单件模式

SINGLETON（单件）

       几乎所有面向对象的程序中，总有一些类的对象需要是唯一的，例如，通过数据库句柄到数据库的连接是独占的。您希望在应用程序中共享数据库句柄，因为在保持连接打开或关闭时，它是一种开销。再如大家最经常用的IM,如QQ，在同一台电脑，一个帐号只能有唯一的登录。

1. 问题

怎样确保一个特殊类的实例是独一无二的（它是这个类的唯一实例），并且这个实例易于被访问呢？

2. 解决方案

1）全局变量：一个全局变量使得一个对象可以被访问，但它不能防止你实例化多个对象。因为你的任何代码都能修改全局变量，这将不可避免的引起更多调试的意外。换句话说，全局变量的状态总是会出现一些问题的。

2）类构造函数私有和类自身的静态方法：让类自身负责保存它的唯一实例(静态变量)。这个类可以保证没有其他实例可以被创建（通过截取创建新对象的请求） ，并且它可以提供一个访问该实例的方法（静态方法）。这就是Singleton模式。换句话说，创建实例的事情由我自己来做，不允许别人随便创建。

3. 适用性
在下面的情况下可以使用单件模式
1）当类只能有一个实例而且客户可以从一个众所周知的访问点访问它时。

2）当这个唯一实例应该是通过子类化可扩展的，并且客户应该无需更改代码就能使用一个扩展的实例时。

4. 实现：

UML结构：

public sealed class Singleton 
{  
    private static $_instance = null;//静态成员保存唯一实例  
    /** 
     * 私有无参构造函数，保证不能被外部访问 
     */  
    private Singleton() {}   
    /** 
     * 静态属性将创建这个实例的操作并保证只有一个实例被创建，也可以使用静态方法。
     * @return unknown 
     */  
    public static Singleton Instance
    {  
        get
        {
            if (instance == null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    } //使用静态方法可以创建带参数的实例 
}  

几个要点：

（1） 构造函数可以设置为protected，这样允许子类派生做扩展。

（2）一般不支持ICloneable接口，也不支持序列化操作。Clone和序列化反序列化会导致多个对象实例。

（3）模式没有考虑对象销毁管理。

（4）不改造不应用于多线程。同步问题可能导致多个对象实例。见6.

5. 效果

   Singleton模式有许多优点：
1)  对唯一实例的受控访问, 因为Singleton类封装它的唯一实例，所以它可以严格的控制客户怎样以及何时访问它。
2)  缩小名空间,Singleton模式是对全局变量的一种改进。它避免了那些存储唯一实例的全局变量污染名空间。
3)  允许对操作和表示的精化Singleton类可以有子类，而且用这个扩展类的实例来配置一个应用是很容易的。你可以用你所需要的类的实例在运行时刻配置应用。
4)  允许可变数目的实例 这个模式使得你易于改变你的想法，并允许Singleton类的多个实例。此外，你可以用相同的方法来控制应用所使用的实例的数目。只有允许访问 Singleton实例的操作需要改变。

6 .多线程单件模式

public sealed class Singleton 
{  
    private static object lockHelper = new object();
    private static volatile $_instance = null;//静态成员保存唯一实例  
    /** 
     * 私有构造函数，保证不能被外部访问 
     */  
    private Singleton() {}   
    /** 
     * 静态方法将创建这个实例的操作并保证只有一个实例被创建 
     * @return unknown 
     */  
    public static Singleton Instance
    {  
        get
        {
            if (instance == null)
            {
                lock(lockHelper)
                {
                    if (instance == null)//double check机制的使用，往后看
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }  
}  

7.另一种实现

public sealed class Singleton
{
    public static readonly Singleton instance = new Singleton();

    private Singleton()
    {}
}

等价于：

public sealed class Singleton
{
    public static readonly Singleton instance;
    //.NET运行库在第一次调用类成员之前执行静态构造函数，只执行一次，
    //在使用Singleton的实例之前，.NET保证先调用静态构造函数，
    //将实例初始化放到静态构造函数中，保证了实例使用时已经被建，
    //也保证了不使用Singleton实例的话，实例不会被创建，
    //同时，静态构造函数只能由一个线程执行。
    //缺陷：不能带参数，不过能用属性等办法实现。
    static Singleton()
    {
        instance = new Singleton();
    }

    private Singleton()
    {}
}

8.扩展

（1）创建固定数量n个实例，例如对象池的实现

单件模式并不是说一个类只能只有一个实例。假设我们使用在一个web 请求或者进程里面。一个用户id对应的某个类只能有唯一的实例。在下面的例子中，我们的User类，可以有多个实例，每个实例对应一个uid. 实例列表注册到静态变量$_instance并和uid关联起来。最简单的例子是我们前面提到的QQ，在同一台电脑，可以使用多帐号登录, 但一个帐号只能有唯一的登录.

9.应用

在zend framework中的Zend_Controller_Front前端控制器，就是采用单价模式来设计的：

Zend_Controller_Front是Zend_Controller_Controller体系的组织者，它是FrontController设计模式的实现。

Zend_Controller_Front处理服务器接受的所有请求，并最后负责将请求分配给ActionController(Zend_Controller_Action) 

    $frontController = Zend_Controller_Front::getInstance();  
    $frontController->addModuleDirectory( “参数”)；  
    $frontController->dispatch();  

--------------------------------------------------------------------------------------------------------------

另一篇博文，前面几种和上文大同小异，最后一种实现起来不太一样~

Implementing the Singleton Pattern in C#

The singleton pattern is one of the best-known patterns in software engineering. Essentially, a singleton is a class which only allows a single instance of itself to be created, and usually gives simple access to that instance. Most commonly, singletons don't allow any parameters to be specified when creating the instance - as otherwise a second request for an instance but with a different parameter could be problematic! (If the same instance should be accessed for all requests with the same parameter, the factory pattern is more appropriate.) This article deals only with the situation where no parameters are required. Typically a requirement of singletons is that they are created lazily - i.e. that the instance isn't created until it is first needed.

There are various different ways of implementing the singleton pattern in C#. I shall present them here in reverse order of elegance, starting with the most commonly seen, which is not thread-safe, and working up to a fully lazily-loaded, thread-safe, simple and highly performant version. Note that in the code here, I omit the private modifier, as it is the default for class members. In many other languages such as Java, there is a different default, and private should be used.

All these implementations share four common characteristics, however:

A single constructor, which is private and parameterless. This prevents other classes from instantiating it (which would be a violation of the pattern). Note that it also prevents subclassing - if a singleton can be subclassed once, it can be subclassed twice, and if each of those subclasses can create an instance, the pattern is violated. The factory pattern can be used if you need a single instance of a base type, but the exact type isn't known until runtime.
The class is sealed. This is unnecessary, strictly speaking, due to the above point, but may help the JIT to optimise things more.
A static variable which holds a reference to the single created instance, if any.
A public static means of getting the reference to the single created instance, creating one if necessary.
Note that all of these implementations also use a public static method GetInstance as the means of accessing the instance. In all cases, the method could easily be converted to a property with only an accessor, with no impact on thread-safety or performance.

First version - not thread-safe

public sealed class Singleton
{ 
      static Singleton instance=null; 
      Singleton() 
      { 
      }
      public static Singleton GetInstance() 
      {
            if (instance==null) 
            instance = new Singleton(); 
            return instance; 
      }
 } 

As hinted at before, the above is not thread-safe. Two different threads could both have evaluated the test if (instance==null) and found it to be true, then both create instances, which violates the singleton pattern. Note that in fact the instance may already have been created before the expression is evaluated, but the memory model doesn't guarantee that the new value of instance will be seen by other threads unless suitable memory barriers have been passed.

Second version - simple thread-safety

public sealed class Singleton
{ 
      static Singleton instance=null; 
      static readonly object padlock = new object(); 
      Singleton() 
      { 
      }
       public static Singleton GetInstance() 
      { 
            lock (padlock) 
            { 
                  if (instance==null) 
                  instance = new Singleton(); 
                  return instance;
             } 
      }
}

This implementation is thread-safe. The thread takes out a lock on a shared object, and then checks whether or not the instance has been created before creating the instance. This takes care of the memory barrier issue (as locking makes sure that all reads occur logically after the lock acquire, and unlocking makes sure that all writes occur logically before the lock release) and ensures that only one thread will create an instance (as only one thread can be in that part of the code at a time - by the time the second thread enters it,the first thread will have created the instance, so the expression will evaluate to false). Unfortunately, performance suffers as a lock is acquired every time the instance is requested.

Note that instead of locking on typeof(Singleton) as some versions of this implementation do, I lock on the value of a static variable which is private to the class. Locking on objects which other classes can access and lock on (such as the type) risks performance issues and even deadlocks. This is a general style preference of mine - wherever possible, only lock on objects specifically created for the purpose of locking, or which document that they are to be locked on for specific purposes (e.g. for waiting/pulsing a queue). Usually such objects should be private to the class they are used in. This helps to make writing thread-safe applications significantly easier.

Third version - attempted thread-safety using double-check locking

public sealed class Singleton
{ 
      static Singleton instance=null; 
      static readonly object padlock = new object(); 
      Singleton() 
      { 
      } 
      public static Singleton GetInstance() 
      { 
            if (instance==null) 
            { 
                  lock (padlock) 
                  {
                        if (instance==null) 
                        instance = new Singleton(); 
                  } 
            } 
            return instance;
      }
} 

This implementation attempts to be thread-safe without the necessity of taking out a lock every time. Unfortunately, there are four downsides to the pattern:

It doesn't work in Java. This may seem an odd thing to comment on, but it's worth knowing if you ever need the singleton pattern in Java, and C# programmers may well also be Java programmers. The Java memory model doesn't ensure that the constructor completes before the reference to the new object is assigned to instance. The Java memory model is going through a reworking for version 1.5, but double-check locking is anticipated to still be broken after this.
It almost certainly doesn't work in .NET either. Claims have been made that it does, but without any convincing evidence. Various people who are rather more trustworthy, however, such as Chris Brumme, have given convincing reasons why it doesn't. Given the other disadvantages, why take the risk? I believe it can be fixed by making the instance variable volatile, but that slows the pattern down more. (Of course, correct but slow is better than incorrect but broken, but when speed was one of the reasons for using this pattern in the first place, it looks even less attractive.) It can also be fixed using explicit memory barriers, but experts seem to find it hard to agree on just which memory barriers are required. I don't know about you, but when experts disagree about whether or not something should work, I try to avoid it entirely.
It's easy to get wrong. The pattern needs to be pretty much exactly as above - any significant changes are likely to impact either performance or correctness.
It still doesn't perform as well as the later implementations.
 
Fourth version - not quite as lazy, but thread-safe without using locks

public sealed class Singleton
{ 
      static readonly Singleton instance=new Singleton(); 
      // Explicit static constructor to tell C# compiler
      // not to mark type as beforefieldinit 
      static Singleton() 
      { 
      } 
      Singleton() 
      { 
      } 
      public static Singleton GetInstance() 
      { 
            return instance; 
      }
}

As you can see, this is really is extremely simple - but why is it thread-safe and how lazy is it? Well, static constructors in C# are specified to execute only when an instance of the class is created or a static member is referenced, and to execute only once per AppDomain. Given that this check for the type being newly constructed needs to be executed whatever else happens, it will be faster than adding extra checking as in the previous examples. There are a couple of wrinkles, however:

It's not as lazy as the other implementations. In particular, if you have static members other than GetInstance, the first reference to those members will involve creating the instance. This is corrected in the next implementation.
There are complications if one static constructor invokes another which invokes the first again. Look in the .NET specifications (currently section 9.5.3 of partition II) for more details about the exact nature of type initializers - they're unlikely to bite you, but it's worth being aware of the consequences of static constructors which refer to each other in a cycle.
The laziness of type initializers is only guaranteed by .NET when the type isn't marked with a special flag called beforefieldinit. Unfortunately, the C# compiler (as provided in the .NET 1.1 runtime, at least) marks all types which don't have a static constructor (i.e. a block which looks like a constructor but is marked static) as beforefieldinit. I now have a discussion page with more details about this issue. Also note that it affects performance, as discussed near the bottom of this article.

One shortcut you can take with this implementation (and only this one) is to just make instance a public static readonly variable, and get rid of the method entirely. This makes the basic skeleton code absolutely tiny! Many people, however, prefer to have a method in case further action is needed in future, and JIT inlining is likely to make the performance identical. (Note that the static constructor itself is still required if you require laziness.)

Fifth version - fully lazy instantiation

public sealed class Singleton
{ 
      Singleton() 
      { 
      } 
      public static Singleton GetInstance() 
      { 
            return Nested.instance; 
      } 
      class Nested 
      {
            // Explicit static constructor to tell C# compiler 
            // not to mark type as beforefieldinit 
            static Nested() 
            { 
            }
             internal static readonly Singleton instance = new Singleton(); 
      }
} 

Here, instantiation is triggered by the first reference to the static member of the nested class, which only occurs in GetInstance. This means the implementation is fully lazy, but has all the performance benefits of the previous ones. Note that although nested classes have access to the enclosing class's private members, the reverse is not true, hence the need for instance to be internal here. That doesn't raise any other problems, though, as the class itself is private. The code is a bit more complicated in order to make the instantiation lazy, however.

Performance vs laziness
In many cases, you won't actually require full laziness - unless your class initialization does something particularly time-consuming, or has some side-effect elsewhere, it's probably fine to leave out the explicit static constructor shown above. This can increase performance as it allows the JIT compiler to make a single check (for instance at the start of a method) to ensure that the type has been initialized, and then assume it from then on. If your singleton instance is referenced within a relatively tight loop, this can make a significant performance difference. You should decide whether or not fully lazy instantiation is required, and document this decision appropriately within the class. Conclusion
There are various different ways of implementing the singleton pattern in C#. The final two are generally best, as they are thread-safe, simple, and perform well. I would personally use the fourth implementation unless I had some other static members which really shouldn't trigger instantiation, simply because it's the simplest implementation, which means I'm more likely to get it right. The laziness of initialization can be chosen depending on the semantics of the class itself.