http://www.lai18.com/content/9755931.html
Spring 容器(Spring 的上下文)
https://my.oschina.net/jast90/blog/282773
http://www.deroneriksson.com/tutorials/java/spring/introduction-to-the-spring-framework/configuring-root-web-application-context-in-spring-web-application
http://stackoverflow.com/questions/33163441/spring-why-root-application-context-and-servlet-application-context-are-cre