[concurrency-interest] Visibility of array initialization (Raph Frank)

Valentin Kovalenko valentin.male.kovalenko at gmail.com
Sun Aug 12 16:56:46 EDT 2018


Vladimir,

Yes, I have missed this part :( Thank you! This is a good reminder of how
careful one should be when thinking about which writes are there, and which
writes are allowed to be observed.

Regards,
Valentin
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On Sun, 12 Aug 2018 at 14:36, Vladimir Sitnikov <sitnikov.vladimir at gmail.com>
wrote:

> Valentin>Having said that, the anwser to your question "Is it possible
> for the 2nd thread to print zero?  Is it guaranteed to print -1 or 2?" is:
> The second thread is allowed to print either -1 or 3 and nothing else (NPE
> is also not allowed).
>
> I would argue there.
>
> TL;DR: -1, 0, 3 are allowed. NPE is forbidden.
>
>
> 1) NPE is not allowed (since there's only one write of null to `array`
> field which is implicit write of default value).
>
> 2) 0 is also allowed since Thread 2 could see updated value of `array`
> (that is it could see the reference to the array that was set after the
> thread started).
> Apparently, there are "implicit writes of 0 values of the elements in the
> new array", and nothing prevents Thread 2 from observing those writes.
>
> I'm not a lawyer, but
> https://docs.oracle.com/javase/specs/jls/se6/html/expressions.html#15.10.1
> seems to address that:
>
> JLS6> and each component of the array is initialized to its default value
> (ยง4.12.5).
>
> In other words, new int[] {1, 2, 3}; does not mean the array is created
> "atomically".
> It is equivalent to the   x=new int[3]; x[0]=1; x[1]=2; x[2]=3;
> Obviously that can cause Thread2 to observe 0 as well.
>
>
> Vladimir
>
>
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