Re: Singleton Question

From: Chris Smith (cdsmith_at_twu.net)
Date: 06/30/04 

Date: Wed, 30 Jun 2004 07:07:46 -0600

Dave Monroe wrote:
> Can somebody point me to a spec for singleton implementation?

Singleton is a pattern, not a specification. There is a description of
the general pattern on page 127 of GoF. In general, Java
implementations use the static initialization time feature to avoid
overhead when retrieving an object, as such:

public class MySingletonClass
{
    private static MySingletonClass instance = new MySingletonClass();

    public static MySingletonClass getInstance()
    {
        return instance;
    }

    private MySingletonClass() { }

    ...
}

Unlike similar-looking code in other languages, this will *not* create
an instance of MySingletonClass until the first time getInstance is
called (or the first time MySingletonClass is loaded directly from a
classloader with an explicit request to initialize it). Essentially,
lazy initialization is provided for you as a feature.

> I'm working with an application that has a couple of singletons and
> obviously the member variables are kept separate in the threads that
> use the singleton, but I can't find any documentation that explains
> the rules.

What you're describing doesn't really sound like a Singleton at all.
Singletons are shared objects, and are intended that way. They
generally have little or no state at all, and thread-local state is
certainly not a part of the pattern. 

-- 
www.designacourse.com
The Easiest Way to Train Anyone... Anywhere.
Chris Smith - Lead Software Developer/Technical Trainer
MindIQ Corporation

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