Re: Singleton Pattern

From:
Rajeev <rajeev.nospam@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 14 Aug 2011 06:37:57 -0700 (PDT)
Message-ID:
<00b9d6ee-ee12-4d87-8238-fdad501b0ec1@g5g2000prn.googlegroups.com>
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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