Annotations

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.

Exercise 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 }).

Show example 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 })
}

Test yourself

Complete the annotation class definition
```
____ class Label
```
Show answer
```
annotation class Label
```
Match the annotation parts

@Target(...)

Placement
annotation class

Declaration
@Label

Usage
findAnnotation<Label>()

Search
Which property reads annotations on a property?
- .methods
- .fields
- .constructors
- .annotations
Show answer

.annotations — annotations gives the annotation list.

Next lesson: Summary - Reflection

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