并发编程陷阱系列(五)double check

public static Singleton getInstance()
{
  if (instance == null)
  {
    synchronized(Singleton.class) {  //1
      if (instance == null)          //2
        instance = new Singleton();  //3
    }
  }
  return instance;
}

The theory behind double-checked locking is perfect. Unfortunately, reality is entirely different. The problem with double-checked locking is that there is no guarantee it will work on single or multi-processor machines. 

 

推荐的做法：

    public class Singleton {  
        /**  
         * 类级的内部类，也就是静态的成员式内部类，该内部类的实例与外部类的实例  
         * 没有绑定关系，而且只有被调用到时才会装载，从而实现了延迟加载  
         */  
        private static class SingletonHolder{  
            /**  
             * 静态初始化器，由JVM来保证线程安全  
             */  
            private static Singleton instance = new Singleton();  
        }  
        /**  
         * 私有化构造方法  
         */  
        private Singleton(){  
        }  
        public static  Singleton getInstance(){  
            return SingletonHolder.instance;  
        }  
    } 

 参考：http://www.ibm.com/developerworks/java/library/j-dcl/index.html