Annotations

Declaring an annotation

Annotations are means of attaching metadata to code. To declare an annotation, put the annotation modifier in front of a class:

annotation class Strippable

Annotations can have meta-anotations:

    @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
    annotation class Strippable

Annotations, like other classes, can have constructors:

annotation class Strippable(val importanceValue: Int)

But unlike other classes, is limited to the following types:

types that correspond to Java primitive types (Int, Long etc.);
strings
classes ( Foo:: class)
enums
other annotations
arrays of the types listed above

Meta-annotations

When declaring an annotation, meta-info can be included using the following meta-annotations:

@Target: specifies the possible kinds of elements which can be annotated with the annotation (classes, functions, properties, expressions etc.)
@Retention specifies whether the annotation is stored in the compiled class files and whether it’s visible through reflection at runtime (by default, both are true.)
@Repeatable allows using the same annotation on a single element multiple times.
@MustBeDocumented specifies that the annotation is part of the public API and should be included in the class or method signature shown in the generated API documentation.

Example:

@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
        AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Fancy