Kotlin is a powerful and expressive programming language that offers a range of features to make your code concise, readable, and maintainable. One of these features is annotations, which allow you to add metadata to your code that can be used by the compiler, tools, and frameworks to simplify and automate common tasks. In this blog post, we’ll explore Kotlin annotations and how you can use them to enhance your code and make development faster and easier.

What is Kotlin Annotation?

Kotlin annotations can help you simplify your code and reduce boilerplate by automating common tasks such as serialization, validation, and dependency injection. They can also help you enforce coding standards and best practices by providing additional context and constraints on how your code is used. Additionally, annotations can be used to generate documentation, tests, and other artifacts based on your code.

annotation class CustomAnnotation

If you want to add more stuff to your annotation, you can do that by putting some extra annotations on top of the annotation class. Those are called meta-annotation.

• @Target annotation is used to specify the possible targets of the annotation. The target determines the elements of the code that the annotation can be applied to (such as classes, functions, properties, and expressions);
• @Retention annotation is used to specify the retention policy of the annotation. The retention policy determines how long the annotated metadata should be retained. Kotlin provides three retention policies:
  – SOURCE: Annotations with this retention policy are only retained at
  compile time and are not available at runtime. This means that they can
  be used by tools like IDEs and code generators to perform static
  analysis and generate code, but they cannot be accessed via reflection
  at runtime.
  – BINARY: Annotations with this retention policy are retained in the
  compiled class files, but are not available at runtime. This means that
  they can be used by other tools and frameworks that operate on the
  bytecode, but they cannot be accessed via reflection at runtime.
  – RUNTIME: Annotations with this retention policy are retained in the
  compiled class files and are available at runtime through reflection.
  This means that they can be used to provide runtime behavior, such as
  configuring dependency injection, serialization, or logging.
• @Repeatable annotation is used to specify that an annotation can be applied multiple times to the same target element. By default, annotations in Kotlin can only be applied once to a target element.
• @MustBeDocumented annotation is used to indicate that an annotation should be documented in the generated documentation. By default, Kotlin annotations are not included in the generated documentation unless you explicitly document them. This can be problematic if you want to create reusable annotations that are intended to be used in multiple projects or by other developers.

How to build custom annotation?

You can create custom annotations using the annotation class keyword. Here’s an example of how to define a custom annotation:

@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class MyAnnotation(val someValue: String)

In this example, we’ve defined an annotation called MyAnnotation that takes a single parameter called someValue of type String.

The @Target annotation is used to specify where the annotation can be used. In this case, we’ve set the target to AnnotationTarget.CLASS, which means the annotation can only be applied to classes.

The @Retention annotation is used to specify the retention policy of the annotation. In this case, we’ve set the retention policy to AnnotationRetention.RUNTIME, which means the annotation will be available at runtime through reflection.

To use the custom annotation, simply apply it to a class like this:

@MyAnnotation("Hello, World!")
class MyClass {
  // class implementation here
}

In this example, we’ve applied the MyAnnotation annotation to the MyClass class and provided a value for the someValue parameter.

To access the value of the annotation at runtime, you can use reflection:

val annotation = MyClass::class.annotations.find { it is MyAnnotation } as MyAnnotation
println(annotation.someValue) // prints "Hello, World!"

This code retrieves the annotation applied to MyClass and casts it to MyAnnotation so that we can access its properties. In this case, we’re printing the value of the someValue parameter.

This article was previously published on proandroiddev.com