Error setting a default null value for an annotation's field

Question

Why am I getting an error \"Attribute value must be constant\". Isn\'t null constant???

@Target(ElementType.TYPE)
@Retention(RetentionPolicy         


        

Answer 1

It would seem this is illegal, although the JLS is very fuzzy on this.

I wracked my memory to try and think of an existing annotation out there that had a Class attribute to an annotation, and remembered this one from the JAXB API:

@Retention(RUNTIME) @Target({PACKAGE,FIELD,METHOD,TYPE,PARAMETER})        
public @interface XmlJavaTypeAdapter {
    Class type() default DEFAULT.class;

    static final class DEFAULT {}    
}

You can see how they've had to define a dummy static class to hold the equivalent of a null.

Unpleasant.