Annotations on Interfaces?

Question

I can't figure out a use case for being able to annotate interfaces in Java.

Maybe someone could give me an example?

Answer 1

I've used it in Spring to annotate interfaces where the annotation should apply to all subclasses. For example, say you have a Service interface and you might have multiple implementations of the interface but you want a security annotation to apply regardless of the annotation. In that case, it makes the most sense to annotate the interface.

Answer 2

A use case that I am working with is javax/hibernate bean validation, we are using interfaces to help us on avoiding to define validations on every specific class.

public interface IUser {
    @NotNull Long getUserId();
    ...
}

public class WebUser implements IUser {
    private Long userId;

    @Override
    public Long getUserId(){
        return userId;
    }
    ...
}