SpringMVC源码(五)异常处理

SpringMVC除了对请求URL的路由处理特别方便外&＃xff0c;还支持对异常的统一处理机制&＃xff0c;可以对业务操作时抛出的异常&＃xff0c;unchecked异常以及状态码的异常进行统一处理。SpringMVC既提供简单的配置类&＃xff0c;也提供了细粒度的异常控制机制。

SpringMVC中所有的异常处理通过接口HandlerExceptionResolver来实现&＃xff0c;接口中只定义了一个方法

public interface HandlerExceptionResolver {ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}


方法中接受request和response信息&＃xff0c;以及当前的处理Handler&＃xff0c;和抛出的异常对象。并且提供抽象类AbstractHandlerExceptionResolver&＃xff0c;实现resolveException方法&＃xff0c;支持前置判断和处理&＃xff0c;将实际处理抽象出doResolveException方法由子类来实现。

SimpleMappingExceptionResolver是SpringMVC提供的一个非常便捷的简易异常处理方式&＃xff0c;在XML中进行配置即可使用。

businessEx



这是极简的一种配置&＃xff0c;exceptionMappings配置的是异常同视图之间的映射关系&＃xff0c;它是一个Properties对象&＃xff0c;key-value分别是异常的类路径和视图名称。defaultErrorView表示默认异常视图&＃xff0c;如果抛出的异常没有匹配到任何视图&＃xff0c;即会走默认异常视图。exceptionAttribute表示在视图中获取exception信息变量名&＃xff0c;默认为exception。还有一些其他配置可以查看SimpleMappingExceptionResolver的源码来使用。

SpringMVC提供了一种注解方式来灵活地配置异常处理&＃xff0c;&＃64;ExceptionHandler中可以配置要处理的异常类型&＃xff0c;然后定义在处理此种异常的方法上&＃xff0c;方法只要写在Controller中&＃xff0c;即可对Controller中所有请求方法有效。

我们定义一个BaseController&＃xff0c;并且将需要处理的异常通过&＃64;ExceptionHandler定义好处理方法&＃xff0c;这样业务Controller只需要继承这个基类就可以了。处理方法中支持Request/Response/Sessioin等相关的参数绑定。

[&＃64;Controller](https://my.oschina.net/u/1774615)
public class BaseController {&＃64;ExceptionHandler(RuntimeException.class)public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){return new ModelAndView("error");}
}


但是继承的方式还是对业务代码造成侵入&＃xff0c;Spring非常重要的特性就是非侵入性&＃xff0c;因而SpringMVC提供了&＃64;ControllerAdvice&＃xff0c;简单来说就是Controller的切面&＃xff0c;支持对可选择的Controller进行统一配置&＃xff0c;用于异常处理简直再合适不过了&＃xff0c;我们只需要将BaseController稍稍改一下。

&＃64;ControllerAdvice
public class AdviceController {&＃64;ExceptionHandler(RuntimeException.class)public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){return new ModelAndView("error");}
}


只需要在统一配置类上加上&＃64;ControllerAdvice注解&＃xff0c;支持包路径&＃xff0c;注解等过滤方式&＃xff0c;即可完成对所有业务Controller进行控制&＃xff0c;而业务Controller不用做anything。

如果请求为ajax方式&＃xff0c;需要其他格式返回异常&＃xff0c;在方法上加上&＃64;ResponseBody即可。

上面介绍了常用的两种异常处理的配置方式&＃xff0c;所谓知其然要知其所以然&＃xff0c;SpringMVC怎么在请求处理的过程中完成对异常的统一处理的呢&＃xff1f;我们从源码来深度解读。

回到DispatcherServlet的doDispatcher方法

try {processedRequest &＃61; checkMultipart(request);multipartRequestParsed &＃61; (processedRequest !&＃61; request);// Determine handler for the current request.mappedHandler &＃61; getHandler(processedRequest);if (mappedHandler &＃61;&＃61; null || mappedHandler.getHandler() &＃61;&＃61; null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.HandlerAdapter ha &＃61; getHandlerAdapter(mappedHandler.getHandler());if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.mv &＃61; ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {dispatchException &＃61; ex;
}
catch (Throwable err) {dispatchException &＃61; new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);


可以看到对请求处理的核心处理使用一个大的try/catch&＃xff0c;如果出现异常&＃xff0c;统一封装成dispatchException交给processDispatchResult方法进行处理。我们知道processDispatchResult方法用来对返回视图进行操作&＃xff0c;而同时也对异常进行统一处理。

在processDispatchResult中&＃xff0c;首先对异常进行判断。

if (exception !&＃61; null) {if (exception instanceof ModelAndViewDefiningException) {logger.debug("ModelAndViewDefiningException encountered", exception);mv &＃61; ((ModelAndViewDefiningException) exception).getModelAndView();}else {Object handler &＃61; (mappedHandler !&＃61; null ? mappedHandler.getHandler() : null);mv &＃61; processHandlerException(request, response, handler, exception);errorView &＃61; (mv !&＃61; null);}
}


如果不是特殊的ModelAndViewDefiningException&＃xff0c;则由processHandlerException来操作。

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,Object handler, Exception ex) throws Exception {// Check registered HandlerExceptionResolvers...ModelAndView exMv &＃61; null;// 遍历所有注册的异常处理器&＃xff0c;由异常处理器进行处理for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {exMv &＃61; handlerExceptionResolver.resolveException(request, response, handler, ex);if (exMv !&＃61; null) {break;}}// 如果异常视图存在&＃xff0c;则转向异常视图if (exMv !&＃61; null) {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()) {exMv.setViewName(getDefaultViewName(request));}if (logger.isDebugEnabled()) {logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " &＃43; exMv, ex);}WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());return exMv;}throw ex;
}


我们主要关注异常处理器对异常的处理&＃xff0c;SpringMVC通过HandlerExceptionResolver的resolveException调用实现类的实际实现方法doResolveException。

SimpleMappingExceptionResolver

来看SimpleMappingExceptionResolver的实现&＃xff1a;

protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,Object handler, Exception ex) {// Expose ModelAndView for chosen error view.// 根据request和异常对象获取异常视图名称String viewName &＃61; determineViewName(ex, request);if (viewName !&＃61; null) {// Apply HTTP status code for error views, if specified.// Only apply it if we&＃39;re processing a top-level request.Integer statusCode &＃61; determineStatusCode(request, viewName);if (statusCode !&＃61; null) {applyStatusCodeIfPossible(request, response, statusCode);}// 组装异常视图模型ModelAndViewreturn getModelAndView(viewName, ex, request);}else {return null;}
}


determineViewName方法决定异常视图名称&＃xff0c;getModelAndView方法返回ModelAndView对象

protected String determineViewName(Exception ex, HttpServletRequest request) {String viewName &＃61; null;if (this.excludedExceptions !&＃61; null) {for (Class excludedEx : this.excludedExceptions) {if (excludedEx.equals(ex.getClass())) {return null;}}}// Check for specific exception mappings.if (this.exceptionMappings !&＃61; null) {viewName &＃61; findMatchingViewName(this.exceptionMappings, ex);}// Return default error view else, if defined.if (viewName &＃61;&＃61; null && this.defaultErrorView !&＃61; null) {if (logger.isDebugEnabled()) {logger.debug("Resolving to default view &＃39;" &＃43; this.defaultErrorView &＃43; "&＃39; for exception of type [" &＃43;ex.getClass().getName() &＃43; "]");}viewName &＃61; this.defaultErrorView;}return viewName;
}


在determineViewName方法中&＃xff0c;我们配置的defaultErrorView和exceptionMappings都起了作用。更细节的就不深入了&＃xff0c;有兴趣可以自己去看。

ExceptionHandlerExceptionResolver

ExceptionHandlerExceptionResolver支持了&＃64;ExceptionHandler注解的实现。它的抽象基类AbstractHandlerMethodExceptionResolver继承了AbstractHandlerExceptionResolver&＃xff0c;doResolveException方法实际调用ExceptionHandlerExceptionResolver的doResolveHandlerMethodException方法。

protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod, Exception exception) {// 根据HandlerMethod和exception获取异常处理的MethodServletInvocableHandlerMethod exceptionHandlerMethod &＃61; getExceptionHandlerMethod(handlerMethod, exception);if (exceptionHandlerMethod &＃61;&＃61; null) {return null;}// 设置异常处理方法的参数解析器和返回值解析器exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);ServletWebRequest webRequest &＃61; new ServletWebRequest(request, response);ModelAndViewContainer mavContainer &＃61; new ModelAndViewContainer();// 执行异常处理方法try {if (logger.isDebugEnabled()) {logger.debug("Invoking &＃64;ExceptionHandler method: " &＃43; exceptionHandlerMethod);}Throwable cause &＃61; exception.getCause();if (cause !&＃61; null) {// Expose cause as provided argument as wellexceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);}else {// Otherwise, just the given exception as-isexceptionHandlerMethod.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 !&＃61; exception && logger.isWarnEnabled()) {logger.warn("Failed to invoke &＃64;ExceptionHandler method: " &＃43; exceptionHandlerMethod, invocationEx);}// Continue with default processing of the original exception...return null;}// 对返回的视图模型进行处理if (mavContainer.isRequestHandled()) {return new ModelAndView();}else {ModelMap model &＃61; mavContainer.getModel();HttpStatus status &＃61; mavContainer.getStatus();ModelAndView mav &＃61; new ModelAndView(mavContainer.getViewName(), model, status);mav.setViewName(mavContainer.getViewName());if (!mavContainer.isViewReference()) {mav.setView((View) mavContainer.getView());}if (model instanceof RedirectAttributes) {Map flashAttributes &＃61; ((RedirectAttributes) model).getFlashAttributes();request &＃61; webRequest.getNativeRequest(HttpServletRequest.class);RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);}return mav;}
}


我们主要关注的是如何匹配到异常处理方法的

protected ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {Class handlerType &＃61; (handlerMethod !&＃61; null ? handlerMethod.getBeanType() : null);// 从当前Controller中匹配异常处理Methodif (handlerMethod !&＃61; null) {ExceptionHandlerMethodResolver resolver &＃61; this.exceptionHandlerCache.get(handlerType);if (resolver &＃61;&＃61; null) {resolver &＃61; new ExceptionHandlerMethodResolver(handlerType);this.exceptionHandlerCache.put(handlerType, resolver);}Method method &＃61; resolver.resolveMethod(exception);if (method !&＃61; null) {return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);}}// 从ControllerAdvice中匹配异常处理Methodfor (Entry entry : this.exceptionHandlerAdviceCache.entrySet()) {if (entry.getKey().isApplicableToBeanType(handlerType)) {ExceptionHandlerMethodResolver resolver &＃61; entry.getValue();Method method &＃61; resolver.resolveMethod(exception);if (method !&＃61; null) {return new ServletInvocableHandlerMethod(entry.getKey().resolveBean(), method);}}}return null;
}


匹配异常处理方法的来源有两个&＃xff0c;一个是当前Controller&＃xff0c;一个是所有&＃64;ControllerAdvice类。可以看到两种方式都使用了cache的方式&＃xff0c;那么ExceptionHandlerMethod的信息怎么初始化的呢&＃xff1f;

当前Controller

对每个请求HandlerMethod的Controller类型&＃xff0c;都实例化一个ExceptionHandlerMethodResolver来处理异常。ExceptionHandlerMethodResolver的构造函数中初始化了当前Controller中的异常处理配置。

public ExceptionHandlerMethodResolver(Class handlerType) {for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {// detectExceptionMappings方法执行探查for (Class exceptionType : detectExceptionMappings(method)) {addExceptionMapping(exceptionType, method);}}
}private List> detectExceptionMappings(Method method) {List> result &＃61; new ArrayList>();// 探查所有ExceptionHandler注解的方法detectAnnotationExceptionMappings(method, result);if (result.isEmpty()) {for (Class paramType : method.getParameterTypes()) {if (Throwable.class.isAssignableFrom(paramType)) {result.add((Class) paramType);}}}if (result.isEmpty()) {throw new IllegalStateException("No exception types mapped to " &＃43; method);}return result;
}protected void detectAnnotationExceptionMappings(Method method, List> result) {ExceptionHandler ann &＃61; AnnotationUtils.findAnnotation(method, ExceptionHandler.class);result.addAll(Arrays.asList(ann.value()));
}


&＃64;ControllerAdvice类

对&＃64;ControllerAdvice统一切面类的处理&＃xff0c;则是在ExceptionHandlerExceptionResolver的初始化方法afterPropertiesSet中进行处理。

public void afterPropertiesSet() {// Do this first, it may add ResponseBodyAdvice beans// 初始化&＃64;ControllerAdvice中的&＃64;ExceptionHandlerinitExceptionHandlerAdviceCache();if (this.argumentResolvers &＃61;&＃61; null) {List resolvers &＃61; getDefaultArgumentResolvers();this.argumentResolvers &＃61; new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers &＃61;&＃61; null) {List handlers &＃61; getDefaultReturnValueHandlers();this.returnValueHandlers &＃61; new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}
}


initExceptionHandlerAdviceCache方法遍历上下文中所有有&＃64;ControllerAdvice注解的Bean&＃xff0c;然后实例化成ExceptionHandlerMethodResolver类&＃xff0c;在构造函数中初始化所有&＃64;ExceptionHandler。

private void initExceptionHandlerAdviceCache() {if (getApplicationContext() &＃61;&＃61; null) {return;}if (logger.isDebugEnabled()) {logger.debug("Looking for exception mappings: " &＃43; getApplicationContext());}// 查询所有&＃64;ControllerAdvice的BeanList adviceBeans &＃61; ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());AnnotationAwareOrderComparator.sort(adviceBeans);// 遍历&＃xff0c;实例化ExceptionHandlerMethodResolverfor (ControllerAdviceBean adviceBean : adviceBeans) {ExceptionHandlerMethodResolver resolver &＃61; new ExceptionHandlerMethodResolver(adviceBean.getBeanType());if (resolver.hasExceptionMappings()) {this.exceptionHandlerAdviceCache.put(adviceBean, resolver);if (logger.isInfoEnabled()) {logger.info("Detected &＃64;ExceptionHandler methods in " &＃43; adviceBean);}}if (ResponseBodyAdvice.class.isAssignableFrom(adviceBean.getBeanType())) {this.responseBodyAdvice.add(adviceBean);if (logger.isInfoEnabled()) {logger.info("Detected ResponseBodyAdvice implementation in " &＃43; adviceBean);}}}
}


匹配到exceptionHandlerMethod后&＃xff0c;设置一些方法执行的环境&＃xff0c;然后调用ServletInvocableHandlerMethod中的invokeAndHandle去执行&＃xff0c;这个调用过程和正常请求的调用就是一致了。这里也不向下扩展了&＃xff0c;可以参看SpringMVC源码(四)-请求处理。

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 执行请求方法Object returnValue &＃61; invokeForRequest(webRequest, mavContainer, providedArgs);setResponseStatus(webRequest);if (returnValue &＃61;&＃61; null) {if (isRequestNotModified(webRequest) || getResponseStatus() !&＃61; null || mavContainer.isRequestHandled()) {mavContainer.setRequestHandled(true);return;}}else if (StringUtils.hasText(getResponseStatusReason())) {mavContainer.setRequestHandled(true);return;}mavContainer.setRequestHandled(false);try {this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);}catch (Exception ex) {if (logger.isTraceEnabled()) {logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);}throw ex;}
}


至此ExceptionHandlerExceptionResolver的异常处理已经基本完成。SpringMVC还内置了ResponseStatusExceptionResolver和DefaultHandlerExceptionResolver来对状态码异常和常见的请求响应异常进行统一处理。

某些情况下&＃xff0c;SpringMVC的处理并没有异常出现&＃xff0c;但在最终的视图输出时找不到视图文件&＃xff0c;就会显示404错误页面&＃xff0c;非常影响用户体验。我们可以在web.xml中对未捕获的异常以及此种4xx或5xx的异常通过进行处理。

404/404

java.lang.Throwable/500



通常对于系统中的异常&＃xff0c;业务相关的尽量自定义异常处理方式&＃xff0c;而一些系统异常通过统一错误页面进行处理。