I m migrating a test to coroutines 1 6 0 which should check

Question

I m migrating a test to coroutines 1 6 0 which should check that a suspending function throws an exception I m using AssertJ for assertions This is what the test looked like before ``` Test fun `when no gateway from scanning present then throw` every state gateway returns null assertThatIllegalStateException isThrownBy runBlocking useCase execute Params ``` This is what it looks like after the migration according to the official migration guide ``` val exceptions = mutableListOf lt Throwable gt val customCaptor = CoroutineExceptionHandler throwable gt exceptions add throwable Test fun `when no gateway from scanning present then throw` = runTest every state gateway returns null launch customCaptor useCase execute Params advanceUntilIdle assertThat exceptions hasSize 1 first isInstanceOf IllegalStateException class java ``` However the exception is for some reason not caught by the `customCaptor` but thrown in the test and the test fails

Lukasz Kalnik · Answer

I works when I simply replace `runBlocking` by `runTest` but this is not recommended according to the migration guide it could obscure other problems with the test detected by the `runTest` cleanup procedure

Lukasz Kalnik · Answer

I don t use any custom dispatcher in the `useCase` under test Should I inject one and replace it with the `TestScope coroutineContext` for testing

Lukasz Kalnik · Answer

The test still throws the exception even when I set a test dispatcher ``` BeforeTest fun setUp Dispatchers setMain StandardTestDispatcher ```

Dmitry Khalanskiy [JB] · Answer

Won t just ```assertFailsWith lt IllegalStateException gt useCase execute Params ``` without `launch`ing a new coroutine work The reason the whole test fails with an exception is that the coroutine you launch fails with that exception The uncaught exception handler is not used because well the exception is not uncaught it s propagated using structured concurrency

Lukasz Kalnik · Answer

Yes this indeed works Thank you for your quick and helpful answer

Lukasz Kalnik · Answer

But I still don t understand the explanation in the migration guide

Lukasz Kalnik · Answer

It says valid uses of TestCoroutineExceptionHandler include Accessing `uncaughtExceptions` when the uncaught exceptions are actually expected In this case `cleanupTestCoroutines` will fail with an exception that is being caught later It would be better in this case to use a custom `CoroutineExceptionHandler` so that actual problems that could be found by the cleanup procedure are not superseded by the exceptions that are expected An example is shown below

Dmitry Khalanskiy [JB] · Answer

I answered some of your questions under the issue you opened but basically the migration guide is for migration from the old test API which in this case you weren t using at all

Lukasz Kalnik · Answer

Yes I understand this now

Lukasz Kalnik · Answer

So the uncaught exceptions in the test would only be exceptions not thrown within a coroutine

Dmitry Khalanskiy [JB] · Answer

Uncaught exceptions are the ones that nobody catches Given that you managed to observe the exception being caught it s not uncaught Uncaught exceptions are in general quite rare and can be considered an obscure edge case

Dmitry Khalanskiy [JB] · Answer

You could create an uncaught exception like this ```launch SupervisorJob launch throw IllegalStateException ```

Lukasz Kalnik · Answer

Yes I guess I misunderstood that exceptions rethrown by coroutines are also uncaught

Dmitry Khalanskiy [JB] · Answer

Children of `SupervisorJob` don t have the right to fail the `SupervisorJob` so they can t propagate the exception upwards so they have to resort to uncaught exception handlers

Lukasz Kalnik · Answer

Because for me uncaught meant kind of crashing the test program

Lukasz Kalnik · Answer

Thank you very much for the explanation

Dmitry Khalanskiy [JB] · Answer

You re welcome Can the issue be closed then

Lukasz Kalnik · Answer

Yes I just closed it