } deferredResult.await()

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... val deferredResult = rootScope.async { throw Exception() } deferredResult.await()

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... val deferredResult = rootScope.async { throw Exception() } rootScope.launch { deferredResult.await() }

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... val deferredResult = rootScope.async { throw Exception() } rootScope.launch { deferredResult.await() }

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... val deferredResult = rootScope.async { throw Exception() } rootScope.launch { try { deferredResult.await() } catch (exception: Exception) { println(“Handled $exception") } }

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... val deferredResult = rootScope.async { throw Exception() } rootScope.launch { try { deferredResult.await() } catch (exception: Exception) { println(“Handled $exception") } }

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... val deferredResult = rootScope.async { throw Exception() } val ceh = CoroutineExceptionHandler { ... } rootScope.launch(ceh) { deferredResult.await() }

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... val deferredResult = rootScope.async { throw Exception() } val ceh = CoroutineExceptionHandler { ... } rootScope.launch(ceh) { deferredResult.await() }

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async{} Child Coroutines

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... val rootScope = CoroutineScope(Job()) rootScope.launch { async { throw Exception() } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { async { throw Exception() } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { async { throw Exception() } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { async { throw Exception() } }

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... val rootScope = CoroutineScope(Job()) rootScope.launch { async { throw Exception() } } async{} child Coroutine launch{} root Coroutine rootScope >_ Exception in thread “DefaultDispatcher- worker-2" java.lang.Exception

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Scoping Function coroutineScope{}

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... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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>_ Exception in thread “DefaultDispatcher-worker-2" java.lang.Exception ... rootScope.launch { try { launch { throw Exception() } } catch (exception: Exception) { println(“Handled $exception") } }

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rootScope.launch { try { coroutineScope { launch { throw Exception() } } } catch (exception: Exception) { println(“Handled $exception") } }

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rootScope.launch { try { coroutineScope { launch { throw Exception() } } } catch (exception: Exception) { println(“Handled $exception") } } >_ Handled exception

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coroutineScope{} re-throws uncaught exceptions of its child coroutine

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coroutineScope{} re-throws uncaught exceptions of its child Coroutines and so we can handle them with a try/catch

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suspend fun suspendFunction() = coroutineScope { // suspend function body }

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suspendFunction() suspend fun suspendFunction() = coroutineScope { // suspend function body }

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suspendFunction() suspend fun suspendFunction() = coroutineScope { // suspend function body }

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try { suspendFunction() } catch (exception: Exception) { println(“Handled $exception") } suspend fun suspendFunction() = coroutineScope { // suspend function body }

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try { suspendFunction() } catch (exception: Exception) { println(“Handled $exception") } suspend fun suspendFunction() = coroutineScope { // suspend function body }

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Scoping Function supervisorScope{}

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supervisorScope{} a new independent child scope in our job hierarchy

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } }

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } } rootScope

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } } Coroutine 1 rootScope

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } } Coroutine 2 Coroutine 1 rootScope

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } } Coroutine 2 SupervisorJob Coroutine 1 rootScope

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rootScope.launch { // Coroutine 1 launch { // Coroutine 2 } supervisorScope { launch { // Coroutine 3 } launch { // Coroutine 4 } } } Coroutine 2 Coroutine 3 SupervisorJob Coroutine 4 Coroutine 1 rootScope supervisorScope

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Coroutine 2 Coroutine 3 Coroutine 4 Coroutine 1 SupervisorJob supervisorScope

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Coroutine 2 Coroutine 3 Coroutine 4 Coroutine 1 SupervisorJob

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Coroutine 2 Coroutine 3 Coroutine 4 Coroutine Exception Handler Coroutine Exception Handler Coroutine 1 SupervisorJob

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Recap

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1) Exceptions can be handled directly in a Coroutine with a try-catch block. This way, the Coroutine doesn’t complete exceptionally. launch { try { throw Exception() } catch (exception: Exception) { println("Handled $exception") } } }

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2) If an exception is thrown in a Coroutine and not handled directly within the Coroutine with a try-catch block, the Coroutine completes exceptionally. launch { throw Exception() } Coroutine

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3) As a result, the exception is propagated up the job hierarchy until it reaches either the RootScope or a SupervisorJob. While the exceptions travels upwards, parent coroutines fail too and sibling coroutines get cancelled. Coroutine 2 Coroutine 4 SupervisorJob Coroutine 5 rootScope Coroutine 1 Coroutine 3 1 3 2 1

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4) When the root coroutine was started with launch{}, the exception will be passed to an installed CoroutineExceptionHandler. When the root coroutine was started with async{}, the exception is encapsulated in the Deferred object. async{} rootScope launch{} 1 Coroutine Exception Handler Deferred re-thrown when calling.await() 2 1 2

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5) The scoping function coroutineScope{} re-throws uncaught exceptions of its child Coroutines and so we can handle them with a try/catch. launch { try { coroutineScope { launch { throw Exception() } } } catch (exception: Exception) { println(“Handled $exception") } }

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6) Keep in mind: • When your Coroutine is not completing exceptionally, parent and sibling coroutines won’t be canceled. • Suspend functions can throw a CancellationException at any point and by catching them, the Coroutine will keep on running.

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Further Resources www.lukaslechner.com/coroutines-exception-handling-cheat-sheet www.lukaslechner.com/why-exception-handling-with-kotlin-coroutines-is-so- hard-and-how-to-successfully-master-it/ github.com/LukasLechnerDev/Kotlin-Coroutine-Use-Cases-on-Android

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Further Resources www.lukaslechner.com/coroutines-on-android

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Why Exception Handling in Kotlin Coroutines is hard … … and how to master it! Lukas Lechner Freelance Developer & Online Instructor www.lukaslechner.com November 17, 2020