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Re: Parametric Initialization On DemandHolder Idiom ?

by Hanson Char :: Rate this Message:

>On 9/5/07, Beverly Sanders <sanders@...> wrote:
> Most of the (interesting) discussion of this example has focused on
> the value of f that can be seen when a reference is leaked in the
> constructor. But suppose we got rid of badHolder and eliminated that
> problem. Wouldn't this program have a race on goodHolder
> itself unless something is done to create hb edges?

Yes, this program would have a data race.

>For example,
> by making goodHolder volatile, or ensuring that goodHolder is assigned
> by some thread before other threads that read it are started, or
> otherwise "safely publish" the reference? But if we still need
> safe publication, then the special semantics for final fields don't
> add anything. What am I missing?

True, but we don't need safe publication for (correctly constructed)
immutable object in the first place.

> In this example, safe publication is needed (it is safe when
> goodHolder is volatile)

It is safe publication in your example (with the volatile) but it is
not needed in this case, given FinalHolder is immutable and is
correctly/safely constructed.

Cheers,
Hanson Char

>
> class FinalHolder {
> static [volatile] FinalHolder goodHolder;
>
> final int f;
> FinalHolder() {
> f = 1;
> }
> void printFinal() {
> System.out.println(f);
> }
>
>
> Thread1:
> Thread2.start();
> goodHolder = new FinalHolder();
>
> Thread2:
> while(goodHolder==null){}
> //termination guaranteed only if goodHolder volatile
> goodHolder.print();
> //if thread gets here, it will print 1 due to
> //correct construction (no leak of ref from consructor)
> //and final field semantics.
> //But if goodHolder volatile, f doesn't need to be
> //final except to ensure that the value is not modified--special
> //treatment in the memory model is not needed.
>
>
>
> Thanks,
> Beverly Sanders

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