I want to detect if a JVM Class is a Kotlin class or not

Question

I want to do special functionality if I encounter a Kotlin class as compared to a generic Java class. How can I detect if it is a Kotlin class?

I was hoping that calling someClass.kotlin would throw an exception or fail if the class wasn't Kotlin. But it wraps Java classes just fine. Then I noticed that if I do someClass.kotlin.primaryConstructor it seems to be null for all java classes even if they have a default constructor, is that a good marker? But can that return null for a Kotlin class as well?

What is the best way to say "is this a Kotlin class?"

Answer 1

Kotlin adds an annotation to all of its classes, and you can safely check for its existence by name. This is an implementation detail and could change over time, but some key libraries use this annotation so it is likely to be ok indefinitely.

fun Class<*>.isKotlinClass(): Boolean {
    return this.declaredAnnotations.any {
        it.annotationClass.qualifiedName == "kotlin.Metadata"
    }
}

Can be used as:

someClass.isKotlinClass()

The class kotlin.Metadata is not accessed directly because it is marked internal in the Kotlin runtime.