Custom annotations in Kotlin can be incredibly useful for AOP (Aspect-Oriented Programming), compile-time processing, or marking elements for specific frameworks. In this article, I'll guide you through the process of creating and using custom annotations in Kotlin.
Understanding Annotations
Annotations are metadata that provide information to the compiler or runtime about the code they are associated with. In Kotlin, annotations start with the @ character followed by the annotation name.
Examples of common annotations are @JvmStatic, @Deprecated, and @Test. But when existing annotations don't cover your needs, you can define your own.
Creating a Custom Annotation
To create a custom annotation in Kotlin, you need to use the annotation class declaration.
// Define a custom annotation
annotation class MyCustomAnnotation
Annotations can also take parameters.
// Define an annotation with parameters
target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class Info(val author: String, val version: Int)
Applying Custom Annotations
Once you have defined your annotation, you can apply it to various elements of your Kotlin code.
@Info(author = "Jane Doe", version = 1)
class ExampleClass {
@MyCustomAnnotation
fun markedFunction() {
println("This function is marked with MyCustomAnnotation")
}
}
Reflecting on Annotations
Reflection is often used to handle annotations at runtime. You can check if an element has a specific annotation and retrieve its parameters.
fun main() {
val example = ExampleClass::class
example.annotations.forEach { annotation ->
println("Found annotation: "+annotation)
if (annotation is Info) {
println("Author: "+annotation.author+", Version: "+annotation.version)
}
}
}
Retention and Target
Annotations can have different retention policies, which define how long the annotations are retained:
AnnotationRetention.SOURCE- discarded during compilationAnnotationRetention.BINARY- stored in the binary output but unavailable for reflectionAnnotationRetention.RUNTIME- available at runtime via reflection
You can also specify where your annotation can be applied using the @Target annotation. Options include CLASS, FUNCTION, PROPERTY, etc.
As we have shown above, Kotlin makes it simple yet powerful to create and use custom annotations that meet your program-specific needs. Always remember to define the right target and retention to optimize their usage.
