11.3.1 Annotation Creation Explained

Creating custom annotations in Java SE 11 is a powerful way to add metadata to your code. Custom annotations allow you to define specific behaviors or attributes that can be processed by tools, frameworks, or your own custom code. Understanding how to create and use custom annotations is essential for advanced Java development.

Key Concepts

1. Defining an Annotation

An annotation is defined using the @interface keyword. This keyword indicates that the interface is an annotation type. The annotation can have elements that define its behavior.

Example

        public @interface MyCustomAnnotation {
            String value();
            int count() default 1;
        }
    

2. Annotation Elements

Annotation elements are methods declared in the annotation interface. These methods can have default values, which are specified using the default keyword. Elements can be of various types, such as String, int, Class, or arrays.

Example

        public @interface MyCustomAnnotation {
            String name();
            int age() default 18;
            Class<?> type() default Object.class;
        }
    

3. Meta-Annotations

Meta-annotations are annotations that are applied to other annotations. They define how the target annotation should be processed. Common meta-annotations include @Retention, @Target, @Documented, and @Inherited.

Example

        @Retention(RetentionPolicy.RUNTIME)
        @Target(ElementType.METHOD)
        public @interface MyCustomAnnotation {
            String value();
        }
    

4. Retention Policies

Retention policies define the lifecycle of an annotation. The three retention policies are SOURCE, CLASS, and RUNTIME. They determine whether the annotation is available at compile time, class loading time, or runtime.

Example

        @Retention(RetentionPolicy.RUNTIME)
        public @interface MyCustomAnnotation {
            String value();
        }
    

5. Target Types

Target types specify the elements to which an annotation can be applied. These include TYPE, METHOD, FIELD, PARAMETER, and others. The @Target meta-annotation is used to define these targets.

Example

        @Target(ElementType.METHOD)
        public @interface MyCustomAnnotation {
            String value();
        }
    

Examples and Analogies

Think of custom annotations as sticky notes with specific instructions. These notes can be attached to different parts of your code, such as methods, classes, or fields. For example, you might create an annotation called @PerformanceCritical to indicate that a method is performance-sensitive and should be optimized.

For instance, if you are developing a logging framework, you could create a custom annotation called @Loggable to indicate that a method should be logged. This annotation could have elements like logLevel and message to specify the logging details.

By mastering custom annotation creation, you can enhance the functionality and maintainability of your Java SE 11 applications, making your code more expressive and easier to manage.