Hi Folks, i currently struggle with Kotlin Annotat...

# reflect

[Timo Gruen]

Hi Folks,
i currently struggle with Kotlin Annotations and the Annotation Processor. I brought up a custom Annotation Property

@Target(AnnotationTarget.PROPERTY, AnnotationTarget.PROPERTY_GETTER)
@Retention(AnnotationRetention.RUNTIME)
annotation class Property

When using the annotation on a property e.g.:

data class Example(@Property val key: String)

and introspecting the class with a AnnotationProcessor i can find three different elements which are related to that key property.
The first one is key , the second one is getKey and the third (which is currently probably the reason for my confusion) is the getKey$annotations() one, which is the only element having the annotation from java annotation processor perspective.

I would have assumed that either key or getKey would also be annotated on byte-code level. Any guidance on how i can access the element getKey OR key using the annotation processor api?

My actual goal is to retrieve the name key as well as the type String of the annotated element

[Timo Gruen]

tl;tr:

Have found the issue (but found another one).
If the @target is only AnnotationTarget.Field it just works fine - but then i’m having issues with the Kotlin Reflection not having the annotation at runtime. (Even if the AnnotationRetention is Runtime )

Any advice here?

[Marc Knaup]

It’s safer to use Kotlin Metadata to inspect Kotlin types as it contains more information.

Check out my library https://github.com/fluidsonic/fluid-meta which helps with that, using Meta.of(yourElement).

Here I use it in my JSON annotation processor:
https://github.com/fluidsonic/fluid-json/blob/accd358687a1247785366625341eeb7ca6be0b99/annotation-processor/sources/jvm-jdk8/collection/CollectionPhase.kt?ts=4#L61

[Marc Knaup]

Using Metadata you can make such links:

[Timo Gruen]

Actually pretty nice library. Thanks @Marc Knaup - gonna give it a try for some other modules I have to write in the next few days

But for the fix I rather used a mapping for all xy$annotations() to xy . Enables me to be compliant with java and kotlin and not just kotlin.

[Marc Knaup]

What do you mean by “with Java”?
There is no $annotations hack in Java 🤔

[Timo Gruen]

That’s true actually - but currently i’m using the same AnnotationProcessor for both - java and kotlin. The $annotations are only present if it’s processing a kotlin file but still, i can use the entire code for both (as long as i’m sanitizing that $annotations thingy when approaching kotlin).

[Timo Gruen]

btw. you got any reference what that $annotations? actually is? Haven’t found any documentation

[Marc Knaup]

If I recall correctly it’s because Java doesn’t have properties.

Kotlin puts field annotation on the JVM field.
Kotlin puts getter annotation on the JVM getter.
Kotlin puts property annotation on a synthetic JVM getter.

[Timo Gruen]

Oh ok. So it’s just synthetic to have a dedicated method to put the property annotations to instead of putting them directly on the field (backed by the getter or similar).

Thanks!