Kotlin - How to get KClass<*> annotation parameter from annotation processor

Question

I have the following annotation:

@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Model(
    val owner: KClass<*>,
    val consumer: KClass<*>
)

@Model(DataOwner::class, DataConsumer::class)
interface Student {
    val name: String
    val group: Group
}

I need to get the value of owner and consumer in my annotation processor.

I've tried this approach:

private inline fun <reified T> findAnnotationValue(
    element: Element,
    annotationClass: KClass<*>,
    valueName: String
): T? {
    return element.annotationMirrors.map {
        it to it.annotationType.asElement() as TypeElement
    }.firstOrNull { (_, element) ->
        element.qualifiedName.contentEquals(annotationClass.qualifiedName)
    }?.let { (mirror, _) ->
        extractValue(mirror, valueName)
    }
}

private inline fun <reified T> extractValue(
    annotationMirror: AnnotationMirror,
    valueName: String
): T? {
    return annotationMirror.elementValues.toList()
        .firstOrNull { (key, _) ->
            key.simpleName.contentEquals(valueName)
        }?.let { (_, value) ->
            value.value as T
        }
}


val ownerClass: KClass<*> = findAnnotationValue(
    element,
    Model::class,
    "owner"
)

But it gave me this error:

e: [kapt] An exception occurred: java.lang.ClassCastException: com.sun.tools.javac.code.Type$ClassType cannot be cast to kotlin.reflect.KClass

I also tried this:

val ownerClass: KClass<*> = element.getAnnotation(Model::class.java).owner

But it gave me this error:

e: [kapt] An exception occurred: javax.lang.model.type.MirroredTypeException: Attempt to access Class object for TypeMirror inc.ahmedmourad.systems.tutors.domain.model.DataOwner

inc.ahmedmourad.systems.tutors.domain.model.DataOwner is the owner value passed to the annotation.

So this's where i'm stuck right now, any help is appreciated. Thank you!

Answer 1

Let's start with the reason the second approach doesn't work:

The annotation returned by this method could contain an element whose value is of type Class. This value cannot be returned directly: information necessary to locate and load a class (such as the class loader to use) is not available, and the class might not be loadable at all. Attempting to read a Class object by invoking the relevant method on the returned annotation will result in a MirroredTypeException, from which the corresponding TypeMirror may be extracted. Similarly, attempting to read a Class[]-valued element will result in a MirroredTypesException.

which obviously applies to KClass as well.

So for classes in annotations you can only get a TypeMirror (implemented in this case by Type.ClassType , but that's an internal detail you shouldn't rely on) and not a KClass . Either by the first approach or

inline fun <reified T : Annotation> Element.getAnnotationClassValue(f: T.() -> KClass<*>) = try { 
    getAnnotation(T::class.java).f() 
    throw Exception("Expected to get a MirroredTypeException")
} catch (e: MirroredTypeException) {
    e.typeMirror
}

which can be used as

element.getAnnotationClassValue<Model> { owner }

and returns the TypeMirror for DataOwner .

Answer 2

If you are using KSP then the solution can look something like this:

Using the experimental APIs and assuming the KClass you reference in the annotation is available:

resolver.getSymbolsWithAnnotation("com.example.Model")
    .map {
        val consumerType = try {
            it.getAnnotationsByType(Model::class).single().consumer
            null
        } catch (e: KSTypeNotPresentException) {
            e.ksType
        }
        val consumerDeclaration = consumerType!!.declaration as KSClassDeclaration // etc
    }

If you can't do this, then maintaining a KSP view of the annotation is more flexible:

resolver.getSymbolsWithAnnotation("com.example.Model")
    .map { ksAnnotated ->
        val args = ksAnnotated.annotations.single {
            it.shortName.asString() == "Model" && it.annotationType.resolve().declaration.qualifiedName?.asString() == "com.example.Model"
        }.arguments

        val consumerType = args.single { it.name?.asString() == "owner" }.value as KSType
        val consumerDeclaration = consumerType.declaration as KSClassDeclaration // etc 
    }