我的java DCL-based Singleton

1）instance完全创建好了，另一个线程得到的还是null：用volatile解决

2）对象内存块地址赋给了instance，但对象还没完全construct完毕：先访问一下对象，再赋instance,

class Singleton {private static volatile Singleton instance = null;private Singleton() {}public static Singleton getInstance() {if (instance == null) {synchronized (Singleton.class) {if (instance == null) {Singleton ins = new Singleton();/* ensure object has been fully constructed before assigned to instance * rule: in a thread, construction of an object -> access to the object. */ins.hashCode(); instance = ins;}}}return instance;}}