Calling a method from Annotation using reflection

Question

I have Sample class with Size annotation

case class Sample(
  attr: SomeTypeA
  @Size(value =         


        

Answer 1

If you annotate like

@Size(50) name: String

rather than with meta-annotations like

@(Size @getter @setter @field)(50) name: String

then @Size remains only on constructor parameter, not field, getter or setter. So you need to use constructor of A.

Try

class Extractor[A](implicit
                   tt: TypeTag[A],
                   ct: ClassTag[A]
                  ) {
  val annotationTree = typeOf[A]
    .decl(termNames.CONSTRUCTOR).asMethod
    .paramLists.flatten
    .flatMap(_.annotations)
    .map(_.tree)
    .filter(_.tpe =:= typeOf[Size])
    .head 

  annotationTree match {
    case q"new $_($value)" => println(value) //50
  }
}

if you need the value or

import scala.tools.reflect.ToolBox
val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()

tb.eval(tb.untypecheck(annotationTree)).asInstanceOf[Size].getValue //50

if you really want to call getValue.

By the way, if you have access to Size and can make it a case class (or case-class-like) then you can do the same mostly at compile time

import shapeless.Annotations

Annotations[Size, Sample].apply() // None :: Some(Size(50)) :: HNil