javac .java——>编译成.class文件(字节码)
参考:
https://www.cnblogs.com/yxwkf/p/3855363.html
https://www.jianshu.com/p/04b300481ea3
https://blog.csdn.net/ning0323/article/details/75451955
https://www.cnblogs.com/straybirds/p/8513870.html
