Scala example source code file (Annotations.scala)

This example Scala source code file (Annotations.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

annotation, arrayargument, collection, immutable, javaargument, list, literalargument, nestedargument, null, option, tree, use

The Annotations.scala Scala example source code

package scala
package reflect
package api

import scala.collection.immutable.ListMap

/**
 *  <span class="badge badge-red" style="float: right;">EXPERIMENTAL</span>
 *
 *  This trait provides annotation support for the reflection API.
 *
 *  In Scala, annotations belong to one of the two categories:
 *
 *  <ul>
 *  <li>''Java annotations'': annotations on definitions produced by the Java compiler, i.e., subtypes of [[java.lang.annotation.Annotation]]
 *  attached to program definitions. When read by Scala reflection, the [[scala.annotation.ClassfileAnnotation]] trait
 *  is automatically added as a subclass to every Java annotation.</li>
 *  <li>''Scala annotations'': annotations on definitions or types produced by the Scala compiler.</li>
 *  </ul>
 *
 *  When a Scala annotation that inherits from [[scala.annotation.StaticAnnotation]] or [[scala.annotation.ClassfileAnnotation]] is compiled,
 *  it is stored as special attributes in the corresponding classfile, and not as a Java annotation. Note that subclassing
 *  just [[scala.annotation.Annotation]] is not enough to have the corresponding metadata persisted for runtime reflection.
 *
 *  Both Java and Scala annotations are represented as typed trees carrying constructor invocations corresponding
 *  to the annotation. For instance, the annotation in `@ann(1, 2) class C` is represented as `q"@new ann(1, 2)"`.
 *
 *  Unlike Java reflection, Scala reflection does not support evaluation of constructor invocations stored in annotations
 *  into underlying objects. For instance it's impossible to go from `@ann(1, 2) class C` to `ann(1, 2)`, so one
 *  has to analyze trees representing annotation arguments to manually extract corresponding values. Towards that end,
 *  arguments of an annotation can be obtained via `annotation.tree.children.tail`.
 *
 *  For more information about `Annotation`s, see the [[http://docs.scala-lang.org/overviews/reflection/annotations-names-scopes.html Reflection Guide: Annotations, Names, Scopes, and More]]
 *
 *  @contentDiagram hideNodes "*Api"
 *  @group ReflectionAPI
 */
trait Annotations { self: Universe =>

  /** Information about an annotation.
   *  @template
   *  @group Annotations
   */
  type Annotation >: Null <: AnyRef with AnnotationApi

  /** The constructor/extractor for `Annotation` instances.
   *  @group Extractors
   */
  val Annotation: AnnotationExtractor

  /** An extractor class to create and pattern match with syntax `Annotation(tpe, scalaArgs, javaArgs)`.
   *  Here, `tpe` is the annotation type, `scalaArgs` the payload of Scala annotations, and `javaArgs` the payload of Java annotations.
    *  @group Extractors
    */
  abstract class AnnotationExtractor {
    def apply(tree: Tree): Annotation = treeToAnnotation(tree)

    @deprecated("Use `apply(tree: Tree): Annotation` instead", "2.11.0")
    def apply(tpe: Type, scalaArgs: List[Tree], javaArgs: ListMap[Name, JavaArgument]): Annotation

    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def unapply(ann: Annotation): Option[(Type, List[Tree], ListMap[Name, JavaArgument])]
  }

  /** The API of `Annotation` instances.
   *  The main source of information about annotations is the [[scala.reflect.api.Annotations]] page.
   *  @group API
   */
  trait AnnotationApi {
    /** The tree underlying the annotation. */
    def tree: Tree = annotationToTree(this.asInstanceOf[Annotation])

    /** The type of the annotation. */
    @deprecated("Use `tree.tpe` instead", "2.11.0")
    def tpe: Type

    /** Payload of the Scala annotation: a list of abstract syntax trees that represent the argument.
     *  Empty for Java annotations.
     */
    @deprecated("Use `tree.children.tail` instead", "2.11.0")
    def scalaArgs: List[Tree]

    /** Payload of the Java annotation: a list of name-value pairs.
     *  Empty for Scala annotations.
     */
    @deprecated("Use `tree.children.tail` instead", "2.11.0")
    def javaArgs: ListMap[Name, JavaArgument]
  }

  protected[scala] def annotationToTree(ann: Annotation): Tree
  protected[scala] def treeToAnnotation(tree: Tree): Annotation

  /** A Java annotation argument
   *  @template
   *  @group Annotations
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  type JavaArgument >: Null <: AnyRef with JavaArgumentApi

  /** Has no special methods. Is here to provides erased identity for `CompoundType`.
   *  @group API
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  trait JavaArgumentApi

  /** A literal argument to a Java annotation as `"Use X instead"` in `@Deprecated("Use X instead")`
   *  @template
   *  @group Annotations
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  type LiteralArgument >: Null <: LiteralArgumentApi with JavaArgument

  /** The constructor/extractor for `LiteralArgument` instances.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  val LiteralArgument: LiteralArgumentExtractor

  /** An extractor class to create and pattern match with syntax `LiteralArgument(value)`
   *  where `value` is the constant argument.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  abstract class LiteralArgumentExtractor {
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def apply(value: Constant): LiteralArgument
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def unapply(arg: LiteralArgument): Option[Constant]
  }

  /** The API of `LiteralArgument` instances.
   *  The main source of information about annotations is the [[scala.reflect.api.Annotations]] page.
   *  @group API
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  trait LiteralArgumentApi {
    /** The underlying compile-time constant value. */
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def value: Constant
  }

  /** An array argument to a Java annotation as in `@Target(value={TYPE,FIELD,METHOD,PARAMETER})`
   *  @template
   *  @group Annotations
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  type ArrayArgument >: Null <: ArrayArgumentApi with JavaArgument

  /** The constructor/extractor for `ArrayArgument` instances.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  val ArrayArgument: ArrayArgumentExtractor

  /** An extractor class to create and pattern match with syntax `ArrayArgument(args)`
   *  where `args` is the argument array.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  abstract class ArrayArgumentExtractor {
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def apply(args: Array[JavaArgument]): ArrayArgument
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def unapply(arg: ArrayArgument): Option[Array[JavaArgument]]
  }

  /** API of `ArrayArgument` instances.
   *  The main source of information about annotations is the [[scala.reflect.api.Annotations]] page.
   *  @group API
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  trait ArrayArgumentApi {
    /** The underlying array of Java annotation arguments. */
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def args: Array[JavaArgument]
  }

  /** A nested annotation argument to a Java annotation as `@Nested` in `@Outer(@Nested)`.
   *  @template
   *  @group Annotations
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  type NestedArgument >: Null <: NestedArgumentApi with JavaArgument

  /** The constructor/extractor for `NestedArgument` instances.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  val NestedArgument: NestedArgumentExtractor

  /** An extractor class to create and pattern match with syntax `NestedArgument(annotation)`
   *  where `annotation` is the nested annotation.
   *  @group Extractors
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  abstract class NestedArgumentExtractor {
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def apply(annotation: Annotation): NestedArgument
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def unapply(arg: NestedArgument): Option[Annotation]
  }

  /** API of `NestedArgument` instances.
   *  The main source of information about annotations is the [[scala.reflect.api.Annotations]] page.
   *  @group API
   */
  @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
  trait NestedArgumentApi {
    /** The underlying nested annotation. */
    @deprecated("Use `Annotation.tree` to inspect annotation arguments", "2.11.0")
    def annotation: Annotation
  }
}