意图
保证一个类只有一个实例，并提供访问它的全局访问点。
场景
我们现在要做一个网络游戏的服务端程序，需要考虑怎么样才能承载大量的用户。在做WEB程序的时候有各种负载均衡的方案，不管是通过硬件实现还是软件实现，基本的思想就是有一个统一的入口，然后由它来分配用户到各个服务器上去。
需要考虑的问题是，即使在多线程的并发状态下，用户只能通过一个唯一的入口来分配，由此引入了Singleton模式来实现这个唯一的入口。
示例代码
using System;
using System.Collections.Generic;
using System.Threading;
namespace SingletonExample
{
class Program
{
static void Main(string[] args)
{
ParameterizedThreadStart ts = new ParameterizedThreadStart(EnterPlayer);
for (int i = 0; i < 20; i++)
{
Thread t = new Thread(ts);
t.Start("player" + i);
}
LoadBalanceServer.GetLoadBalanceServer().ShowServerInfo();
}
static void EnterPlayer(object playerName)
{
LoadBalanceServer lbs = LoadBalanceServer.GetLoadBalanceServer();
lbs.GetLobbyServer().EnterPlayer(playerName.ToString());
}
}
class LoadBalanceServer
{
private const int SERVER_COUNT = 3;
private List<LobbyServer> serverList = new List<LobbyServer>();
private static volatile LoadBalanceServer lbs;
private static object syncLock = new object();
private LoadBalanceServer()
{
for (int i = 0; i < SERVER_COUNT; i++)
{
serverList.Add(new LobbyServer("LobbyServer" + i));
}
}
public static LoadBalanceServer GetLoadBalanceServer()
{
if (lbs == null )
{
lock (syncLock)
{
if (lbs == null )
{
Thread.Sleep(100);
lbs = new LoadBalanceServer();
}
}
}
return lbs;
}
public LobbyServer GetLobbyServer()
{
LobbyServer ls = serverList[0];
for (int i = 1; i < SERVER_COUNT; i++)
{
if (serverList[i].PlayerList.Count < ls.PlayerList.Count)
ls = serverList[i];
}
return ls;
}
public void ShowServerInfo()
{
foreach (LobbyServer ls in serverList)
{
Console.WriteLine("=================" + ls.ServerName + "=================");
foreach (string player in ls.PlayerList)
{
Console.WriteLine(player);
}
}
}
}
class LobbyServer
{
private List<string> playerList = new List<string>();
public List<string> PlayerList
{
get { return playerList; }
}
private string serverName;
public string ServerName
{
get { return serverName; }
}
public LobbyServer(string serverName)
{
this.serverName = serverName;
}
public void EnterPlayer(string playerName)
{
playerList.Add(playerName);
}
}
}