Best way to support "Extendable enums" in annotations?

Question

In my library there is a concept of "user levels". I have provided several default levels but for various reasons want to give the user the option of using their own levels.

Currently this is implemented as

public interface AdminLevel {
    public void name();
}

public enum StandardAdminLevels implements AdminLevel {
    ADMIN,
    ANONYMOUS
}

The problem is that the user is usually going to be passing their required user level in an annotation. Things I have tried and failed:

• Using AdminLevel as the type - Fails with "invalid type for annotation member"
• Using String as the type but setting the value with StandardAdminLevels.ADMIN.name() - Fails with "attribute value must be constant"
• Making StandardAdminLevels a final class that doesn't implement anything with public static final field's for each of the levels (essentially an enum) - Fails with "invalid type for annotation member"

Is there any other way I can't think of to have extendable enums in annotations? I'm trying to stick with enums due to their clarity and basic protection against invalid values, but the only other way I can think of is String constants. The problem I have is that would require verification at every single point user levels are used, potentially even in client code

Any ideas?

Answer 1

One idea: every possible AdminLevel is its own class. Pass the Class of that class to the annotation.

public final class Admin implements AdminLevel { ... }
public final class Anonymous implements AdminLevel { ... }

@Requires(Admin.class) public void protectedMethod() { ... }

@interface Requires {
  Class<? extends AdminLevel> value();
}