Difference between revisions of "Singleton"
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+ | [[BTP600 | Go Back Pattern List]] | ||
==Introduction== | ==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. | 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. | ||
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*If an instance already exists, it simply returns a reference to that object. | *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. | *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]] |
Latest revision as of 09:52, 2 April 2007
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
Sample Code
Singleton pattern in C#:
// 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(); } } } }
References
http://en.wikipedia.org/wiki/Singleton_pattern
http://www.dofactory.com/Patterns/PatternSingleton.aspx
Go Back Pattern List