Annotations are a way to attach meta info to your code (such as documentation, configuration, and others).
Let's look at the following example code:
annotation class Tasty
An annotation itself can be annotated to modify its behavior:
@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class Tasty
In this case, the Tasty annotation can be set on classes, interfaces, and objects, and it can be queried at runtime.
For a complete list of options, check the Kotlin documentation.
Annotations can have parameters with one limitation, they can't be nullable:
@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class Tasty(val tasty:Boolean = true)
@Tasty(false)
object ElectricOven : Oven {
override fun process(product: Bakeable) {
println(product.bake())
}
}
@Tasty
class CinnamonRoll : Roll("Cinnamon")
@Tasty
interface Fried {
fun fry(): String
}
To query annotation values at runtime, we must use the reflection API (kotlin-reflect.jar must be in your classpath):
fun main(args: Array<String>) {
val annotations: List<Annotation> = ElectricOven::class.annotations
for (annotation in annotations) {
when (annotation) {
is Tasty -> println("Is it tasty? ${annotation.tasty}")
else -> println(annotation)
}
}
}