abstract type Constant >: Null <: Universe.ConstantApi
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:
- Literals of primitive value classes (
Byte
,Short
,Int
,Long
,Float
,Double
,Char
,Boolean
andUnit
) - represented directly as the corresponding type - String 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 case _ => sys.error("Not a constant") } val classRef = jarg("classRef").value.asInstanceOf[Type] // ideally one should match instead of casting println(showRaw(classRef)) // TypeRef(ThisType(), 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 }
- Source
- Constants.scala
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abstract
def
tpe: Universe.Type
Scala type that describes the constant.
Scala type that describes the constant. It is generated automatically based on the type of the value.
- Definition Classes
- ConstantApi
-
abstract
val
value: Any
Payload of the constant, that can be accessed directly or pattern matched against.
Payload of the constant, that can be accessed directly or pattern matched against.
- Definition Classes
- ConstantApi
Concrete Value Members
-
final
def
!=(arg0: Any): Boolean
- Definition Classes
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-
final
def
##(): Int
- Definition Classes
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final
def
==(arg0: Any): Boolean
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final
def
asInstanceOf[T0]: T0
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def
clone(): AnyRef
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- protected[java.lang]
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final
def
eq(arg0: AnyRef): Boolean
- Definition Classes
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def
equals(arg0: Any): Boolean
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-
def
finalize(): Unit
- Attributes
- protected[java.lang]
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- @throws( classOf[java.lang.Throwable] )
-
final
def
getClass(): Class[_]
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def
hashCode(): Int
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final
def
isInstanceOf[T0]: Boolean
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final
def
ne(arg0: AnyRef): Boolean
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final
def
notify(): Unit
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final
def
notifyAll(): Unit
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final
def
synchronized[T0](arg0: ⇒ T0): T0
- Definition Classes
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def
toString(): String
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final
def
wait(): Unit
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final
def
wait(arg0: Long, arg1: Int): Unit
- Definition Classes
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- @throws( ... )
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final
def
wait(arg0: Long): Unit
- Definition Classes
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