Is NotNull needed on Kotlin?

Question

I have a class:

class User(

    var name: String

)

And a mapped post request:

@PostMapping("/user")
fun test(@Valid @RequestBody user: User) {
    //...
}

What if a client will send a JSON of a user with name: null? Will it be rejected by the MVC Validator or an exception will be throwed? Should I annotate name with @NotNull? Unfortunately, I cannot check that because only can write tests (it is not available to create User(null)).

Answer 1

You can avoid using @NotNull, as it'll never get triggered for non-nullable property (bean validation only kicks in after Jackson deserializes the JSON - which is where this will fail).

Assuming you're using jackson-module-kotlin, then you should get an exception with MissingKotlinParameterException as the root cause, which you can then handle in a @ControllerAdvice. In your advice you can handle normal bean validation exceptions (e.g. ConstraintViolationExceptioncaused by @Size) and missing non-null constructor params (MissingKotlinParameterException) and return an API response indicating the fields in error.

The only caveat with this is that the jackson-kotlin-module fails on the first missing property, unlike bean validation in Java, which will return all violations.

Answer 2

Since name is a not-null parameter of User, it cannot accept nulls, ever.

To guarantee that Kotlin compiler:

• inserts a null check inside the User constructor which throws an exception if some Java code tries to pass a null.

• annotates name with@NotNull in order to stay consistent with Java APIs

• does not add any @NotNull annotation since there isn't one which everybody agrees on :(

Here are the corresponding docs

Update

I have rechecked it, and Kotlin compiler v1.0.6 does insert @Nullable and @NotNull from org.jetbrains.annotations. I have updated the answer accordingly.

Answer 3

As I tested, @NotNull doesn't affect on the MVC validation at all. You will only receive a WARN message in console which is normal:

Failed to read HTTP message: org.springframework.http.converter.HttpMessageNotReadableException: Could not read document: Instantiation of...