Spring代码加载的入口：
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
context.getBean(XXX.class);
ClassPathXmlApplicationContext 继承关系
参考
http://www.cnblogs.com/xrq730/p/6285358.html、
https://yikun.github.io/2015/05/29/Spring-IOC核心源码学习/
