Suppose I have a (spring boot controller method): ```getLineById(...) = runBlocking { Deferred<...

# coroutines

[Pihentagy]

Suppose I have a (spring boot controller method):

getLineById(...) = runBlocking {
  Deferred<SomeType> someDeferred = async { dependant.thatCanThrow() }

  SomeType? res = someDeferred.awaitOrNull();
}

suspend inline fun <T> Deferred<T>.awaitOrNull(): T? = runCatching { this.await() }.getOrNull()

Shouldn't my awaitOrNull catch all exceptions and convert them to null?

The problem arises when testing:

@MockkBean
private lateinit var dependent: Dependent

   @SpykBean
    private lateinit var controller: MainController

every { dependent.thatCanThrow() } throws Exception("omg") 

controller.getLineById(...)

then the test just throws the Exception("omg"), seemingly pointing to the line:

every { dependent.thatCanThrow() } throws Exception("omg")

[Sam]

An exception from a failed async job always propagates to the containing scope (in this case the runBlocking scope) in addition to being thrown from await. So I would expect getLineById to fail if the async job fails.

[Pihentagy]

Ok, so what should I do if I would like to ignore all errors thrown by someDeferred and just get a null then?

[Sam]

You could put the error handling inside the async job itself, and make it a Deferred<SomeType?>

[Pihentagy]

So instead of

Deferred<SomeType> someDeferred = async { dependant.thatCanThrow() }

I should write

Deferred<SomeType> someDeferred = async { try { dependant.thatCanThrow() } catch(e: Exception) { null } }

?

Can I write an extension method to make it more compact?

[Pihentagy]

say asyncOrNull?

[Pihentagy]

Is this method correct?

public fun <T> CoroutineScope.asyncOrNull(
        context: CoroutineContext = EmptyCoroutineContext,
        start: CoroutineStart = CoroutineStart.DEFAULT,
        block: suspend CoroutineScope.() -> T
    ): Deferred<T?> = async(context, start) {
        try {
            block()
        } catch (e: Exception) {
            null
        }
    }

[Sam]

Looks good to me 👍 😎