Annotations in Kotlin are a way of adding metadata to our code. They can be used to provide additional information to the compiler, to generate code at compile time, or to be processed by third-party tools. In Kotlin, we can create our own annotations using the ‘@interface‘ keyword. Let’s see an example:
@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
@MustBeDocumented
annotation class Entity(val tableName: String)Let’s explain the above code:
- We use the ‘@interface‘ keyword to define the annotation class ‘Entity‘.
- The ‘@Target‘ annotation specifies where this annotation can be used. In this case, we have defined that this annotation can only be applied to classes using the ‘AnnotationTarget.CLASS‘.
- The ‘@Retention‘ annotation specifies how the annotation is stored. In this case, we have set ‘AnnotationRetention.RUNTIME‘, which means that the annotation can be accessed at runtime (useful for reflection).
- The ‘@MustBeDocumented‘ annotation indicates that this annotation should be included in the generated documentation.
The ‘Entity‘ annotation has a parameter ‘tableName‘ that takes a string value. We can use this annotation as follows:
@Entity(tableName = "users")
class User(val id: Int, val name: String)This code applies the ‘@Entity‘ annotation to the ‘User‘ class and sets the ‘tableName‘ parameter to ‘"users"`. We can now access this information at runtime using reflection:
val user = User(1, "John")
val entityAnnotation = user.javaClass.getAnnotation(Entity::class.java)
println("Table name: ${entityAnnotation?.tableName}")This would output ‘Table name: users‘, which is the value we set in the ‘@Entity‘ annotation.
In summary, annotations are an important tool in Kotlin that allow us to add metadata to our code. We can create our own annotations using the ‘@interface‘ keyword and apply them to classes, functions, or properties using their respective ‘@Target‘ annotations. We can access this metadata at runtime using reflection, which allows us to dynamically generate code or process our code in different ways.