Kotlin filter memberProperties by present annotation class

Question

I've got an extension method running on Any type. On that extension method (where this refers to the target instance), I'm trying to filter the memberProperties based on annotation presence.

this::class.memberProperties
        .filter{ it.annotations.map { ann -> ann.annotationClass }.contains(ValidComponent::class)}

But it.annotations is always of size 0

Example of variable declaration on the instance: @ValidComponent var x: SomeType = constructorParam.something or @ValidComponent lateinit var x: SomeType

score 0 · Accepted Answer · answered Jul 01 '18 at 05:09

This is an issue with what the annotations are applied to. If you had an annotation with a target of "property", your code would list them just fine:

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.PROPERTY)
annotation class ValidComponent

I assume your annotation has a "field" target instead, in which case you'll have to jump through Java reflection to list the annotations on your properties:

this::class.memberProperties
        .filter { property ->
            val fieldAnnotations = property.javaField?.annotations
            fieldAnnotations != null && fieldAnnotations.map { ann -> ann.annotationClass }.contains(ValidComponent::class)
        }

Kotlin filter memberProperties by present annotation class

1 Answers1