JAVA存取对象属性时，如果开程多线程，记得对相关存取方法作原子化操作定义

最显著的应用当然是银行存款和取款，不要存在存取数字和实际发生不一样的情况。

ryan is about to withdraw

ryan is going to sleep

ryan woke up.

ryan completes the withdrawl

sorry, not enough for monica