Singleton

Definition

Ensure a class has only one instance, and provide a global point of access to it. [GoF, p127]
Encapsulated "just-in-time initialization" or "initialization on first use".

UML class diagram

Participants

The classes and/or objects participating in this pattern are :

Singleton defines an Instance operation that lets clients access its unique instance. Instance is a class operation. responsible for creating and maintaining its own unique instance.

Sample code

This code demonstrates the Singleton pattern which assures only a single instance (the singleton) of the class can be created.

Java:

public class Singleton {
     private static Singleton _instance= null;

     /**
      * La présence d'un constructeur privé supprime
      * le constructeur public par défaut.
      */
     private Singleton() {}

     /**
      * Le mot-clé synchronized sur la méthode de création
      * empêche toute instanciation multiple même par
      * différents threads.
      * Retourne l'instance du singleton.
      */
     public synchronized static Singleton getInstance() {
         if (_instance== null)
             _instance= new Singleton();
         return _instance;
     }
 }

C#:

// --------------------------------------------------------
//
//  Creational Patterns : Singleton
//
// --------------------------------------------------------

using System;

namespace DesignPatterns
{

    /// <summary>
    /// The singleton class.
    /// A single instance of the class can be created
    /// </summary>
    public class Singleton
    {
        /// <summary>
        ///
        /// </summary>
        private static Singleton instance = null;

        /// <summary>
        /// Private Constructor
        /// </summary>
        private Singleton()
        {
        }

        /// <summary>
        /// gets the singleton instance
        /// </summary>
        /// <returns>The singleton instance</returns>
        public static Singleton Instance()
        {
            // 'Lazy initialization'
            if (instance == null)
            {
                instance = new Singleton();
            }

            return instance;
        }
    } // end class

    /// <summary>
    /// Testing class
    /// </summary>
    public class SingletonTest
    {

        public static void Main()
        {
            // Getting references to the Singleton
            Singleton s1 = Singleton.Instance();
            Singleton s2 = Singleton.Instance();

            if (s1 == s2)
            {
                Console.WriteLine("References are the same. The objects share the same instance");
            }

            // Wait for user
            Console.Read();
        }
    } // end class

}