[concurrency-interest] synchronized constructors

Zhong Yu zhong.j.yu at gmail.com
Fri Dec 16 14:10:25 EST 2011


On Fri, Dec 16, 2011 at 8:38 AM, Yuval Shavit <yshavit at akiban.com> wrote:
> Several people have made the claim that you could see partially initialized
> state even if the constructor were synchronized, and I don't see how this
> could be. Yes, you could assign MyPoint myUnsafeRef before MyPoint()
> finishes -- but if you ever tried to *use* myUnsafeRef, you would run into a
> synchronized block which would then ensure that the constructor
> happened-before whatever method you're writing. Seems to me you should see
> the fully-thread-safe, not-partially-initialized object at that point.

If the reference is unsafely published, another thread can get the
reference early; it then calls an instance method which may obtain the
lock before the creation thread can obtain the lock for the
constructor. Therefore the other thread can observe the blank state.
As Ruslan corrected me, no partial state can be observed though.

Zhong Yu


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