Constants

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.

scala.reflect.api.Constants#Constant instances represent certain kinds of these expressions (with values stored in the value field and its strongly-typed views named booleanValue, intValue etc.), namely:

1. Literals of primitive value classes (bytes, shorts, ints, longs, floats, doubles, chars, booleans and voids).
2. String literals.
3. References to classes (typically constructed with scala.Predef#classOf).
4. References to enumeration values.

Such constants are used to represent literals in abstract syntax trees (the scala.reflect.api.Trees#Literal node) and literal arguments for Java class file annotations (the scala.reflect.api.Annotations#LiteralArgument class).

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).

enum JavaSimpleEnumeration { FOO, BAR }

import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface JavaSimpleAnnotation {
  Class<?> classRef();
  JavaSimpleEnumeration enumRef();
}

@JavaSimpleAnnotation(
  classRef = JavaAnnottee.class,
  enumRef = JavaSimpleEnumeration.BAR
)
public class JavaAnnottee {}

import scala.reflect.runtime.universe._
import scala.reflect.runtime.{currentMirror => cm}

object Test extends App {
  val jann = typeOf[JavaAnnottee].typeSymbol.annotations(0).javaArgs
  def jarg(name: String) = jann(newTermName(name)).asInstanceOf[LiteralArgument].value

  val classRef = jarg("classRef").typeValue
  println(showRaw(classRef))             // TypeRef(ThisType(), JavaAnnottee, List())
  println(cm.runtimeClass(classRef))     // class JavaAnnottee

  val enumRef = jarg("enumRef").symbolValue
  println(enumRef)                       // value BAR

  val siblings = enumRef.owner.typeSignature.declarations
  val enumValues = siblings.filter(sym => sym.isVal && sym.isPublic)
  println(enumValues)                    // Scope{
                                         //   final val FOO: JavaSimpleEnumeration;
                                         //   final val BAR: JavaSimpleEnumeration
                                         // }

  // doesn't work because of https://issues.scala-lang.org/browse/SI-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
}