A brief talk on design pattern: Single case mode (Singleton)

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What is a singleton mode?

• A class has and has only one instance, which is instantiated by the system itself and provided to the entire system through a global access point.

Why use singleton mode?

• Saves memory and does not need to instantiate an object every time it is used
• One instance provides global reuse
• In some environments, it is important to guarantee that a class has and only one instance, such as under Windows Task Manager.

How do I use Singleton mode?
A single example of the pattern of strange, all kinds of writing have, finally escaped the following three points:

• Have and have only one instance
• No manual creation, self-instantiation of the system

• Provides access to the entire system through a single global access point

  In addition, there may be different requirements in different environments: ensure thread safety, ensure serialization, do not produce multiple instances when deserializing, multiple instances of the ClassLoader when loaded, and so on.

The first implementation of:

publicclass Singleton {    privatestaticnull;    privateSingleton(){    }    publicstaticgetInstance(){         ifnull){             new Singleton();        }         return singleton ;    }}

 这一种单例的实例化是在需要用到时才进行实例化，但有明显的缺点：1、线程不够安全 2、每次使用都需要判断是否已经实例化，第一个缺点可以使用 synchronized 来声明 getInstance方法。第二个缺点可以使用第二种实现来避免。

The second implementation:

publicclass Singleton {    privatestaticnew Singleton();    privateSingleton(){    }    publicstaticgetInstance(){         return singleton ;    }}

 这一种单例的实例化基于classloader机制，避免了多线程安全的方式，同时也不需要在实例每一次使用时判断是否已经实例化，属于最常用的方式。类似实现还可以使用静态块来进行单例的实例化，不要累述。

The third implementation:

publicclass Singleton {     privatestaticclass SingletonHolder {          privatestaticnew Singleton();     }     privateSingleton (){}     publicstaticgetInstance() {     return SingletonHolder. INSTANCE;     } }

 这一种实现Singleton类被装载了，instance不一定被初始化。因为SingletonHolder类没有被主动使用，只有显示通过调用getInstance方法时，才会显示装载SingletonHolder类，从而实例化instance。

The fourth implementation:

publicenum  Singleton{    SINGLETON;     publicvoiddoWhatever(){};}

 这一种实现使用枚举来实现，能避免多线程同步问题，还能防止反序列化重复创建对象的问题，但由于枚举是在jdk1.5才加入的新特性，因此目前还很少看到有人在实际项目中使用。

The fifth implementation:

public   Class  Singleton {private  volatile  static  Singleton Singleton; private  singleton  () {} public  static  Singleton getsingleton  () {if  (singleton = =  Null ) {synchronized (Singleton. Class) {if  (Singleton =              = null ) {singleton = new  Singleton ();     }}} return  Singleton; } } 

has been abandoned the use of a writing, specific can be stamped ....
http://blog.csdn.net/kufeiyun/article/details/6166673

In addition, in reading Uncle Bob's article about Singleton and "create only one instance", he offers another way to do this:

publicclass   Singleton {     privateSingleton(){}     publicstaticnew Singleton());}publicclass   NonSingleton {     publicNonSingleton(Singleton singleton) {         null);    }     publicvoiddoWhatever(){    }}

 这一种实现主要是因为uncle bob 认为单例模式自身违背了SRP（单一职责原则）即一个类只做一件事的原则，而他的这种实现即是将单例实例化、以及单例做的事情分割开来。

About design mode: Singleton mode (Singleton)