三种Singleton的实现方式

来源：http://melin.iteye.com/blog/838258

三种Singleton的实现方式，一种是用大家熟悉的DCL，另外两种使用cas特性来实现。

    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线程安全操作，实现单例模式
     *
     * @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();
            }
        }
    }