详解SpringMVC中的异常处理机制
目录
- 开头
- 1.ExceptionHandlerExceptionResolver
- 2. demo
开头
试想一下我们一般怎么统一处理异常呢,答:切面。但抛开切面不讲,如果对每一个controller方法抛出的异常做专门处理,那么着实太费劲了,有没有更好的方法呢?当然有,就是本篇文章接下来要介绍的springmvc的异常处理机制,用到了ControllerAdvice和ExceptionHandler注解,有点切面的感觉哈哈。
1.ExceptionHandlerExceptionResolver
首先从springmvc的异常处理解析器开始讲,当执行完controller方法后,不管有没有异常产生都会调用DispatcherServlet#doDispatch()
方法中的processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
方法,接着会判断是否有异常,若无异常则走正常流程,若有异常则需要进行处理 mv = processHandlerException(request, response, handler, exception);
再接着就是遍历spring已经注册的异常处理解析器直到有处理器返回mav
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; }
其中最重要也是最常使用的一个处理器就是ExceptionHandlerExceptionResolver,下面将着重介绍它,先来看看这个类的继承结构图,实现了InitializingBean接口,在这个bean创建完成之前会调用生命周期初始化方法afterPropertiesSet()
,这里面包含了对@ControllerAdvice
注解的解析,初始化完后的信息供后续解析异常使用。
实现HandlerExceptionResolver
接口,实现解析方法resolveException()
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); } } } }
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
这行代码会解析拥有@ControllerAdvice
注解的class,并且会遍历class中带有 @ExceptionHandler
注解的方法,获取方法注解带有的异常类型,将异常类型和方法放入到mappedMethods
中供后面获取,获取的时候若对应处理此异常类型的method有多个,则需要进行排序,选取一个异常类型与method ExceptionHandler注解异常类型最近的一个(深度最小的那个也即是继承关系最少的那个)具体代码如下:
ExceptionHandlerMethodResolver
@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; } }
exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);
此方法执行完成后已经完成了异常处理方法的调用,若方法返回值为视图ModelAndView或其他视图类型,则还需要借助视图解析器如InternalResourceViewResolver
对视图进行解析渲染,若为其他类型的值则将值写入到response响应中。
2. demo
Controller类方法:
@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"); } }
ExceptionHandlerController异常处理类
@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()); } }
ExceptionHandlerController类中定义了两个异常处理方法,一个处理Exception异常,一个处理RuntimeException异常,那个根据controller方法抛出的异常RuntimeException再结合上面的分析(RuntimeException到RuntimeException深度为0,RuntimeException到Exception中间继承了一次深度为1)可以得出抛出异常类型的处理方法为handlerRuntimeException
方法。 运行程序结果如下:
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