This "virtual" case class represents the reflection interface for literal expressions which can not be further broken down or evaluated, such as "true", "0", "classOf[List]". Such values become parts of the Scala abstract syntax tree representing the program. The constants correspond to section 6.24 "Constant Expressions" of the Scala Language Specification.
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).
Constants 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 certain kinds of these expressions:
Byte
, Short
, Int
, Long
, Float
, Double
, Char
, Boolean
and Unit
) - represented directly as the corresponding typeString literals - represented as instances of the String
.References to classes, typically constructed with scala.Predef#classOf - represented as types.References to enumeration values - represented as symbols. 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 RuntimeMirror
(the simplest way to get such a mirror is using scala.reflect.runtime.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).
Usage example:
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(TermName(name)) match { // Constant is always wrapped into a Literal or LiteralArgument tree node case LiteralArgument(ct: Constant) => value } val classRef = jarg("classRef").value.asInstanceOf[Type] // ideally one should match instead of casting println(showRaw(classRef)) // TypeRef(ThisType(<empty>), JavaAnnottee, List()) println(cm.runtimeClass(classRef)) // class JavaAnnottee val enumRef = jarg("enumRef").value.asInstanceOf[Symbol] // ideally one should match instead of casting println(enumRef) // value BAR val siblings = enumRef.owner.info.decls 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://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 }
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Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala-reflect/scala/reflect/api/Constants.html
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 namedbooleanValue
,intValue
etc.), namely: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).