@Override
protected ModelAndView handleInternal(HttpServletRequest request, 
    HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
  ModelAndView mav;
  checkRequest(request);
  // 如果需要，在同步块中执行invokeHandlerMethod,默认不
  if (this.synchronizeOnSession) {
    HttpSession session = request.getSession(false);
    if (session != null) {
      Object mutex = WebUtils.getSessionMutex(session);
      synchronized (mutex) {
        mav = invokeHandlerMethod(request, response, handlerMethod);
      }
    }
    else {
      // No HttpSession available -> no mutex necessary
      mav = invokeHandlerMethod(request, response, handlerMethod);
    }
  } else {
    // No synchronization on session demanded at all...
    mav = invokeHandlerMethod(request, response, handlerMethod);
  }
  // 处理Cache-Control请求头（若你没有set
  if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
    if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
      applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
    } else {
      prepareResponse(response);
    }
  }
  return mav;
}

  // 它的作用就是执行目标的HandlerMethod，然后返回一个ModelAndView 
  @Nullable
  protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
    ServletWebRequest webRequest = new ServletWebRequest(request, response);
    // 注意：此处只有try-finally 哦
    // 因为invocableMethod.invokeAndHandle(webRequest, mavContainer)是可能会抛出异常的（交给全局异常处理）
    try {
      // 最终创建的是一个ServletRequestDataBinderFactory，持有所有@InitBinder的method方法们
      WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
      // 创建一个ModelFactory，@ModelAttribute啥的方法就会被引用进来
      ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
      // 把HandlerMethod包装为ServletInvocableHandlerMethod，具有invoke执行的能力喽
      // 下面这几部便是一直给invocableMethod的各大属性赋值~~~
      ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
      if (this.argumentResolvers != null) {
        invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
      }
      if (this.returnValueHandlers != null) {
        invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
      }
      invocableMethod.setDataBinderFactory(binderFactory);
      invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
      ModelAndViewContainer mavContainer = new ModelAndViewContainer();
      // 把上个request里的值放进来到本request里
      mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
      // model工厂：把它里面的Model值放进mavContainer容器内（此处@ModelAttribute/@SessionAttribute啥的生效）
      modelFactory.initModel(webRequest, mavContainer, invocableMethod);
      mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
      AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
      asyncWebRequest.setTimeout(this.asyncRequestTimeout);
      WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
      asyncManager.setTaskExecutor(this.taskExecutor);
      asyncManager.setAsyncWebRequest(asyncWebRequest);
      asyncManager.registerCallableInterceptors(this.callableInterceptors);
      asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
      // 它不管是不是异步请求都先用AsyncWebRequest 包装了一下，但是若是同步请求
      // asyncManager.hasConcurrentResult()肯定是为false的~~~
      if (asyncManager.hasConcurrentResult()) {
        Object result = asyncManager.getConcurrentResult();
        mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
        asyncManager.clearConcurrentResult();
        LogFormatUtils.traceDebug(logger, traceOn -> {
          String formatted = LogFormatUtils.formatValue(result, !traceOn);
          return "Resume with async result [" + formatted + "]";
        });
        invocableMethod = invocableMethod.wrapConcurrentResult(result);
      }
      // 此处其实就是调用ServletInvocableHandlerMethod#invokeAndHandle()方法喽
      // 关于它你可以来这里：https://fangshixiang.blog.csdn.net/article/details/98385163
      // 注意哦：任何HandlerMethod执行完后都是把结果放在了mavContainer里（它可能有Model，可能有View，可能啥都木有~~）
      // 因此最后的getModelAndView()又得一看
      invocableMethod.invokeAndHandle(webRequest, mavContainer);
      if (asyncManager.isConcurrentHandlingStarted()) {
        return null;
      }
      return getModelAndView(mavContainer, modelFactory, webRequest);
    } finally {
      webRequest.requestCompleted();
    }
  }
任何处理器执行完后，最终返回的的都是一个ModelAndView对象，这得益于getModelAndView()这个方法的适配处理：
  // @Nullable：表示它返回的可以是个null哦~(若木有视图，就直接不会render啦~因为response已经写入过值了)
  @Nullable
  private ModelAndView getModelAndView(ModelAndViewContainer mavContainer, ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {
    // 把session里面的内容写入
    modelFactory.updateModel(webRequest, mavContainer);
    // Tips：若已经被处理过，那就返回null喽~~（比如若是@ResponseBody这种，这里就是true）
    if (mavContainer.isRequestHandled()) {
      return null;
    }
    // 通过View、Model、Status构造出一个ModelAndView，最终就可以完成渲染了
    ModelMap model = mavContainer.getModel();
    ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, mavContainer.getStatus());
    if (!mavContainer.isViewReference()) { // 是否是String类型
      mav.setView((View) mavContainer.getView());
    }
    // 对重定向RedirectAttributes参数的支持（两个请求之间传递参数，使用的是ATTRIBUTE）
    if (model instanceof RedirectAttributes) {
      Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
      HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
      if (request != null) {
        RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
      }
    }
    return mav;
  }