Nice to @Share?

[Paul Benedict]

I agree with Josh and I strongly agree with his third point. I thought
Sun (Oracle) had the philosophy that annotations should not alter the
semantics of the language? An annotation that tells the compiler to
turns allocation from stack to heap sounds unpalatable. This kind of
behavior should be controlled by a (context-sensitive) keyword.

On Mon, Feb 22, 2010 at 2:38 PM, Joshua Bloch <jjb at google.com> 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
>
>