[concurrency-interest] The Atomic*FieldUpdater situation

Doug Lea dl at cs.oswego.edu
Sat Jul 14 14:18:04 EDT 2012

On 07/14/12 14:01, David M. Lloyd wrote:
> What is the purpose of the access-time access check in the atomic field updater
> classes?

It is because there is no way to check that you haven't handed
your Updater to some untrusted party, so the caller context must
be checked on each use. I agree it is very annoying and slow.

> I mean I'm using field updaters in the first place because I can't afford to
> have a jillion Atomic* objects floating around; obviously performance is
> critical here. And to add to this problem, you can't even rely on using Unsafe,
> even though it's becoming more and more prevalent in JDKs, as some platforms may
> not have atomic 64-bit operations, and there's no way I can see to test for that
> other than relying on the AtomicLong implementation to be similar to OpenJDK's.

We stopped using Updaters entirely inside j.u.c as of JDK7 and
use straight Unsafe calls without even checking for 64bit atomics.
Which means that we are relying on every JDK7+ VM to somehow
implement the 64bit version of Unsafe CAS. So if you are targeting JDK7+
only, you might take some comfort that if j.u.c cannot run, then it
probably doesn't matter if your code runs, and so don't worry about
the checks :-)

> Is there any recourse to solve this issue?

Maybe someday. Remi Forax has mentioned a few times that JDK8 method
handle and invokeDynamic support should make this or some variant
API a lot faster. We'll see...


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