Annotations | Reflection | Kotlin Tutorial

Introduction to Annotations

Annotations in Kotlin are a powerful way to attach metadata to your code. They provide a way to indicate how various elements of your program should behave or be treated by the compiler or runtime. Annotations can be used for various purposes, such as code analysis, documentation, or runtime processing.

Defining Annotations

To define an annotation in Kotlin, you use the annotation class keyword. Here's how you can create a simple annotation:

 
                annotation class MyAnnotation(val message: String)

In this example, MyAnnotation is an annotation that takes a single parameter, message, of type String.

Using Annotations

Once you have defined an annotation, you can use it on classes, functions, properties, or other declarations. Here's an example of how to use MyAnnotation:

 
                @MyAnnotation("This is my custom annotation")
                class AnnotatedClass {
                    fun display() {
                        println("Hello, Annotations!")
                    }
                }

In this code, the AnnotatedClass is marked with the MyAnnotation annotation, which can be processed later.

Accessing Annotations

You can access annotations using reflection. Here’s how to retrieve annotations from a class:

 
                val annotations = AnnotatedClass::class.annotations
                annotations.forEach { 
                    println(it)
                }

The above code retrieves all annotations applied to AnnotatedClass and prints them.

Built-in Annotations

Kotlin provides several built-in annotations, such as @Deprecated, @Override, and @JvmStatic. For example, you can mark a function as deprecated like this:

 
                @Deprecated("Use newFunction() instead")
                fun oldFunction() {
                    println("This function is deprecated.")
                }

This tells developers that oldFunction() should not be used and suggests newFunction() as an alternative.

Conclusion

Annotations are a powerful feature in Kotlin that allow you to add metadata to your code. They can be defined and used in various ways, making them a versatile tool for developers. Understanding how to create and utilize annotations can enhance code readability and maintainability.