[concurrency-interest] The Atomic*FieldUpdater situation

David Holmes davidcholmes at aapt.net.au
Sun Jul 15 22:52:36 EDT 2012


David M. Lloyd writes:
> On 07/15/2012 09:35 PM, David Holmes wrote:
> > David M. Lloyd writes:
> >> On 07/15/2012 06:14 AM, David Holmes wrote:
> >>> David M. Lloyd writes:
> >>>> What is the purpose of the access-time access check in the
> >>>> atomic field updater classes?
> >>>
> >>> Do you mean the ensureProtectedAccess check?
> >>>
> >>> This is to ensure that you can't get an updater for a protected
> >>> inherited field in SubtypeA, then use that updater to
> >>> access/modify the field in an instance of SubtypeB.
> >>
> >> Which is ridiculous because if a subclass "inherited" access
> >> to a normal
> >> protected field on my class, I'd still be able to access it directly in
> >> the base class... because fields aren't actually inherited.
> >
> > I think you are missing the point. Consider this:
> >
> > class Base {
> >    protected volatile int x;
> > }
> >
> > final class Secure extends Base { ... }
> >
> > Instances of Secure are only supposed to be used through their
> public API,
> > as defined by Base and Secure. I hope you will agree that if I have an
> > instance of Secure I shouldn't be able to arbitrarily mess with
> the value of
> > x?
> >
> > But given
> >
> > class BackDoor extends Base {
> >     static AtomicIntegerFieldUpdater<Base>  u = ... (Base.class, "x");
> >
> >     public static void setBaseX(Base target, int newX) {
> >        u.set(target, newX);
> >     }
> > }
> >
> > without these protected access checks, BackDoor.setBaseX would
> allow you to
> > update _any_ instance of Base.
>
> I see your point, but in this case, wouldn't it be better to restrict
> updaters from being created on any class but their own, regardless of
> the access of the field?  In other words, we'd allow:
>
>     static AtomicIntegerFieldUpdater<BackDoor> u = ... (Backdoor.class,
> "x");
>
> but not:
>
>     static AtomicIntegerFieldUpdater<Base> u = ... (Base.class, "x");
>
> from BackDoor.

In hindsight, in the context of the current discussion, it may have been
better to restrict things this way to allow access checks to be elided. But
that has to be weighed up against use-cases where it is an inherited field
that needs the atomic update.

David
-----


> --
> - DML
>



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