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From sebb <seb...@gmail.com>
Subject Re: svn commit: r819141 - in /commons/proper/lang/trunk/src: java/org/apache/commons/lang/concurrent/LazyInitializer.java test/org/apache/commons/lang/concurrent/ test/org/apache/commons/lang/concurrent/LazyInitializerTest.java
Date Mon, 28 Sep 2009 17:26:18 GMT
On 28/09/2009, sebb <sebbaz@gmail.com> wrote:
> On 28/09/2009, Stephen Colebourne <scolebourne@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
>  >
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