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Re: Singleton Pattern

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On Aug 14, 1:56 am, "vbhav...@gmail.com" <vbhav...@gmail.com> wrote:
> People have been coming up with creative solutions to lazily implement
> the singleton pattern in a thread-safe way. We have seen things like
> double-checked locking and creating instance via a single-elemnt enum
> type.
>
> I have thought of yet another way to implement this in a lazy and
> thread-safe way. I haven't seen this proposed anywhere and it seems to
> work unless I am missing something. Here it goes:
>
> public class Singleton {
>
>         private static Singleton _instance;
>         private Singleton(){}
>
>         private synchronized static void createInstance(){
>                 _instance = new Singleton();
>         }
>
>         public static Singleton getInstance(){
>                 if (_instance == null){
>                         createInstance();
>                 }
>                 return _instance;
>         }
>
> }
>
> The synchronized createInstance() method would eliminate the need to
> do double-checked locking and the synchronization would happen only
> when multiple threads call getInstance() before _instance has been
> instantiated.
>
> Anyone see any issues with this?

As clearly pointed out by others, this code is far more vulnerable to
create duplicate instances than the double checked locking algorithm
itself. I think you have misunderstood the fundamental cause of the
problem here. Even I had confused myself (twice) before getting to
understand the solution. See [The "Double-Checked Locking is Broken"
Declaration] below

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

-Rajeev Sreedharan

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Singleton Pattern "vbhavsar@gmail.com" <vbhavsar@gmail.com> - 2011-08-13 13:56 -0700
  Re: Singleton Pattern markspace <-@.> - 2011-08-13 14:41 -0700
  Re: Singleton Pattern Patricia Shanahan <pats@acm.org> - 2011-08-13 14:48 -0700
  Re: Singleton Pattern Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-08-13 16:43 -0700
    Re: Singleton Pattern Ian <m4r35n357@gmail.com> - 2011-08-15 15:00 +0100
      Re: Singleton Pattern Lew <lewbloch@gmail.com> - 2011-08-15 07:56 -0700
        Re: Singleton Pattern Patricia Shanahan <pats@acm.org> - 2011-08-15 22:07 -0700
          Re: Singleton Pattern Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-08-16 00:09 -0700
            Re: Singleton Pattern Patricia Shanahan <pats@acm.org> - 2011-08-16 06:15 -0700
              Re: Singleton Pattern Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-08-16 06:57 -0700
                Re: Singleton Pattern Patricia Shanahan <pats@acm.org> - 2011-08-16 09:25 -0700
                Re: Singleton Pattern Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-08-16 18:10 -0700
  Re: Singleton Pattern Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-13 20:56 -0400
    Re: Singleton Pattern Lew <lewbloch@gmail.com> - 2011-08-13 21:12 -0700
  Re: Singleton Pattern Rajeev <rajeev.nospam@gmail.com> - 2011-08-14 06:37 -0700

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