快精灵印艺坊 您身边的文印专家
广州名片 深圳名片 会员卡 贵宾卡 印刷 设计教程
产品展示 在线订购 会员中心 产品模板 设计指南 在线编辑
 首页 名片设计   CorelDRAW   Illustrator   AuotoCAD   Painter   其他软件   Photoshop   Fireworks   Flash  

 » 彩色名片
 » PVC卡
 » 彩色磁性卡
 » 彩页/画册
 » 个性印务
 » 彩色不干胶
 » 明信片
   » 明信片
   » 彩色书签
   » 门挂
 » 其他产品与服务
   » 创业锦囊
   » 办公用品
     » 信封、信纸
     » 便签纸、斜面纸砖
     » 无碳复印纸
   » 海报
   » 大篇幅印刷
     » KT板
     » 海报
     » 横幅

如何准确实现多线程安全的singleton patterns


Singleton Pattern

Short Introduction











Singleton pattern, described in the GOF Design Patterns book, is one of the most easily understandable and on of the most frequently used pattern. The goal of the singleton pattern is to make sure that there is only one instance of this class in the system and allow other classes access this instance.







Case study







Today I tried to extend a exist MVC system in our project. Because only one instance of the Controller is needed in the system (typical Singleton pattern), a traditional singleton pattern was implemented here. The code is looked like:







public class Controller {







     private static Controller instance;







    public Controller()







    {







            // do something







    }

    public final static Controller getInstance(){ 




    public final static Controller getInstance(){ 




                      (A)







if (instance == null) {







                        (B)







            instance = new Controller();







        }







return instance;







    }







// some more methods







}   







The above shown code is, just like it described in almost all Design patterns books, is beautiful and correct based on the traditional implantation of singleton pattern.







Just at this time, our Team leader came to me, saw the implementation and told me at the first second that the traditional implementation is NOT thread safe! Yes! It is thread unsafe, after I read the code once again. Why? Let us make try: two threads T1 and T2 try to call the class Controller. When T1 goes to the position (B) in the above shown code, it sleeps. Then comes T2 to the position (A) and checks whether an instance of the Controller exists. Because T1 sleeps at the position (B) before an instance will be created, T2 can go into the block and create a new instance of Controller. Now T1 is awake, and what will it do? Just create another instance of Controller, because it is already in the block! So, great! TWO instance are created! It is no more singleton!







The first simple idea that I got in the first second, as you estimated, is adding the key word synchronized  before the method getInstance(). But this brings bad Performance.  As we knew, the normally used solution for such a problem is to moving the synchronizing into the method. That means to building a synchronized block in the method. The code should be looked like this:







public class Controller {







     private static Controller instance;







    public Controller()







    {







            // do something







    }







   







    public final static Controller getInstance(){ 







                        (A)







if (instance == null) {







                        (B)







            synchronized {







                        instance = new Controller();







            }







        }







return instance;







    }







 







// some more methods







}   







But This code is still thread unsafe! Why? Because it is only synchronized when an instance is created, the above described problem DOES still exist. T1 sleeps at (B), T2 goes through and create a new instance, T1 wakes up and create another one! If we move the synchronized block up to contain the whole if block, there is nothing difference with a synchronized method. So, synchronized is a bad idea.







We must find another way.







The Current most used  thread safe solution for this problem is to creating an instance of the class as class’s private property when the class is loaded. The instance can be accessed by calling the public getInstance() method. The correct code should be looked like:







 public class Controller {







     private static Controller instance = new Controller();







    public Controller()







    {







            // do something







    }







   







    public final static Controller getInstance(){ 







return instance;







    }















// some more methods







}   







One drawback of this solution is that an instance is created anyway, even if the instance will be never used.










返回类别: 教程
上一教程: 动态编译JAVA程序
下一教程: Java的秘密:使用全屏幕模式

您可以阅读与"如何准确实现多线程安全的singleton patterns"相关的教程:
· 编写线程安全的JSP程序
· Java开发中的线程安全选择与Swing[Z]
· 小心重写方式,准确实现多态
· java 线程安全
· java中如何实现表格的多表头显示
    微笑服务 优质保证 索取样品