Java singleton 单例

饿汉式，instance在类加载化时完成初始化，线程安全

package cookie;
 
public class SingletonAtOnce {
    private SingletonAtOnce() {
    }
 
    private static final SingletonAtOnce instance = new SingletonAtOnce();
 
    public static SingletonAtOnce getInstance() {
        return instance;
    }
}

懒汉式，双锁检查机制保证线程安全

package cookie;
 
public class SingletonDCL {
    private SingletonDCL() {
    }
 
    private static volatile SingletonDCL instance;
 
    public static SingletonDCL getInstance() {
        if (instance == null) {
            synchronized (SingletonDCL.class) {
                if (instance == null) {
                    instance = new SingletonDCL();
                }
            }
        }
        return instance;
    }
}

私有内部类持有，也属于懒汉式，读ClassHolder中的instance时，导致ClassHolder加载，加载时完成instance的初始化，线程安全

package cookie;
 
public class SingletonClassHolder {
    private SingletonClassHolder() {
    }
 
    private static class Holder {
        public static SingletonClassHolder instance = new SingletonClassHolder();
    }
 
    public static SingletonClassHolder getInstance() {
        return Holder.instance;
    }
}