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

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);
            }
        }
    }
}    
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