public class CustomWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) {
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.register(WebConfig.class);
        DispatcherServlet servlet = new DispatcherServlet(context);
        ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  try {
    Exception dispatchException = null;
    // 1.獲取HandlerMapping（該對象就是當前請求與處理程序的一個映射關系）
    mappedHandler = getHandler(processedRequest);
    // 2.獲取能夠處理上一步得到的處理程序
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    // 3.執行實際的調用（執行實際的處理程序）
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  } catch (Exception ex) {
    // 4.執行過程中發生異常記錄到局部變量中
    dispatchException = ex;
  } catch (Throwable err) {
    dispatchException = new NestedServletException("Handler dispatch failed", err);
  }
  // 5.處理結果
  processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
      @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
      @Nullable Exception exception) throws Exception {

  boolean errorView = false;
  // 1.判斷是否發生異常
  if (exception != null) {
    // 1.1.異常對象是否是該對象
    if (exception instanceof ModelAndViewDefiningException) {
      mv = ((ModelAndViewDefiningException) exception).getModelAndView();
    } else {
      // 如果不是上面的異常對象，則這里獲取具體處理程序的Handler
      // 這里我們只考慮RequestMappingHandlerMapping情況，那么這里獲取的將是HandlerMethod對象
      // 也就是Controller中的具體方法了
      Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
      // 處理異常，查看下面processhandlerException方法
      mv = processHandlerException(request, response, handler, exception);
      errorView = (mv != null);
    }
  }

  // Did the handler return a view to render?
  if (mv != null && !mv.wasCleared()) {
    render(mv, request, response);
    if (errorView) {
      WebUtils.clearErrorRequestAttributes(request);
    }
  }
  else {
    if (logger.isTraceEnabled()) {
      logger.trace("No view rendering, null ModelAndView returned.");
    }
  }

  if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
    // Concurrent handling started during a forward
    return;
  }

  if (mappedHandler != null) {
    // Exception (if any) is already handled..
    mappedHandler.triggerAfterCompletion(request, response, null);
  }
}

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
      @Nullable Object handler, Exception ex) throws Exception {
  ModelAndView exMv = null;
  // 判斷當前的異常解析器是否存在；也就是從容器中獲取所有HandlerExceptionResolver類型對象
  // 這里我們就不展開了，你可以在DispatcherServlet中查看初始化過程
  // 默認情況下，這里集合中有如下圖1中所示
  if (this.handlerExceptionResolvers != null) {
    // 遍歷每一個異常處理器，誰能返回ModelAndView就結束循環
    // 由于DefaultErrorAttributes內部方法直接返回了null，所以這里返回的是HandlerExceptionResolverComposite
    // 這是聚合類，聚合了其它3個具體的解析器，所以時間處理的還是其它類并非它
    for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
      // 結合上面說只考慮RequestMappingHandlerMapping處理Controller的情況
      // 那這里合理的解析器是ExceptionHandlerExceptionResolver
      exMv = resolver.resolveException(request, response, handler, ex);
      if (exMv != null) {
        break;
      }
    }
  }
  // 通過上面的執行如果獲取到了ModelAndView對象，下面就是判斷視圖對象不同的情況如何進行處理了
  if (exMv != null) {
    if (exMv.isEmpty()) {
      request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
      return null;
    }
    if (!exMv.hasView()) {
      String defaultViewName = getDefaultViewName(request);
      if (defaultViewName != null) {
        exMv.setViewName(defaultViewName);
      }
    }
    WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
    // 如果存在要想前端展示的視圖，則返回。
    return exMv;
  }
  throw ex;
}