With Java 8 you are able to repeat the same annotation to a declaration or type. For example, to register that one class should only be accessible at runtime by specific roles, you could write something like:
Notice that now @Role is repeated several times. For compatibility reasons, repeating annotations are stored in a container annotation, so instead of writing just one annotation you need to write two, so in the previous case, you need to create @Role and @Roles annotations.
Notice that you need to create two annotations, one which is the "plural" part of the annotation where you set return type of value method to be an array of the annotation that can be used multiple times. The other annotation can be used multiple time in the scope where it is defined and must be annotated with @Repeatable annotation.
This is how I did all the time since Java 8 allows to do it. But last week, during a code review my mate George Gastaldi pointed me out how they are implementing these repeatable annotations in javax.validation spec. Of course, it is not completely different but I think that looks pretty much clear from point of view implementation since everything is implemented within the same archive and also, in my opinion, the name looks much natural.
Notice that now everything is placed in the same archive. Since usually you only need to refer to @Role class, and not @Roles (now @Role.List) annotation you can hide this annotation as an inner annotation. Also in case of defining several annotations, this approach makes everything look more compact, instead of having of populating the hierarchy with "duplicated" classes serving the same purpose, you only create one.
Of course, I am not saying that the approach of having two classes is wrong, at the end is about preferences since both are really similar. But after implementing repeatable annotations in this way, I think that it is cleaner and compact solution having everything defined in one class.
We keep learning,
Alex.
Jo sóc l'hipopòtam, i crec que el lleó, ha de refrescar-se per estar molt millor (El Lleó Vergonyós - El Pot Petit)Music: https://www.youtube.com/watch?v=lYriMzzMsUw
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