Kotlin, Spring Boot and Validations

Spring boot is an excellent choice for backends and with Kotlin it is even better. However, if you are running to validation issues with Kotlin, remember there is a simple issue with Kotlin Annotations.

class User(@NotNull user:String);

@Controller
@RequestMapping("/users")
class UserController {
    @PostMapping
    fun createUser(@Valid user: User, bindingResult: BindingResult): ModelAndView {
        return ModelAndView("someview", "user", user)
    }
}

Kotlin however confuses these sort of annotations and applies them to the constructor instead of the field. Solution for this is very simple.

class User(@field:NotNull user:String);

This tells the Kotlin that the annotation is field specific and not meant for constructor. You code should then work.

Normally I use spring boot as backend of frontend and to render templates and such. This issue took a lot of my time but I solved it eventually.

Reference:

https://stackoverflow.com/questions/52345291/bean-validation-not-working-with-kotlin-jsr-380

https://stackoverflow.com/questions/51350923/request-binding-not-working-for-kotlin-class