Access to annotation value in Scala 3.0

Question

I created annotation in scala and used it as follows:

object Main extends App {
  println(classOf[Annotated].getAnnotations.length)

  import scala.reflect.runtime.universe._
  val mirror = runtimeMirror(cls.getClassLoader)

}


final class TestAnnotation extends StaticAnnotation

@TestAnnotation
class Annotated

As it's a Scala annotation it can not be read using getAnnotations on the other hand, scala-reflect dependency isn't available anymore for scala 3.0, so we have no access to runtimeMirror

Is there any alternative solution to read an annotation value in scala?

Answer 1

You don't need runtime reflection (Java or Scala) since information about annotations exists at compile time (even in Scala 2).

In Scala 3 you can write a macro and use TASTy reflection

import scala.quoted.*

inline def getAnnotations[A]: List[String] = ${getAnnotationsImpl[A]}

def getAnnotationsImpl[A: Type](using Quotes): Expr[List[String]] = {
  import quotes.reflect.*
  val annotations = TypeRepr.of[A].typeSymbol.annotations.map(_.tpe.show)
  Expr.ofList(annotations.map(Expr(_)))
}

Usage:

@main def test = println(getAnnotations[Annotated]) // List(TestAnnotation)

Tested in 3.0.0-RC2-bin-20210217-83cb8ff-NIGHTLY