Spring Refine ModelAttributeMethodProcessor Kotlin exception handling

I have a kotlin class with a non-nullable property, which is not using a primitive type in the jvm, and I'm trying to use the properties of that class as request parameters. If I don't include that property as a request parameter, I would expect a bad request response. However, it instead crashes with this exception:

java.lang.IllegalArgumentException: Parameter specified as non-null is null: method no.finntech.iaap.data_import.Params.<init>, parameter a

This is the code I'm using:

class Params(
        val a: String
)

@SpringBootApplication
@RestController
class Application {
    @GetMapping("/test")
    fun test(params: Params) = ""
}

If a is of type Int instead, or if I use a non-nullable String directly in the arguments to test as a RequestParam, I get a bad request as expected.

Comment From: sdeleuze

First, sorry for the delay for a response.

For Int, in ModelAttributeMethodProcessor#constructAttribute we get a TypeMismatchException: Failed to convert value of type 'null' to required type 'int'; Failed to convert from type [null] to type [int] for value 'null' which makes senses since int is a primitive type so not nullable at Java/JVM level.

For String, the Java type is nullable, so that's only at BeanUtils.instantiateClass(ctor, args) level that Kotlin awareness of nullness is involved and raises this exception that is from what I can see java.lang.NullPointerException: Parameter specified as non-null is null: method com.example.demokotlinnative.Params.<init>, parameter a.

I will discuss with the team if there is something to refine here or not.

Comment From: sdeleuze

We can potentially try to wrap it for Kotlin parameter use case, I will give it a try and see how it goes.