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Annotations are a mechanism for attaching metadata to declarations and expressions in Kotlin. They are resolved at compile-time and can be retained in the compiled class files for runtime reflection, but they do not directly alter the execution semantics of the annotated code.

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Declaration

An annotation is defined using the annotation modifier preceding a class declaration. Annotation classes can contain a body, but the language strictly limits its contents. They can include companion objects, nested classes, interfaces, and objects, but cannot contain functions, init blocks, or properties outside the primary constructor. 
annotation class MarkerAnnotation

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Meta-Annotations

The behavior and scope of an annotation are configured using meta-annotations (annotations applied to the annotation class itself).
  • @Target: Defines the syntactic elements to which the annotation can be applied (e.g., CLASS, FUNCTION, PROPERTY, EXPRESSION).
  • @Retention: Determines the lifecycle of the annotation.
    • SOURCE: Discarded by the compiler; not present in the compiled binary.
    • BINARY: Stored in the compiled class file but not visible via reflection at runtime.
    • RUNTIME: (Default) Stored in the compiled class file and accessible via runtime reflection.
  • @Repeatable: Permits the annotation to be applied multiple times to the same element.
  • @MustBeDocumented: Instructs documentation generation tools (like Dokka) to include the annotation in the public API signature.
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
@Repeatable
@MustBeDocumented
annotation class Component

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Constructors and Parameters

Annotations can accept parameters via a primary constructor. All parameters must be declared as read-only (val). The type system strictly limits annotation parameters to the following:
  • Primitive types (Int, Double, Boolean, etc.)
  • String
  • Classes (KClass)
  • Enums
  • Other annotations
  • Arrays of the aforementioned types
Nullable types and default JVM classes (like java.lang.Class) are strictly prohibited. 
enum class Priority { LOW, HIGH }

annotation class Task(
    val id: String,
    val priority: Priority = Priority.LOW,
    val relatedClasses: Array<KClass<*>>
)

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Application Syntax

Annotations are applied by prefixing the target element with the @ symbol, followed by the annotation name and any required constructor arguments. 
@Task(id = "T-101", relatedClasses = [String::class, Int::class])
fun processData() { TODO() }
If an annotation is used as a parameter within another annotation, the @ prefix is omitted for the nested annotation. 
annotation class TaskGroup(val primaryTask: Task)

@TaskGroup(primaryTask = Task(id = "T-102", relatedClasses = []))
class Worker

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Use-Site Targets

Because a single Kotlin property declaration can generate multiple Java bytecode elements (a backing field, a getter, a setter, and a constructor parameter), Kotlin provides use-site targets to explicitly specify which generated element receives the annotation. The syntax follows the pattern @target:AnnotationName. Supported use-site targets include:
  • file: Applies to the entire file (must be placed at the top of the file, before package).
  • property: Applies to the Kotlin property (invisible to Java reflection).
  • field: Applies to the generated Java backing field.
  • get / set: Applies to the generated property getter or setter.
  • receiver: Applies to the receiver parameter of an extension function or property.
  • param: Applies to the constructor parameter.
  • setparam: Applies to the parameter of the property setter.
  • delegate: Applies to the field storing the delegate instance for a delegated property.
class User(
    @field:MarkerAnnotation val name: String,   // Annotates the backing field
    @get:MarkerAnnotation val age: Int,         // Annotates the getter method
    @param:MarkerAnnotation val role: String    // Annotates the constructor parameter
)

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Expression and Lambda Annotations

Annotations can be applied directly to expressions or lambda functions, provided the annotation’s @Target explicitly includes AnnotationTarget.EXPRESSION. 
@Target(AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
annotation class InlineMarker

val multiplier = @InlineMarker { x: Int -> x * 2 }
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