Spring MVC（4）2018-08-09

 我们接着Sprring MVC(3)篇章接着看DispatcherServlet中的doDispatch方法：

1、DispatcherServlet中的doDispatch：

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 {
                //1、文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；  
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                 //2.通过HandlerMapping，将请求映射到处理器（返回一个HandlerExecutionChain，它包括一个处理器、多个HandlerInterceptor拦截器）；
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                //确定当前请求的处理程序适配器。
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                //得到当前的http方法。      
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                 //如果是GET请求或者head请求，如果内容没有变化的话，则直接返回。
                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;
                    }
                }
                    //调用执行器链中的拦截器（前置）
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.
                //执行handle处理器
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //如果返回的ModelAndView不为null，并且没有设置view的话，这设置默认的view。
                applyDefaultViewName(processedRequest, mv);
              //处理拦截器的postHandle。（后置）
                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);
            }
             //如果出现异常，返回异常页面。如果没有异常，ModelAndView不为null，则正常渲染页面，调用拦截器的afterCompletion方法。
            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);
                }
            }
        }
    }

最后我们来看下DispatcherServlet的请求处理流程：

DispatcherServlet 2018-08-11 下午4.59.59.png