[[BTP600 | Go Back Pattern List]]
==Introduction==
Singleton pattern Ensure a class only has one instance, and provide a global point of access to it. This is useful when exactly one object is needed to coordinate actions across the system.
==Implementation==
**Singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist.**If an instance already exists, it simply returns a reference to that object.**To make sure that the object cannot be instantiated any other way, the constructor is made either private or protected. ==UML Diagram==[[Image:Singleton_UML.png]] ==Sample Code==Singleton pattern in C#:<pre>// Singleton pattern -- Real World example using System;using System.Collections;using System.Threading; namespace DoFactory.GangOfFour.Singleton.RealWorld{ // MainApp test application class MainApp { static void Main() { LoadBalancer b1 = LoadBalancer.GetLoadBalancer(); LoadBalancer b2 = LoadBalancer.GetLoadBalancer(); LoadBalancer b3 = LoadBalancer.GetLoadBalancer(); LoadBalancer b4 = LoadBalancer.GetLoadBalancer(); // Same instance? if (b1 == b2 && b2 == b3 && b3 == b4) { Console.WriteLine("Same instance\n"); } // All are the same instance -- use b1 arbitrarily // Load balance 15 server requests for (int i = 0; i < 15; i++) { Console.WriteLine(b1.Server); } // Wait for user Console.Read(); } } // "Singleton" class LoadBalancer { private static LoadBalancer instance; private ArrayList servers = new ArrayList(); private Random random = new Random(); // Lock synchronization object private static object syncLock = new object(); // Constructor (protected) protected LoadBalancer() { // List of available servers servers.Add("ServerI"); servers.Add("ServerII"); servers.Add("ServerIII"); servers.Add("ServerIV"); servers.Add("ServerV"); } public static LoadBalancer GetLoadBalancer() { // Support multithreaded applications through // 'Double checked locking' pattern which (once // the instance exists) avoids locking each // time the method is invoked if (instance == null) { lock (syncLock) { if (instance == null) { instance = new LoadBalancer(); } } } return instance; } // Simple, but effective random load balancer public string Server { get { int r = random.Next(servers.Count); return servers[r].ToString(); } } }} </pre> ==References==http://en.wikipedia.org/wiki/Singleton_pattern<br/>http://www.dofactory.com/Patterns/PatternSingleton.aspx<br/><br/>[[BTP600 | Go Back Pattern List]]