How to create annotations and get them in scala

Question

I want to define some annotations and use them in Scala.

I looked into the source of Scala, found in scala.annotation package, there are some annotation

Answer 1

With scala 2.11.6, this works to extract values of a annotation:

case class MyAnnotationClass(id: String) extends scala.annotation.StaticAnnotation

val myAnnotatedClass: ClassSymbol = u.runtimeMirror(Thread.currentThread().getContextClassLoader).staticClass("MyAnnotatedClass")
val annotation: Option[Annotation] = myAnnotatedClass.annotations.find(_.tree.tpe =:= u.typeOf[MyAnnotationClass])
val result = annotation.flatMap { a =>
  a.tree.children.tail.collect({ case Literal(Constant(id: String)) => doSomething(id) }).headOption
}

Answer 2

Could it have something to do with retention? I bet @tailrec is not included in the bytecode getting generated.

If I try to extend ClassfileAnnotation (in order to have runtime retention), Scala tells me that it can't be done, and it has to be done in Java:

./test.scala:1: warning: implementation restriction: subclassing Classfile does not
make your annotation visible at runtime.  If that is what
you want, you must write the annotation class in Java.
class A extends ClassfileAnnotation
      ^

Answer 3

You can find a nice description of how annotations are to be used in Scala in Programming Scala.

So you can define or use annotations in scala. However there is at least one limitation:

Runtime retention is not quite possible. In theory you should subclass ClassFileAnnotation to achieve this, but currently scalac reports the following warning if you do it:

"implementation restriction: subclassing Classfile does not make your annotation visible at runtime. If that is what you want, you must write the annotation class in Java."

It also means that your code is fine as it is (at least as fine as it is currently possible in Scala), but the annotation is on the class only during compile time. So you could use it e.g. in compiler plugins, but you will not be able to access it runtime.

Answer 4

FWIW, you can now define scala annotation in scala 2.10 and use reflection to read them back.

Here are some examples: Reflecting Annotations in Scala 2.10

Answer 5

I think you can only define annotations in Java now.

http://www.scala-lang.org/node/106