RFR (S): JEP-142: Reduce Cache Contention on Specified Fields

[John Rose]

On Nov 27, 2012, at 3:35 AM, Doug Lea wrote:

> And, as someone else noted, sun.misc (where Unsafe lives) is even
> more exotic, and has the audience of low-level/core developers
> that this targets. Placing it there would get us past all the
> concerns (that I honestly think are wrong) about sending the
> wrong message or precluding miraculous automation. So, how
> about we just live with it as follows:

I don't mean to add insult to injury, but I think there is yet another restriction that we should place this annotation under, in its current residence of sun.misc.

The annotation must be documented as being a no-op when it occurs inside class files that are loaded outside the boot class path.

This is consistent with comments made by several other people, that these tools are intended, at least at first, for developers of core classes.  This is also consistent with Unsafe, which (however popular it is) is designed to be used core classes only, on the BCP (boot class path).[1]

For JSR 292 we are likely to introduce @ForceInline, @Stable, and some other annotations into sun.misc.  They will be restricted to BCP classes only, and may go away in the future.  How fast do you think people would pollute their code with @ForceInline if it was effective outside of core classes?  We need hooks for system-specific tuning (when the JIT is part of the tuned system) but we do not want a "register" keyword[2] to become part of Java lore.

At the same time the annotation is sufficiently understood to be exported more widely, it can be enabled off the BCP.  I think we need a module system in place to do a good job of this.  (And, yes, Unsafe should also be importable as a module of some sort.)

The bottom line is that sun.misc is a good place for vetting experimental annotations.  And, unless there is a strong reason otherwise, they should be no-ops outside of the core classes (i.e., the BCP).

I now don my asbestos smoking jacket.

— John

[1] Yes I know Unsafe is sometimes broken into by a Field.setAccessible hack.  Which is an implementation artifact that could easily break by accident.  If you use that hack, you should move your code to BCP.

[2] http://www.cplusplus.com/forum/general/8225/