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/Scala 2.13 Reflection

Trait scala.reflect.api.Mirrors

trait Mirrors extends AnyRef

EXPERIMENTAL

This trait provides support for Mirrors in the Scala Reflection API.

Mirrors are a central part of Scala Reflection. All information provided by reflection is made accessible through Mirrors. Depending on the type of information to be obtained, or the reflective action to be taken, different flavors of mirrors must be used. "Classloader" mirrors can be used to obtain representations of types and members. From a classloader Mirror, it's possible to obtain more specialized "invoker" Mirrors (the most commonly-used mirrors), which implement reflective invocations, such as method/constructor calls and field accesses.

The two flavors of mirrors:

  • “Classloader” mirrors. These mirrors translate names to symbols (via methods staticClass/staticModule/staticPackage).
  • "Invoker” mirrors. These mirrors implement reflective invocations (via methods MethodMirror.apply, FieldMirror.get, etc). These "invoker" mirrors are the types of mirrors that are most commonly used.
Compile-time Mirrors

Compile-time Mirrors make use of only classloader Mirrors to load Symbols by name.

The entry point to classloader Mirrors is via scala.reflect.macros.blackbox.Context#mirror or scala.reflect.macros.whitebox.Context#mirror. Typical methods which use classloader Mirrors include scala.reflect.api.Mirror#staticClass, scala.reflect.api.Mirror#staticModule, and scala.reflect.api.Mirror#staticPackage. For example:

import scala.reflect.macros.blackbox.Context

case class Location(filename: String, line: Int, column: Int)

object Macros {
  def currentLocation: Location = macro impl

  def impl(c: Context): c.Expr[Location] = {
    import c.universe._
    val pos = c.macroApplication.pos
    val clsLocation = c.mirror.staticModule("Location") // get symbol of "Location" object
    c.Expr(Apply(Ident(clsLocation), List(Literal(Constant(pos.source.path)), Literal(Constant(pos.line)), Literal(Constant(pos.column)))))
  }
}

Of Note: There are several high-level alternatives that one can use to avoid having to manually lookup symbols. For example, typeOf[Location.type].termSymbol (or typeOf[Location].typeSymbol if we needed a ClassSymbol), which are type safe since we don’t have to use Strings to lookup the Symbol.

Runtime Mirrors

Runtime Mirrors make use of both classloader and invoker Mirrors.

The entry point to Mirrors for use at runtime is via ru.runtimeMirror(<classloader>), where ru is scala.reflect.runtime.universe.

The result of a scala.reflect.api.JavaUniverse#runtimeMirror call is a classloader mirror, of type scala.reflect.api.Mirrors#ReflectiveMirror, which can load symbols by names as discussed above (in the “Compile-time” section).

A classloader mirror can create invoker mirrors, which include: scala.reflect.api.Mirrors#InstanceMirror, scala.reflect.api.Mirrors#MethodMirror, scala.reflect.api.Mirrors#FieldMirror, scala.reflect.api.Mirrors#ClassMirror and scala.reflect.api.Mirrors#ModuleMirror.

Examples of how these two types of Mirrors interact are available below.

Types of Mirrors, Their Use Cases & Examples

scala.reflect.api.Mirrors#ReflectiveMirror. Used for loading Symbols by name, and as an entry point into invoker mirrors. Entry point: val m = ru.runtimeMirror(<classloader>). Example:

scala> val ru = scala.reflect.runtime.universe
ru: scala.reflect.api.JavaUniverse = ...

scala> val m = ru.runtimeMirror(getClass.getClassLoader)
m: reflect.runtime.universe.Mirror = JavaMirror ...

scala.reflect.api.Mirrors#InstanceMirror. Used for creating invoker Mirrors for methods and fields and for inner classes and inner objects (modules). Entry point: val im = m.reflect(<value>). Example:

scala> class C { def x = 2 }
defined class C

scala> val im = m.reflect(new C)
im: reflect.runtime.universe.InstanceMirror = instance mirror for [email protected]

scala.reflect.api.Mirrors#MethodMirror. Used for invoking instance methods (Scala only has instance methods-- methods of objects are instance methods of object instances, obtainable via ModuleMirror.instance). Entry point: val mm = im.reflectMethod(<method symbol>). Example:

scala> val methodX = typeOf[C].decl(TermName("x")).asMethod
methodX: reflect.runtime.universe.MethodSymbol = method x

scala> val mm = im.reflectMethod(methodX)
mm: reflect.runtime.universe.MethodMirror = method mirror for C.x: scala.Int (bound to [email protected])

scala> mm()
res0: Any = 2

scala.reflect.api.Mirrors#FieldMirror. Used for getting/setting instance fields (Scala only has instance fields-- fields of objects are instance methods of object instances obtainable via ModuleMirror.instance). Entry point: val fm = im.reflectMethod(<field or accessor symbol>). Example:

scala> class C { val x = 2; val y = 3 }
defined class C

scala> val m = ru.runtimeMirror(getClass.getClassLoader)
m: reflect.runtime.universe.Mirror = JavaMirror ...

scala> val im = m.reflect(new C)
im: reflect.runtime.universe.InstanceMirror = instance mirror for [email protected]

scala> val fieldX = typeOf[C].decl(TermName("x")).asTerm.accessed.asTerm
fieldX: reflect.runtime.universe.TermSymbol = value x
scala> val fmX = im.reflectField(fieldX)
fmX: reflect.runtime.universe.FieldMirror = field mirror for C.x (bound to [email protected])

scala> fmX.get
res0: Any = 2

scala> fmX.set(3) // NOTE: can set an underlying value of an immutable field!

scala> val fieldY = typeOf[C].decl(TermName("y")).asTerm.accessed.asTerm
fieldY: reflect.runtime.universe.TermSymbol = variable y

scala> val fmY = im.reflectField(fieldY)
fmY: reflect.runtime.universe.FieldMirror = field mirror for C.y (bound to [email protected])

scala> fmY.get
res1: Any = 3

scala> fmY.set(4)

scala> fmY.get
res2: Any = 4

scala.reflect.api.Mirrors#ClassMirror. Used for creating invoker mirrors for constructors. Entry points: for static classes val cm1 = m.reflectClass(<class symbol>), for inner classes val mm2 = im.reflectClass(<class symbol>). Example:

scala> case class C(x: Int)
defined class C

scala> val m = ru.runtimeMirror(getClass.getClassLoader)
m: reflect.runtime.universe.Mirror = JavaMirror ...

scala> val classC = typeOf[C].typeSymbol.asClass

classC: reflect.runtime.universe.Symbol = class C

scala> val cm = m.reflectClass(classC)
cm: reflect.runtime.universe.ClassMirror = class mirror for C (bound to null)

scala> val ctorC = typeOf[C].decl(ru.nme.CONSTRUCTOR).asMethod
ctorC: reflect.runtime.universe.MethodSymbol = constructor C

scala> val ctorm = cm.reflectConstructor(ctorC)
ctorm: reflect.runtime.universe.MethodMirror = constructor mirror for C.<init>(x: scala.Int): C (bound to null)

scala> ctorm(2)
res0: Any = C(2)

scala.reflect.api.Mirrors#ModuleMirror. Used for getting singleton instances of objects. Entry points: for static objects (modules) val mm1 = m.reflectModule(<module symbol>), for inner objects (modules) val mm2 = im.reflectModule(<module symbol>). Example:

scala> object C { def x = 2 }
defined module C

scala> val m = ru.runtimeMirror(getClass.getClassLoader)
m: reflect.runtime.universe.Mirror = JavaMirror ...

scala> val objectC = typeOf[C.type].termSymbol.asModule
objectC: reflect.runtime.universe.ModuleSymbol = object C

scala> val mm = m.reflectModule(objectC)
mm: reflect.runtime.universe.ModuleMirror = module mirror for C (bound to null)

scala> val obj = mm.instance
obj: Any = [email protected]

For more information about Mirrorss, see the Reflection Guide: Mirrors

Self Type
Universe
Source
Mirrors.scala
Linear Supertypes
AnyRef, Any
Known Subclasses
JavaUniverse, Universe, Universe

Type Members

trait ClassMirror extends Universe.TemplateMirror

A mirror that reflects the instance parts of a runtime class. See the overview page for details on how to use runtime reflection.

trait FieldMirror extends AnyRef

A mirror that reflects a field. See the overview page for details on how to use runtime reflection.

trait InstanceMirror extends AnyRef

A mirror that reflects a runtime value. See the overview page for details on how to use runtime reflection.

trait MethodMirror extends AnyRef

A mirror that reflects a method. See the overview page for details on how to use runtime reflection.

abstract type Mirror >: Null <: api.Mirror[Mirrors.this.type]

The base type of all mirrors of this universe.

This abstract type conforms the base interface for all mirrors defined in scala.reflect.api.Mirror and is gradually refined in specific universes (e.g. Mirror of a scala.reflect.api.JavaUniverse is capable of reflection).

trait ModuleMirror extends Universe.TemplateMirror

A mirror that reflects a Scala object definition or the static parts of a runtime class. See the overview page for details on how to use runtime reflection.

trait ReflectiveMirror extends api.Mirror[Mirrors.this.type]

A mirror that reflects instances and static classes. See the overview page for details on how to use runtime reflection.

abstract type RuntimeClass >: Null <: AnyRef

trait RuntimeClassApi extends AnyRef

Has no special methods. Is here to provides erased identity for RuntimeClass.

trait RuntimeMirror extends api.Mirror[Mirrors.this.type] with Universe.ReflectiveMirror

The API of a mirror for a reflective universe. See the overview page for details on how to use runtime reflection.

trait TemplateMirror extends AnyRef

A mirror that reflects the instance or static parts of a runtime class. See the overview page for details on how to use runtime reflection.

Abstract Value Members

abstract val rootMirror: Universe.Mirror

The root mirror of this universe. This mirror contains standard Scala classes and types such as Any, AnyRef, AnyVal, Nothing, Null, and all classes loaded from scala-library, which are shared across all mirrors within the enclosing universe.

Concrete Value Members

final def !=(arg0: Any): Boolean

Definition Classes
AnyRef → Any

final def ##(): Int

Definition Classes
AnyRef → Any

def +(other: String): String

Implicit
This member is added by an implicit conversion from Mirrors toany2stringadd[Mirrors] performed by method any2stringadd in scala.Predef.
Definition Classes
any2stringadd

def ->[B](y: B): (Mirrors, B)

Implicit
This member is added by an implicit conversion from Mirrors toArrowAssoc[Mirrors] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc
Annotations
@inline()

final def ==(arg0: Any): Boolean

Definition Classes
AnyRef → Any

final def asInstanceOf[T0]: T0

Definition Classes
Any

def clone(): AnyRef

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.CloneNotSupportedException]) @native()

def ensuring(cond: (Mirrors) => Boolean, msg: => Any): Mirrors

Implicit
This member is added by an implicit conversion from Mirrors toEnsuring[Mirrors] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring

def ensuring(cond: (Mirrors) => Boolean): Mirrors

Implicit
This member is added by an implicit conversion from Mirrors toEnsuring[Mirrors] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring

def ensuring(cond: Boolean, msg: => Any): Mirrors

Implicit
This member is added by an implicit conversion from Mirrors toEnsuring[Mirrors] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring

def ensuring(cond: Boolean): Mirrors

Implicit
This member is added by an implicit conversion from Mirrors toEnsuring[Mirrors] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring

final def eq(arg0: AnyRef): Boolean

Definition Classes
AnyRef

def equals(arg0: AnyRef): Boolean

Definition Classes
AnyRef → Any

def finalize(): Unit

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.Throwable])

def formatted(fmtstr: String): String

Implicit
This member is added by an implicit conversion from Mirrors toStringFormat[Mirrors] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
@inline()

final def getClass(): Class[_ <: AnyRef]

Definition Classes
AnyRef → Any
Annotations
@native()

def hashCode(): Int

Definition Classes
AnyRef → Any
Annotations
@native()

final def isInstanceOf[T0]: Boolean

Definition Classes
Any

final def ne(arg0: AnyRef): Boolean

Definition Classes
AnyRef

final def notify(): Unit

Definition Classes
AnyRef
Annotations
@native()

final def notifyAll(): Unit

Definition Classes
AnyRef
Annotations
@native()

final def synchronized[T0](arg0: => T0): T0

Definition Classes
AnyRef

def toString(): String

Definition Classes
AnyRef → Any

final def wait(): Unit

Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException])

final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException])

final def wait(arg0: Long): Unit

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

© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala-reflect/scala/reflect/api/Mirrors.html