Spring MVC 前端控制器 (DispatcherServlet)处理流程解析

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {

     try {

         try {

             // 文件上传相关

             processedRequest = checkMultipart(request);

             multipartRequestParsed = (processedRequest != request);

             // DispatcherServlet收到请求调用处理器映射器HandlerMapping。

             // 处理器映射器根据请求url找到具体的处理器，生成处理器执行链HandlerExecutionChain(包括处理器对象和处理器拦截器)一并返回给DispatcherServlet。

             mappedHandler = getHandler(processedRequest);

             if (mappedHandler == null ) {

                 noHandlerFound(processedRequest, response);

                 return ;

             }

             4 .DispatcherServlet根据处理器Handler获取处理器适配器HandlerAdapter,

             HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

             // Process last-modified header, if supported by the handler.  HTTP缓存相关

             String method = request.getMethod();

             boolean isGet = HttpMethod.GET.matches(method);

             if (isGet || HttpMethod.HEAD.matches(method)) {

                 long lastModified = ha.getLastModified(request, mappedHandler.getHandler());

                 if ( new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {

                     return ;

                 }

             }

             // 前置拦截器

             if (!mappedHandler.applyPreHandle(processedRequest, response)) {

                 // 返回false就不进行后续处理了

                 return ;

             }

             // 执行HandlerAdapter处理一系列的操作，如：参数封装，数据格式转换，数据验证等操作

             // 执行处理器Handler(Controller，也叫页面控制器)。

             // Handler执行完成返回ModelAndView

             // HandlerAdapter将Handler执行结果ModelAndView返回到DispatcherServlet

             mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

             if (asyncManager.isConcurrentHandlingStarted()) {

                 return ;

             }

             // 如果没有视图，给你设置默认视图  json忽略

             applyDefaultViewName(processedRequest, mv);

             //后置拦截器

             mappedHandler.applyPostHandle(processedRequest, response, mv);

         }

         catch (Exception ex) {

             dispatchException = ex;

         }

         catch (Throwable err) {

             // As of 4.3, we're processing Errors thrown from handler methods as well,

             // making them available for @ExceptionHandler methods and other scenarios.

             dispatchException = new NestedServletException( "Handler dispatch failed" , err);

         }

         // DispatcherServlet将ModelAndView传给ViewReslover视图解析器

         // ViewReslover解析后返回具体View

         // DispatcherServlet对View进行渲染视图（即将模型数据model填充至视图中）。

         // DispatcherServlet响应用户。

         processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

     }

     catch (Exception ex) {

         triggerAfterCompletion(processedRequest, response, mappedHandler, ex);

     }

     catch (Throwable err) {

         triggerAfterCompletion(processedRequest, response, mappedHandler,

                                new NestedServletException( "Handler processing failed" , err));

     }

     finally {

         if (asyncManager.isConcurrentHandlingStarted()) {

             // Instead of postHandle and afterCompletion

             if (mappedHandler != null ) {

                 mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);

             }

         }

         else {

             // Clean up any resources used by a multipart request.

             if (multipartRequestParsed) {

                 cleanupMultipart(processedRequest);

             }

         }

     }

}