Scala Reflection Library2.12.21 < Back
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# 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.

  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(TermName(name)).asInstanceOf[LiteralArgument].value

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

        val enumRef = jarg("enumRef").symbolValue
        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
      }

  Definition Classes
    api
- Constant
- ConstantApi
- ConstantExtractor

a

scala.reflect.api.Constants

# Constant 

###
Companion class ConstantExtractor

#### 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:

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 the `String`.
3. References to classes, typically constructed with scala.Predef#classOf - represented as types.
4. 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(<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
    }

Source
  Constants.scala

Linear Supertypes
Universe.ConstantApi, AnyRef, Any
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### Abstract Value Members

1.  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
2.  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

1.  final def !=(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
2.  final def ##(): Int
   Definition Classes
     AnyRef → Any
3.  final def ==(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
4.  final def asInstanceOf[T0]: T0
   Definition Classes
     Any
5.  def clone(): AnyRef
   Attributes
     protected[lang]
   Definition Classes
     AnyRef
   Annotations
     @throws( ... ) @native()
6.  final def eq(arg0: AnyRef): Boolean
   Definition Classes
     AnyRef
7.  def equals(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
8.  def finalize(): Unit
   Attributes
     protected[lang]
   Definition Classes
     AnyRef
   Annotations
     @throws( classOf[java.lang.Throwable] )
9.  final def getClass(): Class[_]
   Definition Classes
     AnyRef → Any
   Annotations
     @native()
10.  def hashCode(): Int
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
11.  final def isInstanceOf[T0]: Boolean
    Definition Classes
      Any
12.  final def ne(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
13.  final def notify(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
14.  final def notifyAll(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
15.  final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
      AnyRef
16.  def toString(): String
    Definition Classes
      AnyRef → Any
17.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... )
18.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... )
19.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... ) @native()

### Inherited from Universe.ConstantApi

### Inherited from AnyRef

### Inherited from Any

### Ungrouped