Annotations
- Kotlin course
- Advanced
- Lesson 65/83
- 5 min
Learn to read annotations with reflection.
Annotations and reading
An annotation adds metadata to code. @Target specifies where it can be placed. Reflection can read annotations.
Example: @Target(AnnotationTarget.PROPERTY) annotation class Label. A property is marked with @Label. prop.annotations gives the list; prop.findAnnotation<Label>() finds a specific one. This pattern branches on metadata at runtime.
Searching with findAnnotation
findAnnotation<T>() searches for a specific annotation on an element. If found, it returns the annotation; otherwise null.
Example: val label = prop.findAnnotation<Label>(). If @Label is on prop, label is non-null; otherwise it is null. This pattern is a clean way to branch on annotation presence.
Practice Define and read the Label annotation
Define @Target(AnnotationTarget.PROPERTY) annotation class Label. Write class Item(@Label val name: String). In main take the reference val prop = Item::name and call println(prop.annotations.any { it is Label }).
Starter code
// Label annotation: PROPERTY target
// Item class: name property has @Label
// in main take prop, search annotations for Label and printSample solution
@Target(AnnotationTarget.PROPERTY)
annotation class Label
class Item(@Label val name: String)
fun main() {
val prop = Item::name
println(prop.annotations.any { it is Label })
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the annotation class definition
____ class Labelannotation class LabelExplanation: annotation class Label declares an annotation.
2 Match the annotation parts
| Term | Meaning |
|---|---|
@Target(...) |
Placement |
annotation class |
Declaration |
@Label |
Usage |
findAnnotation<Label>() |
Search |
3 Which property reads annotations on a property?
- .methods
- .fields
- .constructors
- .annotations Correct answer
Explanation: annotations gives the annotation list.