系统变量和环境变量：https://blog.xiayf.cn/2019/06/25/java-prop-env/
读取配置文件；https://my.oschina.net/u/4332949/blog/3449852
值传递还是引用传递：
https://blog.csdn.net/w372426096/article/details/82216742
https://blog.csdn.net/javazejian/article/details/51192130
