Defining Annotations

Occasionally, you may have the need to create your own custom annotations.

In Kotlin, annotations are simply classes defined using the annotation keyword:

annotation class MyAnnotation

@MyAnnotation
class Foo

You are welcome to add constructor parameters to your annotation:

annotation class MyAnnotation(val isThisGood: Boolean)

@MyAnnotation(isThisGood = true)
class Foo

However, there are limits on the data types. For Kotlin/JVM development, Kotlin annotations get converted into Java annotations, and those have limits on their data types. You can use:

• Int and Long
• Float and Double
• Boolean
• String
• any enum that you define
• any other annotation
• classes (data type is KClass)