scala

# annotation

#### package annotation

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Visibility

1. Public
2. All

### Type Members

1. #### abstract class Annotation extends AnyRef

   A base class for annotations.

   A base class for annotations. Annotations extending this class directly are not preserved for the Scala type checker and are also not stored as Java annotations in classfiles. To enable either or both of these, one needs to inherit from scala.annotation.StaticAnnotation or/and scala.annotation.ClassfileAnnotation.

   Version

     1.1, 2/02/2007

   Since

     2.4

2. #### trait ClassfileAnnotation extends Annotation with StaticAnnotation

   A base class for classfile annotations.

   A base class for classfile annotations. These are stored as Java annotations] in classfiles.

   Version

     1.1, 2/02/2007

   Since

     2.4

3. #### trait StaticAnnotation extends Annotation

   A base class for static annotations.

   A base class for static annotations. These are available to the Scala type checker, even across different compilation units.

   Version

     1.1, 2/02/2007

   Since

     2.4

4. #### trait TypeConstraint extends Annotation

   A marker for annotations that, when applied to a type, should be treated as a constraint on the annotated type.

   A marker for annotations that, when applied to a type, should be treated as a constraint on the annotated type.

   A proper constraint should restrict the type based only on information mentioned within the type. A Scala compiler can use this assumption to rewrite the contents of the constraint as necessary. To contrast, a type annotation whose meaning depends on the context where it is written down is not a proper constrained type, and this marker should not be applied. A Scala compiler will drop such annotations in cases where it would rewrite a type constraint.

   Version

     1.1, 2007-11-5

   Since

     2.6

5. #### final class compileTimeOnly extends Annotation with StaticAnnotation

   An annotation that designates that an annottee should not be referred to after type checking (which includes macro expansion).

   An annotation that designates that an annottee should not be referred to after type checking (which includes macro expansion).

   Examples of potential use: 1) The annottee can only appear in the arguments of some other macro that will eliminate it from the AST during expansion. 2) The annottee is a macro and should have been expanded away, so if hasn't, something wrong has happened. (Comes in handy to provide better support for new macro flavors, e.g. macro annotations, that can't be expanded by the vanilla compiler).

   Annotations
     @getter() @setter() @beanGetter() @beanSetter() @companionClass() @companionMethod()
   Since

     2.11.0

6. #### final class elidable extends Annotation with StaticAnnotation

   An annotation for methods whose bodies may be excluded from compiler-generated bytecode.

   An annotation for methods whose bodies may be excluded from compiler-generated bytecode.

   Behavior is influenced by passing `-Xelide-below <arg>` to `scalac`. Calls to methods marked elidable (as well as the method body) will be omitted from generated code if the priority given the annotation is lower than that given on the command line.

       @elidable(123)           // annotation priority
       scalac -Xelide-below 456 // command line priority

   The method call will be replaced with an expression which depends on the type of the elided expression. In decreasing order of precedence:

       Unit            ()
       Boolean         false
       T <: AnyVal     0
       T >: Null       null
       T >: Nothing    Predef.???

   Complete example:

       import scala.annotation._, elidable._
       object Test extends App {
         def expensiveComputation(): Int = { Thread.sleep(1000) ; 172 }

         @elidable(WARNING) def warning(msg: String) = println(msg)
         @elidable(FINE) def debug(msg: String)      = println(msg)
         @elidable(FINE) def computedValue           = expensiveComputation()

         warning("Warning! Danger! Warning!")
         debug("Debug! Danger! Debug!")
         println("I computed a value: " + computedValue)
       }
       % scalac example.scala && scala Test
       Warning! Danger! Warning!
       Debug! Danger! Debug!
       I computed a value: 172

       // INFO lies between WARNING and FINE
       % scalac -Xelide-below INFO example.scala && scala Test
       Warning! Danger! Warning!
       I computed a value: 0

   Since

     2.8

7. #### final class implicitNotFound extends Annotation with StaticAnnotation

   To customize the error message that's emitted when an implicit of type C[T1,..., TN] cannot be found, annotate the class C with @implicitNotFound.

   To customize the error message that's emitted when an implicit of type C[T1,..., TN] cannot be found, annotate the class C with @implicitNotFound. Assuming C has type parameters X1,..., XN, the error message will be the result of replacing all occurrences of ${Xi} in the string msg with the string representation of the corresponding type argument Ti. *

   Since

     2.8.1

8. #### class strictfp extends Annotation with StaticAnnotation

   If this annotation is present on a method or its enclosing class, the strictfp flag will be emitted.

   If this annotation is present on a method or its enclosing class, the strictfp flag will be emitted.

   Version

     2.9

   Since

     2.9

9. #### final class switch extends Annotation with StaticAnnotation

   An annotation to be applied to a match expression.

   An annotation to be applied to a match expression. If present, the compiler will verify that the match has been compiled to a tableswitch or lookupswitch and issue an error if it instead compiles into a series of conditional expressions. Example usage:

       val Constant = 'Q'
       def tokenMe(ch: Char) = (ch: @switch) match {
         case ' ' | '\t' | '\n'  => 1
         case 'A' | 'Z' | '$'    => 2
         case '5' | Constant     => 3  // a non-literal may prevent switch generation: this would not compile
         case _                  => 4
       }

   Since

     2.8

10. #### final class tailrec extends Annotation with StaticAnnotation

    A method annotation which verifies that the method will be compiled with tail call optimization.

    A method annotation which verifies that the method will be compiled with tail call optimization.

    If it is present, the compiler will issue an error if the method cannot be optimized into a loop.

    Since

      2.8

11. #### class unspecialized extends Annotation with StaticAnnotation

    A method annotation which suppresses the creation of additional specialized forms based on enclosing specialized type parameters.

    A method annotation which suppresses the creation of additional specialized forms based on enclosing specialized type parameters.

    Since

      2.10

12. #### final class varargs extends Annotation with StaticAnnotation

    A method annotation which instructs the compiler to generate a Java varargs-style forwarder method for interop.

    A method annotation which instructs the compiler to generate a Java varargs-style forwarder method for interop. This annotation can only be applied to methods with repeated parameters.

    Since

      2.9

### Value Members

1. #### object elidable

   This useless appearing code was necessary to allow people to use named constants for the elidable annotation.

   This useless appearing code was necessary to allow people to use named constants for the elidable annotation. This is what it takes to convince the compiler to fold the constants: otherwise when it's time to check an elision level it's staring at a tree like

       (Select(Level, Select(FINEST, Apply(intValue, Nil))))

   instead of the number `300`.

   Since

     2.8

2. #### package meta

   When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with `@BeanProperty`, a bean getter and a bean setter.

   When defining a field, the Scala compiler creates up to four accessors for it: a getter, a setter, and if the field is annotated with `@BeanProperty`, a bean getter and a bean setter.

   For instance in the following class definition

       class C(@myAnnot @BeanProperty var c: Int)

   there are six entities which can carry the annotation `@myAnnot`: the constructor parameter, the generated field and the four accessors.

   By default, annotations on (`val`-, `var`- or plain) constructor parameters end up on the parameter, not on any other entity. Annotations on fields by default only end up on the field.

   The meta-annotations in package `scala.annotation.meta` are used to control where annotations on fields and class parameters are copied. This is done by annotating either the annotation type or the annotation class with one or several of the meta-annotations in this package.

   #### Annotating the annotation type

   The target meta-annotations can be put on the annotation type when instantiating the annotation. In the following example, the annotation `@Id` will be added only to the bean getter `getX`.

       import javax.persistence.Id
       class A {
         @(Id @beanGetter) @BeanProperty val x = 0
       }

   In order to annotate the field as well, the meta-annotation `@field` would need to be added.

   The syntax can be improved using a type alias:

       object ScalaJPA {
         type Id = javax.persistence.Id @beanGetter
       }
       import ScalaJPA.Id
       class A {
         @Id @BeanProperty val x = 0
       }

   #### Annotating the annotation class

   For annotations defined in Scala, a default target can be specified in the annotation class itself, for example

       @getter
       class myAnnotation extends Annotation

   This only changes the default target for the annotation `myAnnotation`. When instantiating the annotation, the target can still be specified as described in the last section.

3. #### package unchecked

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