I think you have a potential visibility problem here. You use -1 as the
initial value, but observing threads might see instead the default value
if the initializing write is not visible, and mistakenly think that the
zero default value represents a computed value. (This is different from
the trick employed by String.hashCode(), which does not use an
initializer. Can you get the same result using zero instead?)
The key to making the String.hashCode() trick work is that, while there
is definitely a data race, it is benign because the field can only ever
take on one non-default value, and that value is computed
deterministically from immutable state.
On 2/22/2011 7:29 PM, Mike Duigou wrote:
Daniel Aioanei reported via Josh Bloch a data race issue with the UUID accessor
and hashCode() methods. I've prepared a webrev with the suggested changes:
http://cr.openjdk.java.net/~mduigou/6611830/webrev.0/webrev/
I've tested the change against the standard UUID tests and did a microbenchmark
test of one method, variant(), to see what impact not caching had on
performance. Since there was only negligible change in performance vs. the
existing UUID implementation I am comfortable with eliminating the cache
values. It would appear that a field access plus a shift is not a significant
cost over a field access.
Mike
