Kotlin @Retention Meta-annotation Tutorial 🎯

Welcome to the Kotlin @Retention Meta-annotation Tutorial! In this lesson, we'll explore how to use meta-annotations, specifically the @Retention annotation, to control when annotations are available in your Kotlin code. Let's dive right in! 📝

What are Meta-annotations? 💡

Before we delve into @Retention, let's first understand what meta-annotations are. In Kotlin, annotations are special pieces of code that provide additional information about the source code. Meta-annotations, on the other hand, are annotations that are annotated themselves! They are used to control aspects of the annotation processing system.

Introduction to @Retention 📝

The @Retention annotation is used to control the lifetimes of annotations. By default, annotations are erased during the compilation process, but with @Retention, you can specify when the annotation should be retained.

There are three possible retention policies:

SOURCE: The annotation is kept only in the source code and discarded during compilation.
CLASS: The annotation is retained during compilation and present in the .class files.
RUNTIME: The annotation is retained during compilation and available at runtime.

Creating an Annotation with @Retention 💡

Now, let's create a simple annotation with @Retention.

kotlin

@Retention(AnnotationRetention.RUNTIME)
annotation class MyAnnotation

In the code above, we've created an annotation named MyAnnotation and specified its retention policy to be RUNTIME. This means that MyAnnotation will be available at runtime.

Using the Created Annotation 💡

Now that we have our annotation, let's see how to use it.

kotlin

@MyAnnotation
class MyClass {
    // Your code here
}

In the example above, we've applied the MyAnnotation to a class MyClass. Now, at runtime, you can retrieve the annotation using reflection.

Retention Policy: SOURCE and CLASS 📝

If you want to retain your annotation during compilation but not at runtime, you can set the retention policy to either SOURCE or CLASS. Here's an example:

kotlin

@Retention(AnnotationRetention.SOURCE)
annotation class MySourceAnnotation

@Retention(AnnotationRetention.CLASS)
annotation class MyClassAnnotation

Quiz 💡

Summary 📝

In this tutorial, we learned about meta-annotations and the @Retention annotation in Kotlin. We created an annotation, set its retention policy, and applied it to a class. We also discussed the retention policies SOURCE and CLASS. With this knowledge, you can create your own annotations and control their lifetimes!

Keep practicing, and happy coding! 🎉