Is there a way to force using annotation only on a var property

Question

I'm making an annotation processor for encrypting personal data due to law. So in order to encrypt those field it should be able to set new value so I want my annotation to force using only on a var property.

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.PROPERTY, AnnotationTarget.FIELD, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
annotation class PersonalData {
}

data class CreateAccountCommand(
  @PersonalData
  var fullname: String // it would throw an error if using val since it's not mutable
)

class TestAnnotation {
  @Test
  fun testFindAnnotation() {
    val message = EasyRandom().nextObject(CreateAccountCommand::class.java)
    val wrappedCommand = GenericCommandMessage.asCommandMessage<CreateAccountCommand>(message)
    if (isPersonalDataPresent(wrappedCommand.payload)) {
      encryptData(wrappedCommand.payload)
      wrappedCommand.payload.fullname shouldContain "-encrypted"
    }
  }

  private fun isPersonalDataPresent(payload: Any): Boolean {
    return payload::class.memberProperties.any {
      it.annotations.any { ann -> ann.annotationClass == PersonalData::class }
    }
  }

  private fun encryptData(payload: Any) {
    payload::class.memberProperties.map {
      if (it is KMutableProperty<*>) {
        it.setter.call(payload, "${it.getter.call(payload)}-encrypted")
      }
    }
  }
}

or any better way?

Answer 1

If you only include PROPERTY_SETTER in the annotation targets, the annotation can only be applied to property setters, which is effectively what you want - since a property setter implies a mutable property.

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.PROPERTY_SETTER)
annotation class PersonalData

When using this annotation, you'd usually have to specify the use site target:

data class CreateAccountCommand(
    @set:PersonalData
    var fullname: String
)

Changing it to val would give you an error, just like you expect, because a val does not have a setter.

isPersonalDataPresent also needs a little modification, to check whether the setter has the annotation:

return payload::class.memberProperties
    .filterIsInstance<KMutableProperty<*>>()
    .any {
        it.setter.annotations.any { ann ->
            ann.annotationClass == PersonalData::class
        }
    }