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Adding Code to an Annotation

· Java Zone

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Sometimes you have a framework you would like to be able to add custom annotations to. The problem is; how do you define what this custom annotation does. A possible solution is to use a nested enum.

Nested types

You can nest an interface, a class, an annotation or an enum inside any of these types. e.g.
public class A {
    public interface B {
        public enum C {;
            public @interface D {
                public class E {
                    // etc etc
                }
            }
        }
    }
}
But is this useful or just a novelty?

Adding functionality to an annotation

You can define an interface the framework can call to determine how your custom annotation should behave.
import java.lang.reflect.Member;

/**
* @author peter.lawrey
*/
public interface AnnotationHandler<A> {
    void process(Model model, Member member, A annotation);
}
One way of associating the implementation with the annotation itself is to provide a Singleton enum which implements this interface.
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Member;

/**
* @author peter.lawrey
*/
@Retention(RetentionPolicy.RUNTIME)
public @interface CustomAnnotation {
    public int value() default 0;
    
    enum HandlesAnnotation implements AnnotationHandler<CustomAnnotation> {
        INSTANCE;

        @Override
        public void process(Model model, Member member, CustomAnnotation ca) {
            // do something with a model based on the members details and the annotation state.
        }
    }
}
If an annotation could be handled a number of different ways (depending on what process you are performing), you could provide more than one implementation. Using this approach, a framework which uses annotations can allow developers to add custom annotations is a simple manner. The framework can also make it clear how an annotation behaves, making it easier to create annotations which are similar but need to be different in some way.

Learn more about Kotlin, a new programming language designed to solve problems that software developers face every day brought to you in partnership with JetBrains.

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Published at DZone with permission of Peter Lawrey, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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