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From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Singleton Pattern
Date: Sat, 13 Aug 2011 20:56:12 -0400
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On 8/13/2011 4:56 PM, vbhavsar@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?

     Yes.

	T1: if (_instance == null)
	    "Aha! It's null! Let's go make one."

	** context switch **

	T2: if (_instance == null)
	    "Aha! It's null! Let's go make one."

	T2: _instance = createInstance();  // instance #1

	** context switch **

	T1: _instance = createInstance();  // instance #2

... and the two threads go merrily on their way with references
to two different Singleton instances.  With N threads, you could
get as many as N distinct instances.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid