SpringMVC源码情操陶冶-AbstractHandlerExceptionResolver

springmvc支持服务端在处理业务逻辑过程中出现异常的时候可以配置相应的ModelAndView对象返回给客户端，本文介绍springmvc默认的几种HandlerExceptionResolver类

实际应用

springmvc的xml配置化-Exception配置

<bean id="exceptionHandler" class="org.springframework.web.servlet.handler.SimpleMappingExceptionResolver">
	<!--设置默认返回viewName，通常与freemarker引擎搭配使用-->
	<property name="defaultErrorView" value="error/defaultError" />
	<!--设置默认返回response status-->
	<property name="defaultStatusCode" value="500" />

	<!--配置相应的异常类与viewName的映射-->
	<property name="exceptionMappings">
        <props>
            <prop key="SessionTimeoutException">redirect:../login.html</prop>
			<prop key="AuthenticationException">error/403</prop>
        </props>
	</property>
</bean>

以上的配置会对出SessionTimeoutException异常则跳转至login页面，对AuthenticationException异常则跳转至403页面，对其他的异常则默认跳转至defaultError页面呈现并返回500的错误码

HandlerExceptionResolver-异常解析接口

接口内只有一个方法resolveException()，通过解析异常查询配置以得到符合条件的ModelAndView对象

	/**
	 * 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
	 */
	ModelAndView resolveException(
			HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);

AbstractHandlerExceptionResolver-异常解析抽象基类

所有的spring内置异常解析类都继承于此，直奔主题看resolveException()方法

	@Override
	public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response,
			Object handler, Exception ex) {
		//判断是否需要解析
		if (shouldApplyTo(request, handler)) {
			//此处一般是判断内部属性preventResponseCaching是否为true，是则设置响应包头cache-control:no-store
			prepareResponse(ex, response);
			//使用模板方法doResolveException()方法供子类实现
			ModelAndView result = doResolveException(request, response, handler, ex);
			if (result != null) {
				//日志打印一发
				logException(ex, request);
			}
			return result;
		}
		else {
			return null;
		}
	}

附带着分析下shouldApplyTo()方法

	/**
	**可以配置mappedHandlers和mappedHandlerClasses属性来特定匹配
	**默认情况下两者都为空则直接返回true，表明对所有的handler都进行异常解析
	*/
	protected boolean shouldApplyTo(HttpServletRequest request, Object handler) {
		//此处的handler一般为bean对象
		if (handler != null) {
			if (this.mappedHandlers != null && this.mappedHandlers.contains(handler)) {
				return true;
			}
			if (this.mappedHandlerClasses != null) {
				for (Class<?> handlerClass : this.mappedHandlerClasses) {
					if (handlerClass.isInstance(handler)) {
						return true;
					}
				}
			}
		}
		// Else only apply if there are no explicit handler mappings.
		return (this.mappedHandlers == null && this.mappedHandlerClasses == null);
	}

1. SimpleMappingExceptionResolver-异常映射实现类

比较简单的实现类，可以配绑定viewName和exception以完成简单的异常映射视图页面

	protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,
			Object handler, Exception ex) {

		// Expose ModelAndView for chosen error view.
		String viewName = determineViewName(ex, request);
		if (viewName != null) {
			//如果配置了statusCodes属性，则对此异常的状态码进行设置
			Integer statusCode = determineStatusCode(request, viewName);
			if (statusCode != null) {
				applyStatusCodeIfPossible(request, response, statusCode);
			}
			//创建ModelAndView对象
			return getModelAndView(viewName, ex, request);
		}
		else {
			return null;
		}
	}

针对以上的源码我们分两步去简单分析下

SimpleMappingExceptionResolver#determineViewName()-找寻viewName

	protected String determineViewName(Exception ex, HttpServletRequest request) {
		String viewName = null;
		//判断异常是否属于excludeExceptions集合内，是则直接返回null
		if (this.excludedExceptions != null) {
			for (Class<?> excludedEx : this.excludedExceptions) {
				if (excludedEx.equals(ex.getClass())) {
					return null;
				}
			}
		}
		// Check for specific exception mappings.
		// 从exceptionMappings集合内根据exception获取到相应的viewName
		if (this.exceptionMappings != null) {
			viewName = findMatchingViewName(this.exceptionMappings, ex);
		}
		//当exceptionMappings集合内不存在指定的exception但是默认视图指定则直接返回默认视图
		if (viewName == null && this.defaultErrorView != null) {
			viewName = this.defaultErrorView;
		}
		return viewName;
	}

• excludedExceptions集合可以过滤指定的exception，对其不进行解析直接返回null。可配置

• exceptionMappings绑定了exception与viewName的关系，如果在其集合内没找到相应的viewName，但是defaultErrorView属性指定，则会直接返回defaultErrorView对应的视图

SimpleMappingExceptionResolver#getModelAndView()-创建ModelAndView对象

	protected ModelAndView getModelAndView(String viewName, Exception ex) {
		ModelAndView mv = new ModelAndView(viewName);
		//exceptionAttribute默认为exception
		if (this.exceptionAttribute != null) {
			//将exception信息添加到model中
			mv.addObject(this.exceptionAttribute, ex);
		}
		return mv;
	}

主要就是将exceptionAttribute对应的参数值默认为exception属性添加到视图对象的model中

2. ResponseStatusExceptionResolver-响应状态异常解析类

主要是解析带有@ResponseStatus的异常类，将其中的异常信息描述直接返回给客户端

	@Override
	protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,
			Object handler, Exception ex) {
		//获取相应类上的注解@ResponseStatus
		ResponseStatus responseStatus = AnnotatedElementUtils.findMergedAnnotation(ex.getClass(), ResponseStatus.class);
		if (responseStatus != null) {
			try {
				return resolveResponseStatus(responseStatus, request, response, handler, ex);
			}
			catch (Exception resolveEx) {
				logger.warn("Handling of @ResponseStatus resulted in Exception", resolveEx);
			}
		}
		else if (ex.getCause() instanceof Exception) {
			ex = (Exception) ex.getCause();
			//递归
			return doResolveException(request, response, handler, ex);
		}
		return null;
	}

ResponseStatusExceptionResolver#resolveResponseStatus()-返回异常信息给客户端

读取@ResponseStatus注解信息，返回异常内容给客户端

	protected ModelAndView resolveResponseStatus(ResponseStatus responseStatus, HttpServletRequest request,
			HttpServletResponse response, Object handler, Exception ex) throws Exception {
		//状态码
		int statusCode = responseStatus.code().value();
		//异常原因描述
		String reason = responseStatus.reason();
		if (this.messageSource != null) {
			reason = this.messageSource.getMessage(reason, null, reason, LocaleContextHolder.getLocale());
		}
		//通过response对象直接返回错误信息给客户端
		if (!StringUtils.hasLength(reason)) {
			response.sendError(statusCode);
		}
		else {
			//通过response对象直接返回错误信息给客户端
			response.sendError(statusCode, reason);
		}
		return new ModelAndView();
	}

3. DefaultHandlerExceptionResolver-springmvc默认的异常解析处理

源码就不公布了，读者可自行去查询，基本都是调用response的sendError()方法返回错误信息给客户端。本文对其中的异常归下类

1. 请求方式异常

• HttpRequestMethodNotSupportedException-服务端不支持相应的请求方法
• HttpMediaTypeNotSupportedException/HttpMediaTypeNotAcceptableException-服务端/客户端不支持相应的mediaType，比如application/json
• MissingPathVariableException-@PathVaribale指定参数请求中不包含
• MissingServletRequestParameterException/ServletRequestBindingException-请求参数绑定错误
• MethodArgumentNotValidException-@Valid注解指定的参数校验失败
• AsyncRequestTimeoutException-异步请求超时

1. 消息内容异常

• ConversionNotSupportedException-服务端找寻不到相应的Convert对象来解析javabean
• TypeMismatchException-设置javabean属性类型出错
• HttpMessageNotReadableException/HttpMessageNotWritableException-消息内容不可读/不可写
• MissingServletRequestPartException-文件上传类错误，可能请求没有multipart/form-data或者服务不支持文件上传
• NoHandlerFoundException-handler处理器没有找到，即可能没有对应的请求处理供响应

4. ExceptionHandlerExceptionResolver-处理handlerMethod对象过程中的异常

具体的逻辑本文则不展开了，简述下其中的逻辑：

当处理handlerMethod业务逻辑过程中出现了异常，则此解析器

1. 尝试从handlerMethod所在的class类去找寻是否含有@ExceptionHandler注解的方法

2. 判断@ExceptionHandler指定的exception类与产生的异常一致，一致则执行相应的方法，当有多个@ExceptionHandler(value)，则默认采用第一个

3. 当上述在class类找寻不到则尝试判断class类是否含有@ControllerAdvice注解，有则按照上述第一二步的步骤再次找寻@ControllerAdvice指定的类

小结

springmvc开放了对异常也可以包装成页面显示的功能，通过本文的简单分析可以帮助博主和读者更好的理解springmvc对异常的处理