Can anyone confirm that it not possible to

reflect()

on a lambda specified using the

::methodName

syntax, and that it will return null? I'm experimenting with a library (https://github.com/moia-oss/lambda-kotlin-request-router) and this code passes:

POST("/something") { req: Request<Unit> -> ResponseEntity.ok(Unit) }

But this code fails, and indeed the library seems to suggest that it will always fail because of a limitation in kotlin reflection on lambdas:

val dummy = object {
                fun handler(r: Request<Unit>) = ResponseEntity.ok(Unit)
            }
GET("/some", dummy::handler)

The library uses kotlin reflection while processing the GET() function:

al requestType = handler.reflect()?.parameters?.first()?.type?.arguments?.first()?.type
            ?: throw IllegalArgumentException("reflection failed, try using a real lambda instead of function references (Kotlin 1.6 bug?)")

Clearly the library has chosen not to support the ::method syntax, despite using it in its documentation and samples. Any suggestions? Using Kotlin 1.7.22

You can also do it this way:

val kFunction = when (handler) {
        is KFunction<*> -> handler
        is kotlin.jvm.internal.Lambda<*> -> handler.reflect() ?: error(...)
        else -> error(...)
    }
    val requestType = kFunction.parameters.first().type.arguments.first().type

It uses internal API though,

kotlin.jvm.internal.Lambda

is the class which we use for lambdas with metadata

I've tried this out and it is working in unit tests, but I'll need to raise a pull request with the library - and so far, doesn't seem to be active. Struggling to deploy the library to mavenLocal so can't test it in my app.

val requestType: KType? = try {
            val fk = handler as KFunction<*>
            fk.parameters.first().type.arguments.first().type
        } catch (cce: ClassCastException) {
            log.warn("Unable to to cast handler to KFunction; assuming it's a lambda and reflecting on that")
            handler.reflect()?.parameters?.first()?.type?.arguments?.first()?.type
        }

But I guess doing the type check is better than cast-and-catch.