No2 单例模式 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 class No2TwiceCheckSingleton { private volatile static No2TwiceCheckSingleton instance = null; private static final Object sybObj = new Object(); // 一定记住要私有化构造器，不然人家还是能够创建 private No2TwiceCheckSingleton() { }