https://imaginebrain.github.io/2017/02/05/%E5%8D%95%E4%BE%8B%E6%A8%A1%E5%BC%8F%E5%8F%8C%E9%87%8D%E6%A3%80%E6%9F%A5%E4%B8%ADvolatile%E4%BD%9C%E7%94%A8.html volatile作用 volatile可以保证变量和可见性，让修饰的变量直接从主存中获取值； volatile可以防止指令重排序
https://imaginebrain.github.io/2017/02/05/%E5%8D%95%E4%BE%8B%E6%A8%A1%E5%BC%8F%E5%8F%8C%E9%87%8D%E6%A3%80%E6%9F%A5%E4%B8%ADvolatile%E4%BD%9C%E7%94%A8.html