Java注释属性限制

Question

I don't think this is possible but I wanted to throw the question out there just in case I am missing something. I have an annotation:

@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Auditable {
  enum When{COMPLETE,ENTERING};
  /**
   * The list of attributes in this class to be audited
   * This is used at target level 'Type' only 
   * 
   * @return
   */
  String[] attributes() default {};

  /**
   * enum to dictate when to audit this message
   * This is used at target level 'method' only
   * 
   * @return
   */
  When when() default When.ENTERING;
}

What I would like is a way to add an annotation to limit the Target for the attributes to either Type or Method.

For example the attribute 'when' from above is limited to Method:

  /**
   * enum to dictate when to audit this message
   * This is used at target level 'method' only
   * 
   * @return
   */
  @Target({ElementType.METHOD})
  When when() default When.ENTERING;

again I do not believe this is possible, but it would be nice to have.

Answer 1

No, there is no way to restrict that an annotation's attribute only be usable depending on the annotated target, at compile time. You can always throw exceptions at runtime when processing the annotations.

Otherwise, you'll have to define and use different annotation types.