.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：

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：

// 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();
      }
    }
  }
}