Spring Boot Spring MVC异常处理原理分析

一、Spring MVC为处理异常的前期准备

DispatcherServlet
入口类，是一个Servlet，是所有请求的分发点

初始化

DispatcherServlet在初始化时会触发onRefresh()方法，此方法会调用initStrategies方法（初始化九大组件），完成整个DispatcherServlet的初始化工作，其中initHandlerExceptionResolvers()会初始化HandlerExceptionResolvers对象

protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);// 初始化HandlerExceptionResolvers对象initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);
}

从Spring容器的ApplicationContext中找出所有HandlerExceptionResolvers对象，将保存到对象属性handlerExceptionResolvers 中。从这我们也知道如果要在spring mvc中插入自己的HandlerExceptionResolver也比较简单，只需要类实现接口HandlerExceptionResolver和Ordered，使用类似@Component 的注解注解此类即可

private void initHandlerExceptionResolvers(ApplicationContext context) {this.handlerExceptionResolvers = null;if (this.detectAllHandlerExceptionResolvers) {// 从Spring容器的ApplicationContext中找出所有HandlerExceptionResolvers对象Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);if (!matchingBeans.isEmpty()) {// 将Map转化为List，保存到属性handlerExceptionResolvers 中this.handlerExceptionResolvers = new ArrayList<HandlerExceptionResolver>(matchingBeans.values());// 对HandlerExceptionResolvers使用据order接口里值进行排序AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);}
}
...

到此初始化工作完成

关于HandlerExceptionResolver(异常解析器)以及常用的实现类

HandlerExceptionResolver接口
HandlerExceptionResolver是一个接口，用于处理网络请求过程中抛出的异常，但是不处理异常本身抛出的异常和视图解析过程中抛出的异常

下图是Spring MVC默认实现的HandlerExceptionResolver类

HandlerExceptionResolverComposite
Spring Boot启动时会默认注册HandlerExceptionResolverComposite对象。此类只是一个组合类，并不进行真正的异常处理。当他捕获异常时他只是将异常轮询委托给注册到它属性里的上的HandlerExceptionResolver类来处理异常，如果处理的结果不为null，则转给下一个处理

@Override
public ModelAndView resolveException(HttpServletRequest request,HttpServletResponse response,Object handler,Exception ex) {if (resolvers != null) {for (HandlerExceptionResolver handlerExceptionResolver : resolvers) {ModelAndView mav = handlerExceptionResolver.resolveException(request, response, handler, ex);if (mav != null) {return mav;}}}return null;
}

默认注册到HandlerExceptionResolverComposite 的属性有以下3个HandlerExceptionResolver，按照优先级排列如下：

ExceptionHandlerExceptionResolver

ResponseStatusExceptionResolver

DefaultHandlerExceptionResolver

Spring mvc启动时，初始化所有HandlerExceptionResolver到Spring 容器中，在Spring boot在启动时，会初始化WebMvcConfigurationSupport 里配置的Bean, 会创建HandlerExceptionResolverComposite对象，此对象包括3个HandlerExceptionResolver，当他捕获异常时，会使用这3个HandlerExceptionResolver进行处理，详细如下：

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {…@Beanpublic HandlerExceptionResolver handlerExceptionResolver() {List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<HandlerExceptionResolver>();configureHandlerExceptionResolvers(exceptionResolvers);if (exceptionResolvers.isEmpty()) {// 添加默认HandlerExceptionResolver类    addDefaultHandlerExceptionResolvers(exceptionResolvers);}extendHandlerExceptionResolvers(exceptionResolvers);HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();composite.setOrder(0);composite.setExceptionResolvers(exceptionResolvers);return composite;}//  添加默认HandlerExceptionResolverComposite及注册到此对象中的HandlerExceptionResolverCompositeprotected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {// 创建 ExceptionHandlerExceptionResolver()ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager());exceptionHandlerResolver.setMessageConverters(getMessageConverters());exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());if (jackson2Present) {exceptionHandlerResolver.setResponseBodyAdvice(Collections.<ResponseBodyAdvice<?>>singletonList(new JsonViewResponseBodyAdvice()));}exceptionHandlerResolver.setApplicationContext(this.applicationContext);exceptionHandlerResolver.afterPropertiesSet();exceptionResolvers.add(exceptionHandlerResolver);// 创建ResponseStatusExceptionResolver ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();responseStatusResolver.setMessageSource(this.applicationContext);exceptionResolvers.add(responseStatusResolver);// 创建DefaultHandlerExceptionResolverexceptionResolvers.add(new DefaultHandlerExceptionResolver());}protected ExceptionHandlerExceptionResolver createExceptionHandlerExceptionResolver() {return new ExceptionHandlerExceptionResolver();}
}

二、详细介绍这3个HandlerExceptionResolver的作用

ExceptionHandlerExceptionResolver
使用@ExceptionHandler注解方法处理异常类，使用注解处理异常就有这个类的功劳。默认情况下，这个HandlerExceptionResolver的优先级是最高。
以下是ExceptionHandlerExceptionResolver运行时属性值

属性exceptionHandlerAdviceCache ：存储@Controller里@ExceptionHandler的方法

属性exceptionHandlerAdviceCache：存储@ControllerAdvice里@ExceptionHandler的全局方法

处理异常的关键代码
入口doResolveHandlerMethodException方法会通过 getExceptionHandlerMethod获取对应的@ExceptionHandler方法，如果有找到则执行此方法

  @Overrideprotected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod, Exception exception) {// 杳找对应的方法@ExceptionHandlerServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);if (exceptionHandlerMethod == null) {return null;}
....if (cause != null) {// 执行异常处理方法exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);}else {// 执行异常处理方法exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, handlerMethod);}}
....}

getExceptionHandlerMethod方法：查找特定异常的@ExceptionHandler方法，首先从抛出异常的@Controller类中寻找对应的处理方法，如果没有再从@ControllerAdvice中查找全局的@ExceptionHandler方法，如果找到，则调用这个方法执行处理，否则返回null

protected ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {Class<?> handlerType = (handlerMethod != null ? handlerMethod.getBeanType() : null);if (handlerMethod != null) {// 从抛出异常的@Controller类中自身中寻找对应的处理方法，如果有找到先缓存ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);if (resolver == null) {resolver = new ExceptionHandlerMethodResolver(handlerType);this.exceptionHandlerCache.put(handlerType, resolver);}Method method = resolver.resolveMethod(exception);if (method != null) {// 调用方法，返回结果return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);}}// 如果没有再从@ControllerAdvice中查找全局的@ExceptionHandler方法，如果找到，则调用这个方法执行处理for (Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {if (entry.getKey().isApplicableToBeanType(handlerType)) {ExceptionHandlerMethodResolver resolver = entry.getValue();Method method = resolver.resolveMethod(exception);if (method != null) {return new ServletInvocableHandlerMethod(entry.getKey().resolveBean(), method);}}}return null;
}

ResponseStatusExceptionResolver
使用@ResponseStatus处理异常，将异常转化对应的HTTP的状态码。@ResponseStatus可以定义在Excpetion的子类的类上，也可以定义在被@ExceptionHandler注解的方法上（不过这个需要小心使用，由于ExceptionHandlerExceptionResolver的优先级高，这种方式可能被ExceptionHandlerExceptionResolver覆盖掉）

异常处理入口doResolveException方法会先查找异常上的@ResponseStatus注解信息，如果有ResponseStatus ，则按照ResponseStatus 配置的值处理

//
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,Object handler, Exception ex) {// 获取异常的@ResponseStatus注解信息ResponseStatus responseStatus = AnnotatedElementUtils.findMergedAnnotation(ex.getClass(), ResponseStatus.class);if (responseStatus != null) {try {// 如果有ResponseStatus ，则按照ResponseStatus 配置的值处理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) {…}return null;
}

根据ResponseStatus 的值设置返回的http状态码和原因

protected ModelAndView resolveResponseStatus(ResponseStatus responseStatus, HttpServletRequest request,HttpServletResponse response, Object handler, Exception ex) throws Exception {int statusCode = responseStatus.code().value();String reason = responseStatus.reason();if (!StringUtils.hasLength(reason)) {// 设置返回的http状态码response.sendError(statusCode);}else {String resolvedReason = (this.messageSource != null ?this.messageSource.getMessage(reason, null, reason, LocaleContextHolder.getLocale()) :reason);// 设置返回的http状态码和原因response.sendError(statusCode, resolvedReason);}return new ModelAndView();}

DefaultHandlerExceptionResolver
默认的HandlerExceptionResolver，将特定异常转化为标准的HTTP的状态码。
详细如下：左边是异常名称，右边是http的状态码

通过代码解释此类行为, 只列出NoSuchRequestHandlingMethodException相关的转换http错误码的代码，表格里其他异常处理类似

异常处理入口doResolveException方法，如果发现异常是NoSuchRequestHandlingMethodException，则调用方法handleNoSuchRequestHandlingMethod

 // 对于NoSuchRequestHandlingMethodException进行转化http错误大码
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,Object handler, Exception ex) {try {if (ex instanceof org.springframework.web.servlet.mvc.multiaction.NoSuchRequestHandlingMethodException) {return handleNoSuchRequestHandlingMethod((org.springframework.web.servlet.mvc.multiaction.NoSuchRequestHandlingMethodException) ex,request, response, handler);}else if (ex instanceof HttpRequestMethodNotSupportedException) {…}else if …
}

handleNoSuchRequestHandlingMethod方法返回404错误码和错误信息

protected ModelAndView handleNoSuchRequestHandlingMethod(org.springframework.web.servlet.mvc.multiaction.NoSuchRequestHandlingMethodException ex,HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {pageNotFoundLogger.warn(ex.getMessage());response.sendError(HttpServletResponse.SC_NOT_FOUND);return new ModelAndView();}

Ordered和实现自定义HandlerExceptionResolver类
每个具体的HandlerExceptionResolver都会实现Ordered接口，来定义执行的顺序，order值越小，越是优先执行。
如果要实现自己HandlerExceptionResolver，只需要满足两个条件：

实现接口HandlerExceptionResolver和Ordered

使用类似@Component 的注解注解此类，保证spring启动时创建此类对应的对象即可

三、异常处理流程

当执行@RequestMapping抛出异常，会进入异常处理流程

所有的doPost, doGet等do*的方法都会执行到以下方法：找到真正业务的处理逻辑，并进行处理。

下面的代码是找到本次请求真正要处理的HandlerAdapter 对象，并进行处理，最后调用processDispatchResult对结果进行处理，这是我们关心的内容

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {…
try {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.mappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (logger.isDebugEnabled()) {logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);}if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios.dispatchException = new NestedServletException("Handler dispatch failed", err);}// 处理最后的方法processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}
…
}

处理业务执行的结果，处理结束可能是 ModelAndView，也可能是Exception。如果结果是Exception，就需要通过本文提到的HandlerExceptionResolver转化为ModelAndView。然后根据ModelAndView将结果返回给请求方

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {boolean errorView = false;if (exception != null) {if (exception instanceof ModelAndViewDefiningException) {logger.debug("ModelAndViewDefiningException encountered", exception);mv = ((ModelAndViewDefiningException) exception).getModelAndView();}else {// 如果返回值是异常，通过本文提到的HandlerExceptionResolver转化为ModelAndViewObject handler = (mappedHandler != null ? mappedHandler.getHandler() : null);mv = processHandlerException(request, response, handler, exception);errorView = (mv != null);}}// Did the handler return a view to render?// 根据ModelAndView执行后续操作if (mv != null && !mv.wasCleared()) {render(mv, request, response);if (errorView) {WebUtils.clearErrorRequestAttributes(request);}}…
}

当异常发生时， DispatcherServlet会轮询调用HandlerExceptionResolver，直到异常被转化为ModelAndView

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,Object handler, Exception ex) throws Exception {// Check registered HandlerExceptionResolvers...ModelAndView exMv = null;for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);if (exMv != null) {break;}}….
}

四、流程总结

默认规则
• 默认情况下，Spring Boot提供/error处理所有错误的映射
• 对于机器客户端，它将生成JSON响应，其中包含错误，HTTP状态和异常消息的详细信息。对于浏览器客户端，响应一个“ whitelabel”错误视图，以HTML格式呈现相同的数据

• 要对其进行自定义，添加View解析为error
• 要完全替换默认行为，可以实现 ErrorController 并注册该类型的Bean定义，或添加ErrorAttributes类型的组件以使用现有机制但替换其内容。
• error/下的4xx，5xx页面会被自动解析；

定制错误处理逻辑

自定义错误页

error/404.html error/5xx.html；有精确的错误状态码页面就匹配精确，没有就找4xx.html；如果都没有就触发白页

@ControllerAdvice+@ExceptionHandler处理全局异常；底层是 ExceptionHandlerExceptionResolver 支持的
@ResponseStatus+自定义异常；底层是 ResponseStatusExceptionResolver ，把responsestatus注解的信息底层调用 response.sendError(statusCode, resolvedReason)；tomcat发送的/error
Spring底层的异常，如参数类型转换异常；DefaultHandlerExceptionResolver 处理框架底层的异常。

response.sendError(HttpServletResponse.SC_BAD_REQUEST ,ex.getMessage());
response.sendError会返回tomcat默认的错误页（如下），跟springboot的是不一样的，但springboot底层有个专门的BasicErrorController来处理“"/error" 请求

自定义实现 HandlerExceptionResolver 处理异常；可以作为默认的全局异常处理规则

ErrorViewResolver 实现自定义处理异常；

response.sendError 。error请求就会转给controller

你的异常没有任何人能处理。tomcat底层response.sendError。error请求就会转给controller

basicErrorController要去的页面地址是 ErrorViewResolver ；

异常处理自动配置原理

ErrorMvcAutoConfiguration 自动配置异常处理规则
• 容器中的组件：类型：DefaultErrorAttributes -> id：errorAttributes

public class DefaultErrorAttributes implements ErrorAttributes,HandlerExceptionResolver

DefaultErrorAttributes：定义错误页面中可以包含哪些数据。

容器中的组件：类型：BasicErrorController --> id：basicErrorController（json+白页适配响应）

a.处理默认 /error 路径的请求；页面响应 new ModelAndView(“error”, model)；

b.容器中有组件 View->id是error；（响应默认错误页）

c.容器中放组件 BeanNameViewResolver（视图解析器）；按照返回的视图名作为组件的id去容器中找View对象。 •

容器中的组件：类型：DefaultErrorViewResolver -> id：conventionErrorViewResolver •
a. 如果发生错误，会以HTTP的状态码作为视图页地址（viewName），找到真正的页面 • error/404、5xx.html

b. 如果想要返回页面；就会找error视图【StaticView】。(默认是一个白页) 写出去json 错误页

异常处理步骤流程

执行目标方法，目标方法运行期间有任何异常都会被catch、而且标志当前请求结束；并且用 dispatchException

进入视图解析流程（页面渲染？） processDispatchResult(processedRequest, response,
mappedHandler, mv, dispatchException);

mv =processHandlerException；处理handler发生的异常，处理完成返回ModelAndView；
a. 遍历所有的handlerExceptionResolvers，看谁能处理当前异常【HandlerExceptionResolver处理器异常解析器】

b.系统默认的异常解析器；

一、DefaultErrorAttributes先来处理异常。把异常信息保存到rrequest域，并且返回null；
二、默认没有任何人能处理异常，所以异常会被抛出

如果没有任何人能处理最终底层就会发送 /error请求。会被底层的BasicErrorController处理

解析错误视图；遍历所有的 ErrorViewResolver
看谁能解析。

三、默认的 DefaultErrorViewResolver,作用是把响应状态码作为错误页的地址，或者匹配5xx,4xx（序列码）这种形式找到最终页面，error/500.html
四、模板引擎最终响应这个页面 error/500.html

异常处理顺序
拦截时序图：

图片来自网络

参考文章
参考视频