DispatcherServlet is a core class of Spring MVC.It proccess all request for Spring MVC based application.
Open DispatcherServlet.java,you can learn some core concept fo Spring MVC:
HandlerMapping
HandlerAdapter
ViewResolver
MultipartResolver
LocalResolver
ThemeResolver
All are very important for Spring MVC.And all are configurable.That’s make Spring MVC very flexible.
DispatcherServlet’s initStrategies(ApplicationContext) will init all above core object.
HandlerMapping
HandlerMapping,as it’s name mentioned,it’s used to mapping request to handler.For example,you input a /books/1 in address bar,send the request to the Spring MVC application,how the Spring MVC decide which Controller(Action) to proccess this request,that’s HandlerMapping’s responitity.
Application developer can inherit from this class to customize handler mapping strategy,such as mapping by session or cookie.
There are many implementions by Spring.DefaultAnnotationHandlerMapping(for @RequestMapping),SimpleUrlHandlerMapping,BeanNameUrlHandlerMapping.etc.We will go details later.
HandlerAdapter
HandlerAdapter is an adapter for handler,just a joke.In the privious article,we can see a simple Spring MVC based Controller,it not inherit from any Spring classes,just add some annotation.Why?The interesting thing is in HandlerAdapter.When we find a pure Controller class,we can use HandlerAdapter to wrappe this class.
Adapter is a design pattern,you can refer to GOF’s Design Pattern book.
ViewResolver
Write this code:
1: return new ModelAndView("index");
How can Spring use the view name “index” to find a view file?That’s ViewResolver’s responitity.Spring provided some default implemention:VelocityViewResolver(used for velocity template engine),InternalResourseViewResolver,etc.
Ok,next three object will introduce later.
Order
You can config muliti HandlerMapping,HandlerAdapter and ViewResolver.And you can specify order for these object by assign a order property.
And Spring use OrderComparator to sort these.All handler mappings,handler adapters and view resolvers are implement Ordered interface.
Open OrderComparator,we know the Order value is lower,the priority is higher.
For example,you can assign a lower value for VelocityViewResolver,and a higher InternalResourceViewResolver,Spring will use VelocityViewResolver to find the view file first,if it can not find out,then it will use InternalResourseViewResolver.
That’s all.Next article we will go through the doService method in DispatcherServlet.