Accessing an Annotation Value in Scala

Question

TL;DR: Basically, I\'m looking for the Scala equivalent of the Java:

(MyAnnotation) Thing.getClass().getAnnotations()[0]

D

Answer 1

Without having to depend on scala-compiler, this is my version:

  def fetchAnnotations[T <: Annotation](cls : Class[_]) : List[T]= {
    import scala.reflect.runtime.universe._
    val mirror = runtimeMirror(cls.getClassLoader)
    val clsSymbol = mirror.staticClass(cls.getCanonicalName)
    val annotations = clsSymbol.annotations

    val res = ListBuffer[T]()
    for(annt : Annotation <- annotations) {
      val anntCls = annt.tree.tpe.typeSymbol.asClass
      val classMirror = mirror.reflectClass(anntCls);
      val anntType = annt.tree.tpe
      val constructor = anntType.decl(termNames.CONSTRUCTOR).asMethod;
      val constructorMirror = classMirror.reflectConstructor(constructor);

      val instance = annt.tree match {
        case Apply(c, args : List[Tree])   =>
          val res = args.collect({
            case i: Tree =>
              i match {
                case Literal(Constant(value)) =>
                  value
              }
          })
          constructorMirror(res: _*).asInstanceOf[T]
      }

      res+=(instance)
    }
    res.toList
  }

The previous code will only work when the arguments are primitive I suspect.

If we can afford depending on scala-compiler, then we can do something like:

def fetchAnnotations_toolBox[T <: Annotation](cls : Class[_]) : List[T]= {
    import scala.reflect.runtime.universe._
    val mirror = runtimeMirror(cls.getClassLoader)
    val clsSymbol = mirror.staticClass(cls.getCanonicalName)
    val annotations = clsSymbol.annotations

    val res = ListBuffer[T]()
    for(annt : Annotation <- annotations) {
      import scala.tools.reflect.ToolBox

      val toolbox = mirror.mkToolBox()
      val instance = toolbox.eval(toolbox.untypecheck(toolbox.typecheck(annt.tree))).asInstanceOf[T]
      res+=(instance)
    }
    res.toList
  }