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：

Step 1: First request will be received by DispatcherServlet.Step 2: DispatcherServlet will take the help of HandlerMapping and get to know the Controller class name associated with the given request.Step 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.Step 4: Now DispatcherServlet send the model object to the ViewResolver to get the actual view page.Step 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：

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/javaeehttp://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"><context-param><param-name>contextConfigLocation</param-name><param-value>classpath:/applicationContext.xml</param-value></context-param><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener><servlet><servlet-name>smart</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>smart</servlet-name><url-pattern>/</url-pattern></servlet-mapping><welcome-file-list><welcome-file>index.jsp</welcome-file></welcome-file-list></web-app>

smart-servlet.xml的内容如下：

 <context:component-scan base-package="com.iwill.mvc"/><!-- 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--><bean class="org.springframework.web.servlet.view.InternalResourceViewResolver"p:order="100" p:viewClass="org.springframework.web.servlet.view.JstlView"p:prefix="/WEB-INF/views/" p:suffix=".jsp"/>

UserController.java的代码如下：

package com.iwill.mvc;import org.apache.log4j.Logger;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.servlet.ModelAndView;@Controller
@RequestMapping("/user")
public class UserController {Logger logger = Logger.getLogger(UserController.class);@RequestMapping("register")public String register() {logger.info("invoke register");return "user/register";}@RequestMapping(method = RequestMethod.POST)public ModelAndView createUser(User user) {ModelAndView mav = new ModelAndView();mav.setViewName("user/createSuccess");mav.addObject("user", user);return mav;}
}

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：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = processedRequest != request;// 根据request在handlerMappings中获取HandlerExecutionChainmappedHandler = getHandler(processedRequest, false);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}//根据handler在handlerAdapters中获取HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (logger.isDebugEnabled()) {String requestUri = urlPathHelper.getRequestUri(request);logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);}if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}try {//适配器调用实际的handlermv = ha.handle(processedRequest, response, mappedHandler.getHandler());}finally {if (asyncManager.isConcurrentHandlingStarted()) {return;}}applyDefaultViewName(request, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}//逻辑视图名转换为物理视图名，并进行视图渲染processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Error err) {triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionmappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);return;}// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}

首先获取HandlerExecutionChain（入口：mappedHandler = getHandler(processedRequest, false);），方法如下：

   protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {for (HandlerMapping hm : this.handlerMappings) {if (logger.isTraceEnabled()) {logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");}HandlerExecutionChain handler = hm.getHandler(request);if (handler != null) {return handler;}}return null;}

之后就是根据handler获取HandlerAdapter（入口：HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler())）：

  protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {for (HandlerAdapter ha : this.handlerAdapters) {if (logger.isTraceEnabled()) {logger.trace("Testing handler adapter [" + ha + "]");}if (ha.supports(handler)) {return ha;}}throw new ServletException("No adapter for handler [" + handler +"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");}

handler适配器调用handler的方法（入口：mv = ha.handle(processedRequest, response, mappedHandler.getHandler())）：

protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {ServletHandlerMethodResolver methodResolver = getMethodResolver(handler);Method handlerMethod = methodResolver.resolveHandlerMethod(request);ServletHandlerMethodInvoker methodInvoker = new ServletHandlerMethodInvoker(methodResolver);ServletWebRequest webRequest = new ServletWebRequest(request, response);ExtendedModelMap implicitModel = new BindingAwareModelMap();Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel);ModelAndView mav =methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest);methodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest);return mav;}

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：

 protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,HttpServletRequest request) throws Exception {for (ViewResolver viewResolver : this.viewResolvers) {View view = viewResolver.resolveViewName(viewName, locale);if (view != null) {return view;}}return null;}

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。