Nice to @Share?

[Alex Buckley]

As Neal said, @Shared is mentioned in non-normative text lifted directly 
from John Rose's mail. I have never thought it appropriate for an 
annotation to turn a local variable into a shared variable. That is, 
once a method-scoped variable is morally "shared", it isn't a local 
variable (stack-allocated, single-thread-visibility, no volatility) at 
all. It's an eighth kind of variable in JLS 4.12.3. (And being 'final' 
is orthogonal.) A restricted keyword - suggestions welcome - is more 
than appropriate.

Alex

Joshua Bloch wrote:
> Some say it's nice to @Share, but not me. Draft 0.1.5 of the Project Lambda
> spec contains some discussion of an @Shared annotation to "allow unqualified
> references to upplevel variables (all sorts) if they are marked @Shared."
>  There are several reasons that I believe this not a good idea:
> 
> (1) For fields (instance or static) this annotation is not required today
> (in anonymouns class instance creation expressions), and I believe that
> programmers are likely to see it as an added annoyance (like the current
> situation with final variables).
> 
> (2) For local variables, I see it as too much magic:
> 
>     @Shared int i;
> 
>  has a totally different implementation from:
> 
>     int i;
> 
> The latter creates a "container object" on the heap, the former doesn't. The
> semantics (in particular, the lifetime) and the performance are likely to
> differ significantly.
> 
> (3) I believe it's an abuse of annotations.  Some may liken it to @Override,
> but I believe this is specious. The @Override annotation merely expresses a
> compile-time assertion that the compiler is able to verify. The proposed
> @Shared annotation changes the semantics of the language.
> 
> (4) If, in spite of all the above, it is the consesnsus that such an
> annotation is still a good idea, I believe the name @Shared could hardly be
> more misleading. In practical terms, it means "this variable must *not* be
> shared (without additional synchronization)." It would typically be used
> when the programmer could assert that the variable in question would never
> be sharead among multiple threads.
> 
>             Josh
>