Validator does not work with Kotlin Spring Boot app

Question

Problem

Validation annotations for a Kotlin Spring Boot web project do not appear to be referenced at runtime.

@Min(18)
val age: Int? = null

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Expected Result

Annotated fields should throw an error if the specified validation condition is not met.

There are a lot of questions floating around the web asking about similar issues, but the answers don't seem fix this problem.

Suggestions are typically to use "get" or "field" for the annotations - e.g.

@field:Min(18)
val age: Int? = null

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Reproduction of Problem

To see why this wasn't working for Kotlin I did a comparison for Kotlin vs Java using this tutorial:

https://spring.io/guides/gs/validating-form-input/

I created a Java application here:

https://github.com/lgviii/JavaSpringValidations

I careated the same app in Kotlin here:

https://github.com/lgviii/KotlinSpringValidations

The Java app does validate the input in the webpage that runs on port 8080, but the Kotlin app does not. So the only difference appears the be the language...

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Question

1. Why does the Java app work out of the box while Kotlin does not?
2. What needs to be added to the Kotlin Spring Boot validator template project to get
   the validator to pick up the annotations like in the Java version of the project?

Answer 1

In your Kotlin sample project, the position of the @Valid annotation is incorrect. You use:

personForm: @Valid PersonForm?

This is putting the annotation on the type, a mechanism described in detail here. You're not annotating the parameter itself, which is what Spring MVC expects to trigger the validation. This is possible as the Validation API's @Valid is defined to be usable in exactly that position. If you try to use other parameter-only annotations like @ModelAttribute in that position, the compiler will reject that.

To resolve the problem, switch to the following declaration:

@Valid personForm: PersonForm?

This annotates the parameter, Spring MVC triggers the validation and requests sending inadequate data get redirected to the form as expected.

Alternatively, you can use Spring's @Validated annotation instead of JSR-303's @Valid. It essentially works the same way in this context, but prevents the misplacement as it cannot be used as type-use annotation.