Add Kotlin nullable type support for `AnnotatedService` arguments

Currently, AnnotatedService requires @Nullable annotations(or Optional) to allow null values for method parameters even when it’s used with Kotlin which offers nice null-safety with nullable types.

Example:

@Get("/foo")
fun foo(@Nullable @Param("bar") bar: String?): HttpResponse {
    // ...
}

Even though Kotlin nullable types(?), when compiled to java code, is attached @org.jetbrains.annotations.Nullable annotations, its retention policy is CLASS, which makes it invisible at Java Runtime.

kotlin-reflect seems to be the best bet to make this work.

[ikhoon]

The values of @Metadata seem to be encoded as Protobuf. That are decoded by kotlinx-metadata-jvm.

• https://github.com/JetBrains/kotlin/blob/master/libraries/kotlinx-metadata/jvm/ReadMe.md
• https://github.com/JetBrains/kotlin/blob/7bcd67068c232507921d5e48dbd93044007c1184/core/metadata/src/metadata.proto#L135

[ks-yim]

@ikhoon

Otherwise, is there another API that is only available in Kotlin at runtime time?

I was thinking of using isMarkedNullable API of KType.

I don’t know how exactly isMarkedNullable API works at runtime(maybe by looking at @Metadata annotation values), but we can get nullability of function parameters at runtime.