trait Constants extends AnyRef
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:
- Literals of primitive value classes (bytes, shorts, ints, longs, floats, doubles, chars, booleans and voids).
- String literals.
- References to classes (typically constructed with scala.Predef#classOf).
- 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://issues.scala-lang.org/browse/SI-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 }
- Self Type
- Universe
- Source
- Constants.scala
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Type Members
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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]".
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 asRuntimeMirror
(the simplest way to get such a mirror is usingscala.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://issues.scala-lang.org/browse/SI-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 } - Literals of primitive value classes (
-
abstract
class
ConstantApi
extends AnyRef
The API of Constant instances.
-
abstract
class
ConstantExtractor
extends AnyRef
An extractor class to create and pattern match with syntax
Constant(value)
wherevalue
is the Scala value of the constant.
Abstract Value Members
-
abstract
val
Constant: Universe.ConstantExtractor
The constructor/extractor for
Constant
instances.
Concrete Value Members
-
final
def
!=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
- returns
true
if !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
-
final
def
##(): Int
Equivalent to
x.hashCode
except for boxed numeric types andnull
.Equivalent to
x.hashCode
except for boxed numeric types andnull
. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. Fornull
returns a hashcode wherenull.hashCode
throws aNullPointerException
.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- def +(other: String): String
- def ->[B](y: B): (Constants, B)
-
final
def
==(arg0: Any): Boolean
The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.- returns
true
if the receiver object is equivalent to the argument;false
otherwise.
- Definition Classes
- AnyRef → Any
-
final
def
asInstanceOf[T0]: T0
Cast the receiver object to be of type
T0
.Cast the receiver object to be of type
T0
.Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression
1.asInstanceOf[String]
will throw aClassCastException
at runtime, while the expressionList(1).asInstanceOf[List[String]]
will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.- returns
the receiver object.
- Definition Classes
- Any
- Exceptions thrown
ClassCastException
if the receiver object is not an instance of the erasure of typeT0
.
-
def
clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the
clone
method is platform dependent.- returns
a copy of the receiver object.
- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
- Note
not specified by SLS as a member of AnyRef
- def ensuring(cond: (Constants) ⇒ Boolean, msg: ⇒ Any): Constants
- def ensuring(cond: (Constants) ⇒ Boolean): Constants
- def ensuring(cond: Boolean, msg: ⇒ Any): Constants
- def ensuring(cond: Boolean): Constants
-
final
def
eq(arg0: AnyRef): Boolean
Tests whether the argument (
that
) is a reference to the receiver object (this
).Tests whether the argument (
that
) is a reference to the receiver object (this
).The
eq
method implements an equivalence relation on non-null instances ofAnyRef
, and has three additional properties:- It is consistent: for any non-null instances
x
andy
of typeAnyRef
, multiple invocations ofx.eq(y)
consistently returnstrue
or consistently returnsfalse
. - For any non-null instance
x
of typeAnyRef
,x.eq(null)
andnull.eq(x)
returnsfalse
. null.eq(null)
returnstrue
.
When overriding the
equals
orhashCode
methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2
), they should be equal to each other (o1 == o2
) and they should hash to the same value (o1.hashCode == o2.hashCode
).- returns
true
if the argument is a reference to the receiver object;false
otherwise.
- Definition Classes
- AnyRef
- It is consistent: for any non-null instances
-
def
equals(arg0: Any): Boolean
The equality method for reference types.
-
def
finalize(): Unit
Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the
finalize
method is invoked, as well as the interaction betweenfinalize
and non-local returns and exceptions, are all platform dependent.- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( classOf[java.lang.Throwable] )
- Note
not specified by SLS as a member of AnyRef
- def formatted(fmtstr: String): String
-
final
def
getClass(): Class[_]
Returns the runtime class representation of the object.
Returns the runtime class representation of the object.
- returns
a class object corresponding to the runtime type of the receiver.
- Definition Classes
- AnyRef → Any
-
def
hashCode(): Int
The hashCode method for reference types.
-
final
def
isInstanceOf[T0]: Boolean
Test whether the dynamic type of the receiver object is
T0
.Test whether the dynamic type of the receiver object is
T0
.Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression
1.isInstanceOf[String]
will returnfalse
, while the expressionList(1).isInstanceOf[List[String]]
will returntrue
. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.- returns
true
if the receiver object is an instance of erasure of typeT0
;false
otherwise.
- Definition Classes
- Any
-
final
def
ne(arg0: AnyRef): Boolean
Equivalent to
!(this eq that)
.Equivalent to
!(this eq that)
.- returns
true
if the argument is not a reference to the receiver object;false
otherwise.
- Definition Classes
- AnyRef
-
final
def
notify(): Unit
Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.
- Definition Classes
- AnyRef
- Note
not specified by SLS as a member of AnyRef
-
final
def
notifyAll(): Unit
Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.
- Definition Classes
- AnyRef
- Note
not specified by SLS as a member of AnyRef
-
final
def
synchronized[T0](arg0: ⇒ T0): T0
- Definition Classes
- AnyRef
-
def
toString(): String
Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
- returns
a String representation of the object.
- Definition Classes
- AnyRef → Any
-
final
def
wait(): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long, arg1: Int): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
- def →[B](y: B): (Constants, B)
Constants
API
The methods available for each reflection entity, without the implementation. Since the reflection entities are later overridden by runtime reflection and macros, their API counterparts guarantee a minimum set of methods that are implemented.
Extractors
Extractors provide the machinery necessary to allow pattern matching and construction of reflection entities that is similar to case classes, although the entities are only abstract types that are later overridden.