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)).

Lanoralanose

Are you asking about kotlin in general, or specifically how to handle this situation in Spring MVC (i.e. "as @NotNull is redundant when using Kotlin, how do I validate requests in which a non-null property isn't supplied in the request JSON?") – Conjoint

Answer 1

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...

Lanoralanose

Answer 2

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.

Conjoint

Answer 3

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.

Jerad

Answer 4

My workarround for now (place the file anywhere):

@Order(Ordered.HIGHEST_PRECEDENCE)
@ControllerAdvice
class ExceptionHandler {

    @ResponseStatus(BAD_REQUEST)
    @ResponseBody
    @ExceptionHandler(MissingKotlinParameterException::class)
    fun missingKotlinParameterException(ex: MissingKotlinParameterException): Error? {
        return createMissingKotlinParameterViolation(ex)
    }

    private fun createMissingKotlinParameterViolation(ex: MissingKotlinParameterException): Error{
        val error = Error(BAD_REQUEST.value(), "validation error")
        val errorFieldRegex = Regex("\\.([^.]*)\\[\\\"(.*)\"\\]\$")
        val errorMatch = errorFieldRegex.find(ex.path[0].description)!!
        val (objectName, field) = errorMatch.destructured
        error.addFieldError(objectName.decapitalize(), field, "must not be null")
        return error
    }

    data class Error(val status: Int, val message: String, val fieldErrors: MutableList<CustomFieldError> = mutableListOf()) {
        fun addFieldError(objectName: String, field: String, message: String) {
            val error = CustomFieldError(objectName, field, message)
            fieldErrors.add(error)
        }
    }

    data class CustomFieldError(val objectName: String, val field: String, val message: String)

Collegium

Answer 5

I use following exception handler:

@Order(Ordered.HIGHEST_PRECEDENCE)
@ControllerAdvice
class ExceptionHandlerResolver {

  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ResponseBody
  @ExceptionHandler(MissingKotlinParameterException::class)
  fun missingKotlinParameterException(ex: MissingKotlinParameterException): MyCustomError? {
    return MyCustomError(
      timestamp = LocalDateTime.now(),
      status = HttpStatus.BAD_REQUEST,
      exception = ex,
      validationMessages = listOf(
        ValidationMessage(
          field = ex.path.fold("") { result, segment ->
            if (segment.fieldName != null && result.isEmpty()) segment.fieldName
            else if (segment.fieldName != null) "$result.${segment.fieldName}"
            else "$result[${segment.index}]"
          },
          message = "value is required"
        )
      )
    )
  }
}

It supports exception for any Json path, for example: arrayField[1].arrayItemField.childField

MyCustomError and ValidationMessage classes:

data class MyCustomError(
  val timestamp: LocalDateTime,
  @JsonIgnore val status: HttpStatus,
  @JsonIgnore val exception: Exception? = null,
  val validationMessages: List<ValidationMessage>? = null
) {
  @JsonProperty("status") val statusCode = status.value()
  @JsonProperty("error") val statusReasonPhrase = status.reasonPhrase
  @JsonProperty("exception") val exceptionClass = exception?.javaClass?.name
  @JsonProperty("message") val exceptionMessage = exception?.message
}

data class ValidationMessage(val field: String, val message: String)

Semifinal