Using annotation with @Target(TYPE_USE) on type declaration

[Gunnar Morling]

Thanks, Alex!

> 3. You're now wondering why JLS 9.6.4.1 doesn't "just" make a type declaration context

I see. While 9.7.4 addresses the issue, it seems a bit implicit to me.
A sentence in 9.6.4.1 in that spirit might be helpful, given it's
probably the first place people will check when looking for details
around @Target.

Btw. I had looked for clarification at the JavaDoc of the enum member
ElementType#TYPE_USE, but this just says "Use of a type". Perhaps it'd
be a good idea to move/duplicate some of the very useful info given in
the class comment of ElementType to the specific docs of the TYPE_USE
member?

--Gunnar


2017-06-27 20:47 GMT+02:00 Alex Buckley <alex.buckley at oracle.com>:
> On 6/27/2017 7:54 AM, Gunnar Morling wrote:
>>
>> I've an annotation type whose only supported target is
>> ElementType.TYPE_USE:
>>
>>      @Target(ElementType.TYPE_USE)
>>      public @interface MyAnno {}
>>
>> To my surprise I can use this annotation on the definition of a class,
>> whereas I'd have expected a compilation error:
>>
>>      @MyAnno
>>      public class Foo {}
>>
>> I can't seem to find the JLS wording that'd suggest that TYPE_USE
>> allows usage of the annotation on a type definition.
>>
>> "9.6.4.1 @Target" [1] mentions eight declaration contexts (which don't
>> seem to apply here as per the supported element type) and 16 type
>> contexts which are described in "4.11. Where Types Are Used" [2]. They
>> are represented by TYPE_USE, but none of these seem to match the the
>> declaration of type Foo.
>
>
> 1. JLS 9.6.4.1 is trying to make {declaration,type} contexts in the language
> correspond to constants in the ElementType API. A type declaration context
> corresponds to TYPE, and the 16 type contexts corresponds to TYPE_USE.
> Notice the correspondence is one-way.
>
> 2. The API spec for ElementType is trying to make constants in the
> ElementType API correspond to {declaration,type} contexts in the language.
> The TYPE_USE constant corresponds to more contexts than the JLS said
> correspond to it:
>
>   "The constant TYPE_USE corresponds to the type contexts in JLS 4.11,
>    as well as to two declaration contexts: type declarations
>    (including annotation type declarations) and
>    type parameter declarations."  [Rationale follows]
>
> 3. You're now wondering why JLS 9.6.4.1 doesn't "just" make a type
> declaration context correspond to both TYPE and TYPE_USE. The answer is that
> I wanted to keep the correspondence between contexts and constants as
> straightforward as possible in the JLS. This was one of many techniques for
> managing the complexity of the specification as it gained support for type
> annotations and repeating annotations in SE 8.
>
> 4. OK, so why does the JLS allow @MyAnno on the declaration of class Foo?
> Because JLS 9.7.4 says the following, which "implements" the ElementType API
> spec's belief that TYPE_USE corresponds to type declarations:
>
>   "It is a compile-time error if an annotation of type T is
>    syntactically a modifier for: ... a class, interface, or enum
>    declaration, but T is not applicable to type declarations or
>    type contexts"
>
> 5. The JLS9 draft available at http://cr.openjdk.java.net/~mr/jigsaw/spec/
> has some words in 9.7.4 to explain this multi-role of TYPE_USE.
>
> Alex