PSA don t try catch `IllegalStateException` because a `Cance

Question

PSA don t try catch `IllegalStateException` because a `CancellationException` is a subclass and it puts your application into an unexpected state if you catch it Just found out the hard way

Dico · Answer

Why does it subclass that

Paul Woitaschek · Answer

Well `kotlinx coroutines CancellationException` is a typealias of `java util concurrent CancellationException` and that extends `java lang IllegalStateException`

Paul Woitaschek · Answer

I have shot myself so often in my foot by accidentially swallowing `CancellationException` confused

Paul Woitaschek · Answer

< elizarov> Can t coroutine have their own cancellation exception This makes error handling really hard especially as functions like `error` also throw IllegalStateExeption

Adam Powell · Answer

under what circumstances would you specifically try catch `IllegalStateException` Making assumptions about how to recover from one vs `Exception` or `Throwable` seems quite brittle since you don t know what threw it or why without further info like a more specific exception type

Dico · Answer

any suspend function that may throw IllegalStateException

Paul Woitaschek · Answer

I m working with the samsung health sdk and it randomly throws illegalstateexceptions

Adam Powell · Answer

< Dico> unless you re making unsafe assumptions about what that ISE means it s not really meaningful to catch it any more specifically than `Throwable`

Dico · Answer

We re talking about a very specific use case you re talking about generalizing strategy that is not useful

Dico · Answer

The point is that functions can and will throw concretely IllegalStateException and catching it interferes with coroutines machinery unintentionally

Paul Woitaschek · Answer

< Adam Powell> Don t you agree that the coroutine machinery for cancellation interferes with exception throwing code in an error prone way

Adam Powell · Answer

not in a way that is specific to `IllegalStateException` Job cancellation exists at the library layer not a language layer The mechanism for stack unwinding in such circumstances is to throw `catch t Throwable ` has the same gotcha and requires the same consideration but given the rest of the advantages of the design imo it s worth the tradeoffs

Adam Powell · Answer

changing the exception hierarchy that it uses at this point for it would be a breaking change so I m guessing it s not going to happen

Paul Woitaschek · Answer

I see it as a severe flow and something that you constantly have to be careful with

Adam Powell · Answer

Now I m also curious about the specific samsung health sdk method you re referring to do you have a link to some public docs by any chance

Paul Woitaschek · Answer

I don t think that s relevant here It s simple code that gets broken in a subtle way

Paul Woitaschek · Answer

For example if you take this code ``` try thirdPartyApi doSomethingThatCanThrow catch e IllegalStateException e printStackTrace ```

Paul Woitaschek · Answer

Now you add a simple delay and you have sucessfully shot yourself in the foot ``` try thirdPartyApi doSomethingThatCanThrow delay 100 catch e IllegalStateException e printStackTrace ```

elizarov · Answer

Don t catch ISEs Period also applies to NPEs OOMs etc This simple rule will save you countless hours in the course of your career If you have to do it because of broken 3rd party API Well all bets are off anyway

orafaaraujo · Answer

Hey guys So If I have a `NPE` I can t get it with ` catch e gt emit Result Error Exception e ` Tthis is what is happening here I m receiving an NPE and my `catch` is getting it but it s not emitting my `Result Error` The app is still crashing disappointed