三种Singleton的实现方式

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();
        }
    }
}