Unable to use kotlin value classes as function arguments to functions mapping web endpoints

[junkdog]

I've encountered a bug in Spring Framework version 6.1.0 related to Kotlin value classes in web endpoints. The issue arises when a Kotlin value class is used as a path variable in a Spring Boot controller. This results in a failure to correctly handle the value type, causing runtime exceptions.

Environment

Spring Framework Version: 6.1.0 (with spring-boot 3.2.0)
Kotlin Version: 1.9.20
JVM Version: 17

Steps to reproduce

1. Define a Spring Boot controller with two endpoints, one accepting a Kotlin data class as a path variable and another accepting a Kotlin value class.
2. Create tests for these endpoints using SpringBootTest.
3. Observe that the endpoint with the Kotlin data class functions correctly, while the one with the kotlin value class fails.

Expected behavior

Both endpoints should accept their respective path variables without any issue

Actual Behavior

The endpoint with the Kotlin value class as a path variable fails at runtime with the error message "object is not an instance of declaring class".

Minimal example

@RestController
@RequestMapping(value = ["/exhibit"])
class KotlinCallByBugController {

    @GetMapping("/working-id/{id}")
    fun works(
        @PathVariable id: WorkingId
    ) = Unit

    @GetMapping("/value-id/{id}")
    fun broken(
        @PathVariable id: SomeId
    ) = Unit
}

@JvmInline // "Value classes without @JvmInline annotation are not supported yet"
value class SomeId(val s: String)

// data classes work just fine
data class WorkingId(val s: String)

@SpringBootTest(
    webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT,
    classes = [MyApplication::class])
class KotlinCallByBugControllerTest {
    @LocalServerPort
    protected val port = 0

    @Test
    fun `this works - call endpoint with path variable as data class`() {
        val client = HttpClients.createDefault()
        client.execute(HttpGet("http://localhost:$port/exhibit/working-id/123")) { response ->
            assertThat(response.code).isEqualTo(200)
        }
    }

    @Test
    fun `this does not work - call endpoint with path variable as value class`() {
        // breaks: "object is not an instance of declaring class"
        val client = HttpClients.createDefault()
        client.execute(HttpGet("http://localhost:$port/exhibit/value-id/123")) { response ->
            assertThat(response.code).isEqualTo(200)
        }
    }
}

[CharlyRien]

Hello,

I wanted to mention that I am experiencing the same issue, even though I am not directly using the value class in the controller path variable. If I invoke any bean methods with a value class in the parameters, the outcome remains the same: I receive the error message object is not an instance of declaring class from java.base/jdk.internal.reflect.DirectMethodHandleAccessor.checkReceiver(DirectMethodHandleAccessor.java:197).

We are currently utilizing Java 21 on our end.

Thanks

[sdeleuze]

It looks like Kotlin reflection does not support kotlin.reflect.KCallable#callBy invocation with the data class unwrapped type, I have asked Kotlin team feedback.

[sdeleuze]

Confirmed to be a Kotlin bug, you can follow the resolution of https://youtrack.jetbrains.com/issue/KT-64097.

[junkdog]

While not an ideal solution, one possible workaround for the Kotlin callBy bug could be to manually box arguments that are value class types. One can determine which KParameters correspond to value classes by checking the KClass::isValue property.

A simpler mitigation might be to fallback on the java reflection code path if callBy encounters failures (alt if any KParameters point to value classes) - since it is only aware of the underlying type.

Edit: Just to clarify. We upgraded from 6.0.13, where it worked fine.