Annotations

Annotations are means of attaching metadata to code.

To declare an annotation, put the annotation modifier in front of a class:

annotation class Fancy

Additional attributes of the annotation can be specified by annotating the annotation class with meta-annotations:

@Target specifies the possible kinds of elements which can be annotated with the annotation (such as classes, functions, properties, and expressions);
@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.

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

Usage

@Fancy class Foo {
    @Fancy fun baz(@Fancy foo: Int): Int {
        return (@Fancy 1)
    }
}

If you need to annotate the primary constructor of a class, you need to add the constructor keyword to the constructor declaration, and add the annotations before it:

class Foo @Inject constructor(dependency: MyDependency) { ... }

You can also annotate property accessors:

class Foo {
    var x: MyDependency? = null
        @Inject set
}

Constructors

Annotations can have constructors that take parameters.

annotation class Special(val why: String)

@Special("example") class Foo {}

TODO:

Kotlin Docs

Last modified: 18 March 2024