[JSR308] Annotations on type variable declarations

Rémi Forax forax at univ-mlv.fr
Wed Feb 25 09:19:45 EST 2009

Michael Ernst a écrit :
> This message concerns annotations on type variable declarations, and in
> particular how to specify whether an annotation is permitted there.
> JSR 308 supports annotations on the declarations of type variables, such as
>   class Foo<@Anno1 X> { ... }
> Separately, JSR 308 supports annotations on uses of type variables, such as
>   class Foo<X> {
>     @Anno2 X myVariable = ...;
>   }
> and annotations on uses of types that are not type variables:
>     @NonNull String myVariable = ...;
> JSR 308's new TYPE_USE enum constant of java.lang.annotation.ElementType
> indicates that a given annotation may appear on a use of a type.  For
> example, the NonNull annotation might be declared as follows:
>   @Target(ElementType.TYPE_USE)
>   public @interface NonNull { ... }
> Question:  Is it necessary to add more new enum constants to
> java.lang.annotation.ElementType, to distinguish among the locations for
> annotations that are exemplified by @Anno1, @Anno2, and @NonNull above?
> If so, then the compiler will forbid placing an annotation in the
> disallowed locations whether or not the annotation processor is being run.
> If not, then the annotation processor must check explicitly.  The current
> Java design already requires an annotation processor must do some such
> checks.  For example, Java 5 provides no way to indicate that a particular
> annotation is permitted on class declarations but not permitted on enums;
> but an annotation processor can achieve this same effect by issuing an
> error when it sees an annotation appears in a place that the annotation is
> not supported.
> A key question is whether the need for making the distinctions among the
> annotation locations justify adding more enum constants (and asking users
> of ElementType to understand their meaning).
> Here are some possibilities.
>  * ElementType.TYPE_USE permits @Anno1, @Anno2, and @NonNull.
>    A type variable declaration is not really a use of a type, so in this
>    design the name TYPE_USE is a bit of a misnomer and may be confusing.
>    It could perhaps be renamed to, say, ElementType.TYPE_ANNOTATION.
>  * ElementType.TYPE permits @Anno1.
>    ElementType.TYPE_USE permits @Anno2 and @NonNull.
>    This re-uses the existing ElementType.TYPE constant, which applies to
>    type declarations.  This fits because, just as with class/iterator/enum
>    declarations, a type variable declaration introduces a symbol that may
>    afterward be used as a type.  A difference is that a type variable
>    declaration only introduces a name for an existing type, whereas the
>    other declarations introduce both a new type and a name.
>  * new enum constant ElementType.TYPE_PARAMETER permits @Anno1.
>    ElementType.TYPE_USE permits @Anno2 and @NonNull.
> I don't think it makes sense to distinguish between the locations
> exemplified by @Anno2 and @NonNull.
>                     -Mike
> PS:  JSR 308 supports annotations on type variable declarations because
> people have found uses for it.  If those other people can remind me of
> those uses, I'll note them in the design document.  That will also make our
> discussion more concrete.
> (The checkers that are currently distributed with the Checker Framework
> don't make use of annotations on declarations of type variables.  To make
> sure that the type argument supplied as X has annotation @Anno1, one would
> write
>   class Foo<X extends @Anno1 Object> { ... }
> But other type systems have different needs.)
I vote for proposition 3. I think it's important to distinguish the 
declaration site
from the use site of a type variable.

Here is an example of annotation on type variable:
I want to introduce a new annotation @Reified that explicitly says that
my type variable is reified at runtime (yah an old dream, that one day 
will become true :)

interface WonderfullList<@Reified E> {
   public E get(int index);

Here, the declaration of E can be annotated by @Reified but
be able to annotate the return type of method get() seems really weird.


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