三種Singleton的實現方式

來源：http://melin.iteye.com/blog/838258

三種Singleton的實現方式，一種是用大家熟悉的DCL，另外兩種使用cas特性來實現。

    public class LazySingleton {     
        private static volatile LazySingleton instance;     
             
        public static LazySingleton getInstantce() {     
            if (instance == null) {     
                synchronized (LazySingleton.class) {     
                    if (instance == null) {     
                        instance = new LazySingleton();     
                    }  
                }     
            }     
            return instance;     
        }     
    }    

 

    /** 
     * 利用putIfAbsent執行緒安全操作，實現單例模式 
     *  
     * @author Administrator 
     *  
     */  
    public class ConcurrentSingleton {  
        private static final ConcurrentMap<String, ConcurrentSingleton> map = new ConcurrentHashMap<String, ConcurrentSingleton>();  
        private static volatile ConcurrentSingleton instance;  
      
        public static ConcurrentSingleton getInstance() {  
            if (instance == null) {  
                instance = map.putIfAbsent("INSTANCE", new ConcurrentSingleton());  
            }  
            return instance;  
        }  
    }  

 

    public class AtomicBooleanSingleton {  
        private static AtomicBoolean initialized = new AtomicBoolean(false);  
        private static volatile AtomicBooleanSingleton instance;  
          
        public static AtomicBooleanSingleton getInstantce() {     
            checkInitialized();  
            return instance;     
        }  
          
        private static void checkInitialized() {  
            if(instance == null && initialized.compareAndSet(false, true)) {  
                instance = new AtomicBooleanSingleton();  
            }  
        }  
    }