Use of Double Check Lock and Singleton Pattern in Orbeon

As I'm browsing around the source trying to learn my way around I ran
across the OXFClassLoader class. I didn't make it pass the first
method in the source when I ran into the "Double Check Lock" idiom.
If your wondering why I'm commenting about it then please read the
following two articles about the use of this broken java idiom:

http://www-128.ibm.com/developerworks/java/library/j-dcl.html
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

I'd be interested to hear the main developers perspective on this idiom.

-Brian

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Brian,

Interesting (although the authors of the second link deserve death for
not putting a date on the paper ;-). I didn't have time to read it all,
but this is what the first link says:

"The issue of the failure of double-checked locking is not due to
implementation bugs in JVMs but to the current Java platform memory
model. The memory model allows what is known as "out-of-order writes"
and is a prime reason why this idiom fails."

Now it looks like JSR-133 was meant to address some related issues, and
that these articles are pre-JSR-133:

   http://www.jcp.org/en/jsr/detail?id=133

However I read: "There exist a number of common but dubious coding
idioms, such as the double-checked locking idiom, that are proposed to
allow threads to communicate without synchronization. Almost all such
idioms are invalid under the existing semantics, and are expected to
remain invalid under the proposed semantics."

So still no luck even with JDK 1.5? But I also found this:

   http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl

This seems to say that using a "volatile" variable, things will work,
but then goes on to recommend another, simpler pattern.

So I guess the bottom line is that we shouldn't use this pattern without
using "volatile" and JDK 1.5, or simply not use this pattern at all!

-Erik

Brian Weaver wrote: --
Orbeon - XForms Everywhere:
http://www.orbeon.com/blog/



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