设计模式之单键型模式

Singleton Design Pattern

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definition Ensure a class has only one instance and provide a global point of access to it. Frequency of use:   medium high

UML class diagram

participants

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

• Singleton   (LoadBalancer)
  • 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 in C#

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

// Singleton pattern -- Structural example

using System; namespace DoFactory.GangOfFour.Singleton.Structural {       // MainApp test application    class MainApp   {           static void Main()     {        // Constructor is protected -- cannot use new       Singleton s1 = Singleton.Instance();       Singleton s2 = Singleton.Instance();        if (s1 == s2)       {         Console.WriteLine("Objects are the same instance");       }        // Wait for user       Console.Read();     }   }    // "Singleton"    class Singleton   {      private static Singleton instance;      // Note: Constructor is 'protected'      protected Singleton()     {     }      public static Singleton Instance()     {        // Use 'Lazy initialization'        if (instance == null)       {         instance = new Singleton();       }        return instance;     }   } }

Output Objects are the same instance

// Singleton pattern -- Structural example

using System; namespace DoFactory.GangOfFour.Singleton.Structural {       // MainApp test application    class MainApp   {           static void Main()     {        // Constructor is protected -- cannot use new       Singleton s1 = Singleton.Instance();       Singleton s2 = Singleton.Instance();        if (s1 == s2)       {         Console.WriteLine("Objects are the same instance");       }        // Wait for user       Console.Read();     }   }    // "Singleton"    class Singleton   {      private static Singleton instance;      // Note: Constructor is 'protected'      protected Singleton()     {     }      public static Singleton Instance()     {        // Use 'Lazy initialization'        if (instance == null)       {         instance = new Singleton();       }        return instance;     }   } }

Output Objects are the same instance

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This real-world code demonstrates the Singleton pattern as a LoadBalancing object. Only a single instance (the singleton) of the class can be created because servers may dynamically come on- or off-line and every request must go throught the one object that has knowledge about the state of the (web) farm.

// 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();       }     }   } }

Output Same instance ServerIII ServerII ServerI ServerII ServerI ServerIII ServerI ServerIII ServerIV ServerII ServerII ServerIII ServerIV ServerII ServerIV