Packages

•  package root

  Definition Classes

  root

•  package scala

  Definition Classes

  root

•  package reflect

  Definition Classes

  scala

•  package api

  EXPERIMENTAL

  EXPERIMENTAL

  The Scala Reflection API (located in scala-reflect.jar).

  In Scala 2.10.0, the Scala Reflection API and its implementation have an "experimental" status. This means that the API and the docs are not complete and can be changed in binary- and source-incompatible manner in 2.10.1. This also means that the implementation has some known issues.

  The following types are the backbone of the Scala Reflection API, and serve as a good starting point for information about Scala Reflection:

  • scala.reflect.api.Symbols
  • scala.reflect.api.Types
  • scala.reflect.api.Mirrors
  • scala.reflect.api.Universe

  For more information about Scala Reflection, see the Reflection Guide

  Definition Classes

  reflect

•  trait Constants extends AnyRef

  EXPERIMENTAL

  EXPERIMENTAL

  According to the section 6.24 "Constant Expressions" of the Scala language specification, certain expressions (dubbed constant expressions) can be evaluated by the Scala compiler at compile-time. Examples include "true", "0" and "classOf[List]".

  Constant instances can be matched against and can be constructed directly, as if they were case classes:

  assert(Constant(true).value == true)
  Constant(true) match {
    case Constant(s: String) =>  println("A string: " + s)
    case Constant(b: Boolean) => println("A boolean value: " + b)
    case Constant(x) =>          println("Something else: " + x)
  }

  Constant instances can wrap the following kinds of expressions:

  1. Literals of primitive value classes (Byte, Short, Int, Long, Float, Double, Char, Boolean and Unit) - represented directly as the corresponding type
  2. String literals - represented as instances of String.
  3. References to classes, typically constructed with scala.Predef#classOf - represented as types.
  4. References to enumeration values - represented as symbols.

  Instances are used to represent literals in abstract syntax trees, inside scala.reflect.api.Trees#Literal nodes.

  Example

  The value field deserves some explanation. Primitive and string values are represented as themselves, whereas references to classes and enums are a bit roundabout.

  Class references are represented as instances of scala.reflect.api.Types#Type (because when the Scala compiler processes a class reference, the underlying runtime class might not yet have been compiled). To convert such a reference to a runtime class, one should use the runtimeClass method of a mirror such as scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is using scala.reflect.runtime.package#currentMirror).

  Enumeration value references are represented as instances of scala.reflect.api.Symbols#Symbol, which on JVM point to methods that return underlying enum values. To inspect an underlying enumeration or to get runtime value of a reference to an enum, one should use a scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is again scala.reflect.runtime.package#currentMirror).

  // File "JavaSimpleEnumeration.java"
  enum JavaSimpleEnumeration { FOO, BAR }
  
  // File "JavaSimpleAnnotation.java"
  import java.lang.annotation.*;
  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.TYPE})
  public @interface JavaSimpleAnnotation {
    Class<?> classRef();
    JavaSimpleEnumeration enumRef();
  }
  
  // File "JavaAnnottee.java"
  @JavaSimpleAnnotation(
    classRef = JavaAnnottee.class,
    enumRef = JavaSimpleEnumeration.BAR
  )
  public class JavaAnnottee {}

  val javaArgs = typeOf[JavaAnnottee].typeSymbol.annotations(0).tree.children.tail
  
  def jArg[A](lhs: String): Option[A] = javaArgs
    .map { case NamedArg(lhs, Literal(const)) => (lhs.toString, const) }
    .find(_._1 == lhs)
    .map(_._2.value.asInstanceOf[A])
  
  // class reference, cast to Type
  val classRef = jArg[Type]("classRef").get
  println(showRaw(classRef))             // TypeRef(ThisType(<empty>), JavaAnnottee, List())
  println(cm.runtimeClass(classRef))     // class JavaAnnottee
  // enum value reference, cast to Symbol
  val enumRef = jArg[Symbol]("enumRef").get
  println(enumRef)                       // value BAR
  
  val siblings = enumRef.owner.info.decls
  val enumValues = siblings.filter(_.isJavaEnum)
  println(enumValues)                    // Scope{
                                         //   final val FOO: JavaSimpleEnumeration;
                                         //   final val BAR: JavaSimpleEnumeration
                                         // }
  
  // doesn't work because of https://github.com/scala/bug/issues/6459
  // val enumValue = mirror.reflectField(enumRef.asTerm).get
  val enumClass = cm.runtimeClass(enumRef.owner.asClass)
  val enumValue = enumClass.getDeclaredField(enumRef.name.toString).get(null)
  println(enumValue)                     // BAR

  Definition Classes

  api

• Constant
• ConstantApi
• ConstantExtractor

scala.reflect.api.Constants

ConstantExtractor