On 28/09/2009, sebb <seb...@gmail.com> wrote: > On 28/09/2009, Stephen Colebourne <scolebou...@btopenworld.com> wrote: > > sebb wrote: > > > > > Where is that described? The first reference I found says that the > > > local variable is a performance optimisation; it's the volatile > > > keyword that makes it thread-safe: > > > > > > > > http://technology.amis.nl/blog/4384/the-double-checked-locking-confusion > > > also > > > > > http://jeremymanson.blogspot.com/2008/05/double-checked-locking.html > > > > > > But of course these could be wrong... > > > > > > > > http://java.sun.com/developer/technicalArticles/Interviews/bloch_effective_08_qa.html > > > > "The idiom is very fast but also complicated and delicate, so don't be > > tempted to modify it in any way" > > > > > That does not necessarily mean that the version without the temporary > variable is wrong. > > As I see it, the fact that the variable is volatile is enough to > ensure as-sequential operation, because the write of the volatile > variable happens-before the read. >
I've just discovered that the book also says: "This code may appear a bit convoluted. In particular, the need for the local variable <result> may be unclear ... While not strictly necessary this may improve performance ..." So the local variable is indeed for performance reasons - it reduces the number of reads of the volatile variable to once in the case where the variable has already been initialised. > > Stephen > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: dev-unsubscr...@commons.apache.org > > For additional commands, e-mail: dev-h...@commons.apache.org > > > > > --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@commons.apache.org For additional commands, e-mail: dev-h...@commons.apache.org
