Thursday, March 31, 2016

How does spring's DispatcherServlet work

In Spring webmvc, there is a specifial servlet which is the portal between Servlet container and Spring webmvc framework. It's the DispatcherServlet. A spring web application usually maps all requst to this DispatcherServlet.

A servlet's job is take in a HttpServletRequest, do all the business process and finally return a HttpServletResponse. 

1. Basic process flow of DispatcherServlet

How does DispatcherServlet work

The most important components for understanding DispatcherServlet are 4 interfaces, HandlerMapping, HandlerAdapter, ViewResolver and View. Every interface has ONE core method.  So in short, you can think only 4 methods get main logic done in DispatcherServlet. The lines in above chart also show how previous method affects latter ones.

2. More description

2.1 HandlerMapping

In a spring web application, usually there are a lot of @RequstMapping annotated methods within many @Controller annotated classes. HandlerMapping solve this problem: "Which method in which class should be used to process this current request?" Spring has default HandlerMapping instances built in. For example in spring 4.2.3.RELEASE, BeanNameUrlHandlerMapping and DefaultAnnotationHandlerMapping are used by default. Here are the tyep hierarchy of HandlerMapping.

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In the HandlerMapping hierarchy, 2 classes are default in DispatcherServlet, although one of them is deprecated. Spring document recommand to use the RequestMappingHandlerMapping to replace the deprecated DefaultAnnotationHandlerMapping.

The returned type of getHandler(…) HandlerExecutionChain. HandlerExecutionChain is a combination of 1 handler + 1 or more interceptors. You can think it as a wrapper of the real handler. One thing to notice is the type of handler is Object, which means a handler to process the income request can be any class! That's the ultimate flexibility spring provides.

Also since the handler type is Object, how DispatcherServlet use it, since type Object expose no real business methods. How to use this "Object handler" to process http request? That's exactly what HandlerAdapter does.

2.2 HandlerAdapter

The key method in HandlerAdapter interface is handle(…, Object handler). The last parameter is the "Object handler" that returned from HandlerMapping, through a wrapper class, HandlerExecutionChain. In this handle method, method of a @Controller class finally get invoked.

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One thing need to notice is that HandlerMapping and HandlerAdapter usually are closely coupled. All though the typeof  HandlerMapping returned handler is Object in method signature, it's actually some type that only be recognized by a certain HandlerAdapter implementation.  For example,  the handler that returned by RequestMappingHandlerMapping is actually a instance of org.springframework.web.method.HandlerMethod. Only RequestMappingHandlerAdapter knows how to invoke the HandlerMethod.

The return type of a HandlerAdpater's handle(…) method is ModelAndView. As the name says, it has mode and view( or view name) in it.

2.3 ViewResolver

The task for ViewResolver is to find View instance by view's name. The return type of method resolveViewName(…) is View.

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2.4 View

The key mothod of view instance is render(Map<String,?> model, …), in here the final html result back to user is created. The first parameter is the model data used to create page.  Up to now the main flow of processing an income Http request is over, the created final html page will be send back by servlet container.

3. More

Besides all above interfaces. Another important interface you many come into contact with is HandlerExceptionResolver.

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If you want to set a default view for all Exceptions or set different views for different types of Excpetion,  extend from SimpleMappingExceptionResolver is very convenient.

See tutorial "Exception handling in spring mvc/rest application" on how to take care of exceptions in Spring webmvc.

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Has been a senior software developer, project manager for 10+ years. Dedicate himself to Alcatel-Lucent and China Telecom for delivering software solutions.

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