Custom annotations are only useful if they can be discovered and used. The standard annotations mostly exist for the benefit of the compiler, but custom annotations are commonly used as runtime metadata.

To find the annotations declared on a class, function, or other construct, we can use the annotation property available on KClass, KFunction, KParameter, and KProperty. This property returns a collection that has an instance for each of the defined annotations.

For example, let's create an annotation called Description, which accepts a single parameter of String. This String is used to add a description to a class, which might be used to generate documentation in a web service:

    annotation class Description(val summary: String) 

Then we'll use this to describe a class:

    @Description("This class creates Executor instances") class  Executors 

Now, at runtime, we could look up this annotation and use the value:

    val desc = Executors::class.annotations...