Hey! Is there a way to know the type of a generic ...

# server

[nadi]

Hey!
Is there a way to know the type of a generic annotation in runtime?

if i have the following:

@Target(allowedTargets = [AnnotationTarget.CLASS, AnnotationTarget.FUNCTION])
@Retention(AnnotationRetention.RUNTIME)
annotation class RequiredExternalUserAuth<T>(
    val contextReader: KClass<ContextReader<T>>
)

i want to be able to know what T is in runtime. is it possible?

[David Kubecka]

Not without using reflection.

[nadi]

with reflection of course

[David Kubecka]

I used something like this to get the type parameters of normal classes:

fun Any.getGenericSupertypesArgumentClasses(): List<Class<*>> {
    val superType = this::class.java.genericSuperclass as ParameterizedType
    return superType.actualTypeArguments.map { Class.forName(it.typeName) }
}

For annotations you might have to modify it a bit.

[nadi]

ahh cool, thanks a lot!

[nadi]

btw - is this the right place to ask this question?

[nadi]

is there a more appropriate place?

[David Kubecka]

I think #getting-started would be a better place. But honestly, this isn't much kotlin-specific 🙂

[nadi]

umm actually, how is it possible?
https://kotlinlang.org/docs/generics.html#type-erasure
i see that there is type-erasure in kotlin
im kotlin noobie hehe

[David Kubecka]

Yes, there's type erasure and therefore you have to use reflection. Also note that in contrast to Java there's a way how to sometimes get the type parameters without reflection: https://kotlinlang.org/docs/inline-functions.html#reified-type-parameters. But that works only for functions, not classes.

[nadi]

If there's type-erasure doesnt it mean that the information isn't saved after compilation?

[nadi]

https://stackoverflow.com/questions/34122450/how-to-get-generic-parameter-class-in-kotlin
so its only for reified-type

[David Kubecka]

Yes, I forgot to mention the documentation for that function 🙂

/**
 * If the receiver is a subclass of a generic superclass with type arguments (e.g. `Receiver : Map<Int, String>`)
 * then return the type argument Classes (e.g. `[Int, String]`). Otherwise an exception is thrown.
 */

In other words, this relis on an old Java technique (hack): https://www.baeldung.com/java-generic-type-find-class-runtime#3-using-typetoken