Re: Singleton Pattern

From:

Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 13 Aug 2011 16:43:15 -0700

Message-ID:

<vt-dnf5wc8CJkdrTnZ2dnUVZ_jKdnZ2d@posted.palinacquisition>

On 8/13/11 1:56 PM, vbhavsar@gmail.com wrote:

[...]
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?

It depends on what's legal.

For some kinds of singletons, it is not harmful to initialize the
instance multiple times. There's neither a performance nor interference
issue. For those kinds of singletons, your proposal is fine.

But for others, one of the reasons the class is a singleton in the first
place is that something bad will happen if more than one instance is
even created, never mind used. In those cases, the code you posted is
broken.

Frankly, there is rarely any need to be any more "clever" than to just
create the singleton instance in the static field initializer. Let the
JVM deal with the threading issues automatically and leave it at that.

I'd never even heard of the "single-element enum type" variation of
singleton initialization. That one in particular sounds like a
completely over-engineered approach.

Pete