在分析spring mvc源码之前，先看一张图：

请求处理的过程：

1.DispatcherServelt作为前端控制器，拦截request对象。

2.DispatcherServlet接收到request对象之后，查询HandlerMapping,得到一个HandlerExecutionChain对象。

3.DispatcherServlet将Handler对象交由HandlerAdapter(适配器模式的典型应用)，调用相应的controller方法。

4.Controller方法返回ModelAndView对象，DispatcherServlet将view交由ViewResolver进行解析，得到相应的视图。用model渲染视图。

5.返回响应结果。

整个过程的流程其实就是DispatcherServelt中doDispatch()方法的调用过程。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {//第一步拦截request对象，doDispatch()方法在doService()方法中被调用，request对象是经过处理的。HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {//和文件的上传和下载有关系，判断请求的类型是否是multipart类型processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.//主要看这里，这里是得到HandlerExecutionChain的方法。关于Handler()方法向下看mappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.//这里已经获取到HandlerExecutionChain对象，接下来就要获取HandlerAdapter对象，调用Handler对象的方法。HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler//有关浏览器缓存的设定（304）String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (logger.isDebugEnabled()) {logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);}if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}//pan'duif (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}//解析视图，数据渲染applyDefaultViewName(request, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}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 afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {//遍历HandlerMappingList对象（存储若干个HandlerMapping对象），不断调用，直到不为空为止，否则返回nullfor (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;}

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