Add support for Kotlin inline/value classes in `BeanUtils`

[MrBuddyCasino]

Kotlin value classes currently cannot be used in configuration bindings in Spring Boot 2.6.6.

This is a repro case:

package com.example

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.boot.context.properties.ConfigurationPropertiesScan
import org.springframework.boot.context.properties.ConstructorBinding
import org.springframework.boot.runApplication
import org.springframework.context.annotation.Bean

fun main(args: Array<String>) {
    System.setProperty("SPRING_APPLICATION_JSON", "{ \"predicate\": \"C\" }")
    runApplication<SpringTest>(*args)
}

@SpringBootApplication
@ConfigurationPropertiesScan(basePackages = ["com.example"])
class SpringTest {

    @Bean
    fun someBean(config: PredicateConfiguration): String {
        println("predicate: ${config.predicate.value}")
        return ""
    }

}

@JvmInline
value class Predicate(val value: Char)

@ConfigurationProperties
@ConstructorBinding
data class PredicateConfiguration(val predicate: Predicate)

It fails with the following error message:

Failed to bind properties under '' to com.example.PredicateConfiguration:

    Reason: java.lang.IllegalArgumentException: object is not an instance of declaring class

[wilkinsona]

Thanks for the report. The problem can be reproduced without Spring Boot:

package com.example.gh31398

import org.springframework.beans.BeanUtils

class Gh31398Application

fun main(args: Array<String>) {
	val constructor = PredicateConfiguration::class.java.constructors[0]
	val args = arrayOf(Character.valueOf('c'))
	BeanUtils.instantiateClass(constructor, *args)
}

@JvmInline
value class Predicate(val value: Char)

data class PredicateConfiguration(val predicate: Predicate)

Exception in thread "main" org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.gh31398.PredicateConfiguration]: Illegal arguments for constructor; nested exception is java.lang.IllegalArgumentException: object is not an instance of declaring class
	at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:221)
	at com.example.gh31398.Gh31398ApplicationKt.main(Gh31398Application.kt:10)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at kotlin.reflect.jvm.internal.calls.InlineClassAwareCaller.call(InlineClassAwareCaller.kt:134)
	at kotlin.reflect.jvm.internal.KCallableImpl.call(KCallableImpl.kt:108)
	at kotlin.reflect.jvm.internal.KCallableImpl.callDefaultMethod$kotlin_reflection(KCallableImpl.kt:159)
	at kotlin.reflect.jvm.internal.KCallableImpl.callBy(KCallableImpl.kt:112)
	at org.springframework.beans.BeanUtils$KotlinDelegate.instantiateClass(BeanUtils.java:892)
	at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:196)
	... 1 more

I'm not sure if this is something that can be addressed in Framework's support for instantiating Kotlin types or if it's a bug in Kotlin's reflection support.

We'll transfer this to the Framework team so that they can take a look.

[sdeleuze]

I don't think we support Kotlin inline classes yet in Spring portfolio as such support is not straightforward from a JVM bytecode reflection POV and we lack a global abstraction for Kotlin reflection. We had related discussion with the Spring Data team that shown it can be tricky topic even if this specific case is maybe doable without too much trouble.

Maybe we should at least document at Framework level it is not supported yet. I will create a related documentation issue.

[aseemsavio]

Hello team - can support for Kotlin inline classes be expected in the near future? :)