Annotations
Definition
Annotations associate meta-information with definitions.
A simple annotation has the form @´c´
or @´c(a_1 , \ldots , a_n)´
.
Here, ´c´ is a constructor of a class ´C´, which must conform
to the class scala.Annotation
.
Annotations may apply to definitions, types, or expressions. An annotation of a definition appears in front of that definition. An annotation of a type appears after that type. An annotation of an expression appears after that expression, separated by a colon. More than one annotation clause may apply to an entity. The order in which these annotations are given does not matter.
Examples:
Predefined Annotations
Predefined annotations are found in the scala.annotation
package, and also in the scala
package.
Scala Compiler Annotations
@tailrec
Marks a method which must be transformed by the compiler to eliminate self-recursive invocations in tail position. It is an error if there are no such invocations, or a recursive call not in tail position.@switch
Marks the expression submitted to a match as "switchable", such that the match can be compiled to an efficient form. The compiler will warn if the type of the expression is not a switchable type. Certain degenerate matches may remain unoptimized without a warning.@unchecked
When applied to the selector of amatch
expression, this attribute suppresses any warnings about non-exhaustive pattern matches that would otherwise be emitted. For instance, no warnings would be produced for the method definition below, or the similar value definition.Without the
@unchecked
annotation, a Scala compiler could infer that the pattern match is non-exhaustive and issue a warning becauseOption
is asealed
class.@uncheckedStable
When applied to a value definition, it allows the defined value to appear in a path, even if its type is volatile. For instance, the following member definitions are legal:Without the
@uncheckedStable
annotation, the designatorx
would not be a path since its typeA with B
is volatile. Hence, the referencex.T
would be malformed.When applied to value definitions that have no volatile types, the annotation has no effect.
@specialized
When applied to the definition of a type parameter, this annotation causes the compiler to generate definitions that are specialized for primitive types. An optional list of primitive types may be given, in which case specialization takes into account only those types. For instance, the following code would generate specialized traits forUnit
,Int
andDouble
:Whenever the static type of an expression matches a specialized variant of a definition, the compiler will use the specialized version instead. See the specialization SID for more details of the implementation.
Deprecation Annotations
@deprecated(message: <stringlit>, since: <stringlit>)
Marks a definition as deprecated. Accesses to the defined entity will then cause a deprecated warning mentioning the message<stringlit>
to be issued from the compiler. The argument since documents since when the definition should be considered deprecated.
Deprecated warnings are suppressed in code that belongs itself to a definition that is labeled deprecated.@deprecatedName(name: <stringlit>, since: <stringlit>)
Marks a formal parameter name as deprecated. Invocations of this entity using named parameter syntax referring to the deprecated parameter name cause a deprecation warning.
Java Platform Annotations
The meaning of other annotation clauses is implementation-dependent. On the Java platform, the following annotations have a standard meaning.
@transient
Marks a field to be non-persistent; this is equivalent to thetransient
modifier in Java.@volatile
Marks a field which can change its value outside the control of the program; this is equivalent to thevolatile
modifier in Java.@SerialVersionUID(<longlit>)
Attaches a serial version identifier (along
constant) to a class. This is equivalent to the following field definition in Java:@throws(<classlit>)
A Java compiler checks that a program contains handlers for checked exceptions by analyzing which checked exceptions can result from the execution of a method or constructor. For each checked exception which is a possible result, thethrows
clause for the method or constructor must mention the class of that exception or one of the superclasses of the class of that exception.
Java Beans Annotations
@scala.beans.BeanProperty
When prefixed to a definition of some variableX
, this annotation causes getter and setter methodsgetX
,setX
in the Java bean style to be added in the class containing the variable. The first letter of the variable appears capitalized after theget
orset
. When the annotation is added to the definition of an immutable value definitionX
, only a getter is generated. The construction of these methods is part of code-generation; therefore, these methods become visible only once a classfile for the containing class is generated.@scala.beans.BooleanBeanProperty
This annotation is equivalent toscala.reflect.BeanProperty
, but the generated getter method is namedisX
instead ofgetX
.
User-defined Annotations
Other annotations may be interpreted by platform- or application-dependent
tools. The class scala.annotation.Annotation
is the base class for
user-defined annotations. It has two sub-traits:
scala.annotation.StaticAnnotation
: Instances of a subclass of this trait will be stored in the generated class files, and therefore accessible to runtime reflection and later compilation runs.scala.annotation.ConstantAnnotation
: Instances of a subclass of this trait may only have arguments which are constant expressions, and are also stored in the generated class files.- If an annotation class inherits from neither
scala.ConstantAnnotation
norscala.StaticAnnotation
, its instances are visible only locally during the compilation run that analyzes them.
Host-platform Annotations
The host platform may define its own annotation format. These annotations do not
extend any of the classes in the scala.annotation
package, but can generally
be used in the same way as Scala annotations. The host platform may impose
additional restrictions on the expressions which are valid as annotation
arguments.