详解SpringMVC中的异常处理机制

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

		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				// 执行处理器产生的异常处理
				mv = processHandlerException(request, response, handler, exception);
				// 是否有异常视图返回
				errorView = (mv != null);
			}
		}

		// Did the handler return a view to render? 处理程序是否返回要渲染的视图
		if (mv != null && !mv.wasCleared()) {
			// 渲染视图
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
						"': assuming HandlerAdapter completed request handling");
			}
		}
	}

	@Nullable
	protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
			@Nullable Object handler, Exception ex) throws Exception {

		// Check registered HandlerExceptionResolvers...
		ModelAndView exMv = null;
		if (this.handlerExceptionResolvers != null) {
			for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
				exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
				if (exMv != null) {
					break;
				}
			}
		}
		if (exMv != null) {
			// 无视图view
			if (exMv.isEmpty()) {
				request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
				return null;
			}
			// We might still need view name translation for a plain error model...
			if (!exMv.hasView()) {
				String defaultViewName = getDefaultViewName(request);
				if (defaultViewName != null) {
					exMv.setViewName(defaultViewName);
				}
			}
			WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
			return exMv;
		}

		throw ex;
	}

public interface HandlerExceptionResolver {

	/**
	 * Try to resolve the given exception that got thrown during handler execution,
	 * returning a {@link ModelAndView} that represents a specific error page if appropriate.
	 * <p>The returned {@code ModelAndView} may be {@linkplain ModelAndView#isEmpty() empty}
	 * to indicate that the exception has been resolved successfully but that no view
	 * should be rendered, for instance by setting a status code.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler the executed handler, or {@code null} if none chosen at the
	 * time of the exception (for example, if multipart resolution failed)
	 * @param ex the exception that got thrown during handler execution
	 * @return a corresponding {@code ModelAndView} to forward to,
	 * or {@code null} for default processing in the resolution chain
	 */
	@Nullable
	ModelAndView resolveException(
			HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);

}

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBodyAdvice beans
    // 初始化异常注解 @ControllerAdvice
    initExceptionHandlerAdviceCache();
}

private void initExceptionHandlerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }
    if (logger.isDebugEnabled()) {
        logger.debug("Looking for exception mappings: " + getApplicationContext());
    }

    // 解析有@ControllerAdvice注解的bean，并将这个bean构建成ControllerAdviceBean对象
    List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
    // 将ControllerAdviceBean根据order排序
    AnnotationAwareOrderComparator.sort(adviceBeans);

    for (ControllerAdviceBean adviceBean : adviceBeans) {
        Class<?> beanType = adviceBean.getBeanType();
        if (beanType == null) {
            throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
        }
        ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
        // mappedMethods 映射不为空
        if (resolver.hasExceptionMappings()) {
            // 添加到缓存中
            this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
            if (logger.isInfoEnabled()) {
                logger.info("Detected @ExceptionHandler methods in " + adviceBean);
            }
        }
        // 若实现了ResponseBodyAdvice接口（暂不介绍）
        if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
            this.responseBodyAdvice.add(adviceBean);
            if (logger.isInfoEnabled()) {
                logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean);
            }
        }
    }
}

@Override
@Nullable
protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,
                                                       HttpServletResponse response, @Nullable HandlerMethod handlerMethod, Exception exception) {

    // exception为controller方法抛出的异常
    // 根据异常及其类型从上述的mappedMethods中获取对应的方法，再获取方法所在的对象 封装成ServletInvocableHandlerMethod
    ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
    if (exceptionHandlerMethod == null) {
        return null;
    }

    // 设置参数解析器，主要用来获取方法的参数值的，供后续反射调用方法
    if (this.argumentResolvers != null) {
        exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
    }
    // 设置返回值解析器，当执行完方法后获取返回值，对返回值进行处理 或返回视图或将结果写入到response
    if (this.returnValueHandlers != null) {
        exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
    }

    ServletWebRequest webRequest = new ServletWebRequest(request, response);
    ModelAndViewContainer mavContainer = new ModelAndViewContainer();

    try {
        if (logger.isDebugEnabled()) {
            logger.debug("Invoking @ExceptionHandler method: " + exceptionHandlerMethod);
        }
        Throwable cause = exception.getCause();
        if (cause != null) {
            // Expose cause as provided argument as well
            // 执行异常处理方法，也就是我们的自定义的异常处理方法
            exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);
        }
        else {
            // Otherwise, just the given exception as-is
            exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, handlerMethod);
        }
    }
    catch (Throwable invocationEx) {
        // Any other than the original exception is unintended here,
        // probably an accident (e.g. failed assertion or the like).
        if (invocationEx != exception && logger.isWarnEnabled()) {
            logger.warn("Failed to invoke @ExceptionHandler method: " + exceptionHandlerMethod, invocationEx);
        }
        // Continue with default processing of the original exception...
        return null;
    }

    // 根据后续的返回值解析器设置的，将返回值写入到response中了直接返回空的mav
    if (mavContainer.isRequestHandled()) {
        return new ModelAndView();
    }
    else {
        ModelMap model = mavContainer.getModel();
        HttpStatus status = mavContainer.getStatus();
        ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, status);
        mav.setViewName(mavContainer.getViewName());
        // (this.view instanceof String)
        if (!mavContainer.isViewReference()) {
            mav.setView((View) mavContainer.getView());
        }
        if (model instanceof RedirectAttributes) {
            Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
            RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
        }
        return mav;
    }
}

@Controller
@RequestMapping(value = "test")
public class HelloWorldController{

  @Data
  public static class User {
    private String username;

    private Integer age;

    private String address;
  }


  @RequestMapping(value = "user/get", method = RequestMethod.POST)
  @ResponseBody
  public Object testObject(@RequestBody @Valid User user, @RequestParam String address) {
    user.setAddress(address);
    // 这里特意抛出RuntimeException异常
    throw new RuntimeException("this is a exception");
  }

}

@ControllerAdvice
@ResponseBody
public class ExceptionHandlerController {

  @ExceptionHandler(value = Exception.class)
  public Object handleException(Exception e) {
    return CommonResult.fail("Exception:" + e.getMessage());
  }

  @ExceptionHandler(value = RuntimeException.class)
  public Object handlerRuntimeException(Exception e) {
    return CommonResult.fail("handlerRuntimeException:" + e.getMessage());
  }
}