Android - 单例模式线程安全

https://blog.csdn.net/Mars_idea/article/details/80724404

https://blog.csdn.net/cselmu9/article/details/51366946

https://blog.csdn.net/huachao1001/article/details/51536074

https://zhuanlan.zhihu.com/p/28491630

https://www.hollischuang.com/archives/1144

https://juejin.im/post/5a3d2a6c5188257d6a7ec055