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:

    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(), 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

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(), 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 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[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  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[java.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
    @native() @throws( ... )

Inherited from Universe.ConstantApi

Inherited from AnyRef

Inherited from Any

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