设计模式：单件模式

2009-11-26 11:43 敏捷的水阅读(508) 评论(0) 编辑收藏举报

Singleton模式要求一个类有且仅有一个实例，并且提供了一个全局的访问点。

1. 单线程时方法

public sealed class Singlton
{
    static Singlton instance = null;
    Singlton()
    { }
 
    public static Singlton Instance
    {
        get
        {
            if (instance == null)
            {
                return new Singlton();
            }
            return instance;
        }    
    }    
}

这句if (instance == null)不是线程安全的，可能产生多个实例。

2.线程安全的

public sealed class Singlton
{
    static Singlton instance = null;
 
    static readonly object o = new object();
 
    Singlton()
    { }
 
    public static Singlton Instance
    {
        get
        {
            lock (o)
            {
                if (instance == null)
                {
                    return new Singlton();
                }
                return instance;
            }
        }    
    }    
}

对象实例由最先进入的那个线程创建，后来的线程在进入时（instence == null）为假，不会再去创建对象实例了。但是这种实现方式增加了额外的开销，损失了性能。

3. 双重锁定

public sealed class Singlton
{
    static Singlton instance = null;
 
    static readonly object o = new object();
 
    Singlton()
    { }
 
    public static Singlton Instance
    {
        get
        {
            if (instance == null)
            {
                lock (o)
                {
                    if (instance == null)
                    {
                        return new Singlton();
                    }
                }
            }
            return instance;
        }
    }
}

避免了每个 Instance 属性方法的调用中都出现独占锁定。

4. 静态初始化

public sealed class Singlton
{
    static readonly Singlton instance = new Singlton();
 
    static Singlton()
    { }
 
    public static Singlton Instance
    {
        get
        {
            return instance;
        }
    }
}

5. 延迟静态初始化

public sealed class Singlton
{       
 
    static Singlton()
    { }
     
    public static Singlton Instance
    {
        get
        {
            return CreateSinglton.instance;
        }
    }
     
    class CreateSinglton
    {
        internal static readonly Singlton instance = new Singlton();
     
        static CreateSinglton() { }
    }
}