一、简介
Spring MVC框架在工作中经常用到，配置简单，使用起来也很方便，很多书籍和博客都有介绍其处理流程，但是，对于其原理，总是似懂非懂的样子。我们做技术，需要做到知其然，还要知其所以然。今天我们结合源码来深入了解一下Spring MVC的处理流程。
以上流程图是Spring MVC的处理流程（参考：spring-mvc-flow-with-example），原作者对流程的解释如下：
1Step 1: First request will be received by DispatcherServlet. 2 3Step 2: DispatcherServlet will take the help of HandlerMapping and get to know the Controller class name associated with the given request. 4 5Step 3: So request transfer to the Controller, and then controller will process the request by executing appropriate methods and returns ModelAndView object (contains Model data and View name) back to the DispatcherServlet. 6 7Step 4: Now DispatcherServlet send the model object to the ViewResolver to get the actual view page. 8 9Step 5: Finally DispatcherServlet will pass the Model object to the View page to display the result.
针对以上流程，这里需要更加详细一点：
1、请求被web 容器接收，并且根据contextPath将请求发送给DispatcherServlet
2、DispatcherServlet接收到请求后，会设置一些属性（localeResolver、themeResolver等等），在根据request在handlerMappings中查找对应的HandlerExecutionChain；然后根据HandlerExecutionChain中的handler来找到HandlerAdapter，然后通过反射来调用handler中的对应方法（RequestMapping对应的方法）
3、handler就是对应的controller，调用controller中的对应方法来进行业务逻辑处理，返回ModelAndView（或者逻辑视图名称）
4、ViewResolver根据逻辑视图名称、视图前后缀，来获取实际的逻辑视图
5、获取实际视图之后，就会使用model来渲染视图，得到用户实际看到的视图，然后返回给客户端。
二、Demo样例
我们运行一个小样例（github地址：https://github.com/yangjianzhou/spring-mvc-demo）来了解Spring MVC处理流程，项目结构如下：
web.xml配置如下：
1<?xml version="1.0" encoding="UTF-8"?> 2<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 5 http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"> 6 <context-param> 7 <param-name>contextConfigLocation</param-name> 8 <param-value>classpath:/applicationContext.xml</param-value> 9 </context-param> 10 <listener> 11 <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> 12 </listener> 13 14 <servlet> 15 <servlet-name>smart</servlet-name> 16 <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> 17 <load-on-startup>1</load-on-startup> 18 </servlet> 19 20 <servlet-mapping> 21 <servlet-name>smart</servlet-name> 22 <url-pattern>/</url-pattern> 23 </servlet-mapping> 24 25 <welcome-file-list> 26 <welcome-file>index.jsp</welcome-file> 27 </welcome-file-list> 28 29</web-app>
smart-servlet.xml的内容如下：
1 <context:component-scan base-package="com.iwill.mvc"/> 2 3 <!-- 在使用Excel PDF的视图时，请先把这个视图解析器注释掉，否则产生视图解析问题--> 4 <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver" 5 p:order="100" p:viewClass="org.springframework.web.servlet.view.JstlView" 6 p:prefix="/WEB-INF/views/" p:suffix=".jsp"/>
UserController.java的代码如下：
1package com.iwill.mvc; 2 3import org.apache.log4j.Logger; 4import org.springframework.stereotype.Controller; 5import org.springframework.web.bind.annotation.RequestMapping; 6import org.springframework.web.bind.annotation.RequestMethod; 7import org.springframework.web.servlet.ModelAndView; 8 9@Controller 10@RequestMapping("/user") 11public class UserController { 12 13 Logger logger = Logger.getLogger(UserController.class); 14 15 @RequestMapping("register") 16 public String register() { 17 logger.info("invoke register"); 18 return "user/register"; 19 } 20 21 @RequestMapping(method = RequestMethod.POST) 22 public ModelAndView createUser(User user) { 23 ModelAndView mav = new ModelAndView(); 24 mav.setViewName("user/createSuccess"); 25 mav.addObject("user", user); 26 return mav; 27 } 28}
三、请求接收
DispatcherServlet的类继承关系如下：
可以看出，DispatcherServlet是一个HttpServlet，因此，它可以处理http请求。
在浏览器中输入http://localhost:8080/spring-mvc-demo/user/register，因为在web服务器上配置了spring-mvc-demo的contextPath为spring-mvc-demo，所以/spring-mvc-demo/user/register的请求就会被DispatcherServlet处理，请求处理路径如下：
请求由tomcat传递给了DispatcherServlet了，DispatcherServlet接收后，就开始自己的特殊处理了。
红框中是Spring MVC自己特有的逻辑，主要是与视图、主题有关。
接下来的主要处理逻辑在org.springframework.web.servlet.DispatcherServlet#doDispatch中：
1protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { 2 HttpServletRequest processedRequest = request; 3 HandlerExecutionChain mappedHandler = null; 4 boolean multipartRequestParsed = false; 5 6 WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); 7 8 try { 9 ModelAndView mv = null; 10 Exception dispatchException = null; 11 12 try { 13 processedRequest = checkMultipart(request); 14 multipartRequestParsed = processedRequest != request; 15 16 // 根据request在handlerMappings中获取HandlerExecutionChain 17 mappedHandler = getHandler(processedRequest, false); 18 if (mappedHandler == null || mappedHandler.getHandler() == null) { 19 noHandlerFound(processedRequest, response); 20 return; 21 } 22 23 //根据handler在handlerAdapters中获取HandlerAdapter 24 HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); 25 26 // Process last-modified header, if supported by the handler. 27 String method = request.getMethod(); 28 boolean isGet = "GET".equals(method); 29 if (isGet || "HEAD".equals(method)) { 30 long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); 31 if (logger.isDebugEnabled()) { 32 String requestUri = urlPathHelper.getRequestUri(request); 33 logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified); 34 } 35 if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { 36 return; 37 } 38 } 39 40 if (!mappedHandler.applyPreHandle(processedRequest, response)) { 41 return; 42 } 43 44 try { 45 //适配器调用实际的handler 46 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); 47 } 48 finally { 49 if (asyncManager.isConcurrentHandlingStarted()) { 50 return; 51 } 52 } 53 54 applyDefaultViewName(request, mv); 55 mappedHandler.applyPostHandle(processedRequest, response, mv); 56 } 57 catch (Exception ex) { 58 dispatchException = ex; 59 } 60 //逻辑视图名转换为物理视图名，并进行视图渲染 61 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); 62 } 63 catch (Exception ex) { 64 triggerAfterCompletion(processedRequest, response, mappedHandler, ex); 65 } 66 catch (Error err) { 67 triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err); 68 } 69 finally { 70 if (asyncManager.isConcurrentHandlingStarted()) { 71 // Instead of postHandle and afterCompletion 72 mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); 73 return; 74 } 75 // Clean up any resources used by a multipart request. 76 if (multipartRequestParsed) { 77 cleanupMultipart(processedRequest); 78 } 79 } 80 }
首先获取HandlerExecutionChain（入口：mappedHandler = getHandler(processedRequest, false);），方法如下：
1 protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { 2 for (HandlerMapping hm : this.handlerMappings) { 3 if (logger.isTraceEnabled()) { 4 logger.trace( 5 "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'"); 6 } 7 HandlerExecutionChain handler = hm.getHandler(request); 8 if (handler != null) { 9 return handler; 10 } 11 } 12 return null; 13 }
之后就是根据handler获取HandlerAdapter（入口：HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler())）：
1 protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { 2 for (HandlerAdapter ha : this.handlerAdapters) { 3 if (logger.isTraceEnabled()) { 4 logger.trace("Testing handler adapter [" + ha + "]"); 5 } 6 if (ha.supports(handler)) { 7 return ha; 8 } 9 } 10 throw new ServletException("No adapter for handler [" + handler + 11 "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler"); 12 }
handler适配器调用handler的方法（入口：mv = ha.handle(processedRequest, response, mappedHandler.getHandler())）：
1protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler) 2 throws Exception { 3 4 ServletHandlerMethodResolver methodResolver = getMethodResolver(handler); 5 Method handlerMethod = methodResolver.resolveHandlerMethod(request); 6 ServletHandlerMethodInvoker methodInvoker = new ServletHandlerMethodInvoker(methodResolver); 7 ServletWebRequest webRequest = new ServletWebRequest(request, response); 8 ExtendedModelMap implicitModel = new BindingAwareModelMap(); 9 10 Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel); 11 ModelAndView mav = 12 methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest); 13 methodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest); 14 return mav; 15 }
通过Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel)会调用底层的方法：
红框中，通过反射调用UserController的register方法。这样请求就被传递到了实际的controller方法了。
四、响应返回
UserController#register处理后，就返回逻辑视图名：user/register。在org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.ServletHandlerMethodInvoker#getModelAndView中，就会将String转化为ModelAndView：
在org.springframework.web.servlet.DispatcherServlet#render中，就会将逻辑视图ModelAndView转化物理视图。
resolveViewName就是使用ViewResolver来获取物理视图名：
1 protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale, 2 HttpServletRequest request) throws Exception { 3 4 for (ViewResolver viewResolver : this.viewResolvers) { 5 View view = viewResolver.resolveViewName(viewName, locale); 6 if (view != null) { 7 return view; 8 } 9 } 10 return null; 11 }
物理视图名会被缓存，不需要重复解析，提高性能。
这里就是prefix和suffix的用途了，用于定位实际视图。
获取到了物理视图之后，就进行视图渲染了。
针对jsp格式的视图，我们配置的view是org.springframework.web.servlet.view.JstlView，渲染过程就是将model中的值set到request的attribute中，之后就是使用jsp自己的规则来显示jsp文件就好。