Spring中@ExceptionHandler注解的工作原理详解

    /**
     * 此方法定义一个异常处理器，仅仅处理异常 DemoException , 它使用一个视图对象 
     * DemoExceptionHandlerView 来处理异常 
     * @param e
     * @return
     */
    @ExceptionHandler(DemoException.class)
    public View handleDemoException(DemoException e) {
        return new DemoExceptionHandlerView(e);
    }

    // DispatcherServlet 代码片段，
    // 在 HandlerAdaptor 执行 Handler 之后调用，
    // 如果之前的逻辑有异常，则 exception 不为 null，
    // 如果之前的逻辑执行正常， 则 exception 为 null
	private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable Exception exception) throws Exception {
		boolean errorView = false;
		if (exception != null) {
           // 请求处理出现了异常 
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
              // 这里视图处理该异常：
              // 1. 内部将其完全处理
              // 2. 或者返回一个用于渲染错误的数据模型和视图
				mv = processHandlerException(request, response, handler, exception);
				errorView = (mv != null);
			}
		}
        // ... 省略其他代码
    }        
	@Nullable
	protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
			@Nullable Object handler, Exception ex) throws Exception {
		// Success and error responses may use different content types
		request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
		// Check registered HandlerExceptionResolvers...
		ModelAndView exMv = null;
		if (this.handlerExceptionResolvers != null) {
          // 遍历`handlerExceptionResolvers`中每个`HandlerExceptionResolver`对象
          // 试图对异常 ex 进行处理
			for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
				exMv = resolver.resolveException(request, response, handler, ex);
				if (exMv != null) {
					break;
				}
			}
		}
		if (exMv != null) {
			// ...
			return exMv;
		}
		throw ex;
	}