protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    try {
        try {
            // 文件上傳相關
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);
            // DispatcherServlet收到請求調用處理器映射器HandlerMapping。
            // 處理器映射器根據請求url找到具體的處理器，生成處理器執(zhí)行鏈HandlerExecutionChain(包括處理器對象和處理器攔截器)一并返回給DispatcherServlet。
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }
            4.DispatcherServlet根據處理器Handler獲取處理器適配器HandlerAdapter,
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
            // Process last-modified header, if supported by the handler.  HTTP緩存相關
            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;
                }
            }
            // 前置攔截器
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                // 返回false就不進行后續(xù)處理了
                return;
            }
            // 執(zhí)行HandlerAdapter處理一系列的操作，如：參數封裝，數據格式轉換，數據驗證等操作
            // 執(zhí)行處理器Handler(Controller，也叫頁面控制器)。
            // Handler執(zhí)行完成返回ModelAndView
            // HandlerAdapter將Handler執(zhí)行結果ModelAndView返回到DispatcherServlet
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }
            // 如果沒有視圖，給你設置默認視圖  json忽略
            applyDefaultViewName(processedRequest, mv);
            //后置攔截器
            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將ModelAndView傳給ViewReslover視圖解析器
        // ViewReslover解析后返回具體View
        // DispatcherServlet對View進行渲染視圖（即將模型數據model填充至視圖中）。
        // DispatcherServlet響應用戶。
        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);
            }
        }
    }
}