.Net 单例模式(Singleton)

 

目录

1. 定义
2. UML
3. 参与者
4. 示例

定义：

单例模式的意思就是只有一个实例。单例模式确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。这个类称为单例类。

 

应用：

每台计算机可以有若干个打印机，但只能有一个Printer Spooler， 以避免两个打印作业同时输出到打印机中。每台计算机可以有若干传真卡，但是只应该有一个软件负责管理传真卡，以避免出现两份传真作业同时传到传真卡中的情况。每台计算机可以有若干通信端口，系统应当集中管理这些通信端口，以避免一个通信端口同时被两个请求同时调用。

 

UML:

 

参与者：

Singleton 角色：定义一个实例操作，同一段时间只能被一个客户访问。

 

示例：

代码using System;

 

namespace SingletonPattern

{

  /// <summary>

  /// Singleton Design Pattern.

  /// </summary>

  class MainApp

  {

    static void Main()

    {

      // Constructor is protected -- cannot use new

      Singleton s1 = Singleton.Instance();

      Singleton s2 = Singleton.Instance();

 

      // Test for same instance

      if (s1 == s2)

      {

        Console.WriteLine("同一个实例");

      }

 

      Console.ReadKey();

    }

  }

 

  class Singleton

  {

    private static Singleton _instance;

 

    // Constructor is 'protected'

    protected Singleton()

    {

    }

 

    public static Singleton Instance()

    {

      // Uses lazy initialization.

      // Note: this is not thread safe.

      if (_instance == null)

      {

        _instance = new Singleton();

      }

 

      return _instance;

    }

  }

}

实际运用：

代码// Singleton pattern -- Real World example



using System;

using System.Collections.Generic;

using System.Threading;

 

namespace DoFactory.GangOfFour.Singleton.RealWorld

{

  /// <summary>

  /// MainApp startup class for Real-World

  /// Singleton Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    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");

      }

 

      // Load balance 15 server requests

      LoadBalancer balancer = LoadBalancer.GetLoadBalancer();

      for (int i = 0; i < 15; i++)

      {

        string server = balancer.Server;

        Console.WriteLine("Dispatch Request to: " + server);

      }

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Singleton' class

  /// </summary>

  class LoadBalancer

  {

    private static LoadBalancer _instance;

    private List<string> _servers = new List<string>();

    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();

      }

    }

  }

}