without MVC，we have to wirte every servlet class to finish servlet；wirte servlet by ourself work as follows:
browser->servlet->servletHandler->result->browser
in this case , servlet dependency JSP to return result to browser;

MVC的存在改变了什么？

at above case:

  1. every servlet have to be registered;
  2. return result to browser have to wirte JSP;

if choose uses Spring MVC:

  1. just need register one servlet servletDispatcher
  2. return Model and view;