SpringFramework之ControllerAdvice注解怎么用
这篇文章主要介绍了SpringFramework之ControllerAdvice注解怎么用，具有一定借鉴价值，感兴趣的朋友可以参考下，希望大家阅读完这篇文章之后大有收获，下面让小编带着大家一起了解一下。
SpringFramework版本5.0.9.release。
我们会通过@ControllerAdvice和@ExceptionHandler来处理异常，Springmvc是如何进行处理的呢?
ControllerAdviceBean有个重要的方法findAnnotatedBeans，如下List-1
List-1
public class ControllerAdviceBean implements Ordered { ... public static List findAnnotatedBeans(ApplicationContext applicationContext) { List beans = new ArrayList(); String[] var2 = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(applicationContext, Object.class); int var3 = var2.length; for(int var4 = 0; var4 < var3; ++var4) { String name = var2[var4]; if (applicationContext.findAnnotationOnBean(name, ControllerAdvice.class) != null) { beans.add(new ControllerAdviceBean(name, applicationContext)); } } return beans; } ... 如List-1所示，从applicationContext中获取所有的ControllerAdvice注解的Bean，之后封装到ControllerAdviceBean中。
来看下ExceptionHandlerExceptionResolver，它的类继承图如下图1所示:
图1
ExceptionHandlerExceptionResolver是HandlerExceptionResolver，所以在Springmvc的doDispatch中会调用它。实现了InitializingBean，所以有afterPropertiesSet方法，如下List-2所示:
List-2
@Overridepublic void afterPropertiesSet() { // Do this first, it may add ResponseBodyAdvice beans initExceptionHandlerAdviceCache(); ...}private void initExceptionHandlerAdviceCache() { ... List adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext()); 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); if (resolver.hasExceptionMappings()) { this.exceptionHandlerAdviceCache.put(adviceBean, resolver); if (logger.isInfoEnabled()) { logger.info("Detected @ExceptionHandler methods in " + adviceBean); } } if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) { this.responseBodyAdvice.add(adviceBean); if (logger.isInfoEnabled()) { logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean); } } }} List-2中，initExceptionHandlerAdviceCache方法调用List-1中ControllerAdviceBean的findAnnotatedBeans方法，获取所有ControllerAdvice的bean，之后排序，所以当有多个ControllerAdivce注解的类且需要排序时，可以实现spring的Order接口来实现。
之后遍历ControllerAdviceBean，之后获取Bean的class类，传入到ExceptionHandlerMethodResolver的构造方法中，如下List-3所示:
List-3
public class ExceptionHandlerMethodResolver { ... public static final MethodFilter EXCEPTION_HANDLER_METHODS = method -> (AnnotationUtils.findAnnotation(method, ExceptionHandler.class) != null); ... private final Map, Method> mappedMethods = new HashMap<>(16); ... public ExceptionHandlerMethodResolver(Class handlerType) { for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) { for (Class exceptionType : detectExceptionMappings(method)) { addExceptionMapping(exceptionType, method); } } } ... private List> detectExceptionMappings(Method method) { List> result = new ArrayList<>(); 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 " + method); } return result; } protected void detectAnnotationExceptionMappings(Method method, List> result) { ExceptionHandler ann = AnnotationUtils.findAnnotation(method, ExceptionHandler.class); Assert.state(ann != null, "No ExceptionHandler annotation"); result.addAll(Arrays.asList(ann.value())); } private void addExceptionMapping(Class exceptionType, Method method) { Method oldMethod = this.mappedMethods.put(exceptionType, method); if (oldMethod != null && !oldMethod.equals(method)) { throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" + exceptionType + "]: {" + oldMethod + ", " + method + "}"); } } 找到方法上有ExceptionHandler注解的方法。
detectExceptionMappings方法获取ExceptionHandler的value值，如果我们没有设置ExceptionHandler的value，那么遍历方法的参数，如果参数是Throwable的子类，就将改类型放入result中，所以由此可知道，我们可以不设置ExceptionHandler的value，只需要将方法的参数设置为Throwable的子类即可，spring会自动识别。
addExceptionMapping方法将结果放入mappedMethods这个map中，key是Throwable，而value则是method。
再回到List-2中，initExceptionHandlerAdviceCache方法中，将构造好的ControllerAdviceBean和ExceptionHandlerMethodResolver放入到exceptionHandlerAdviceCache(是个map)中。
HandlerExceptionResolver是个接口，如下List-4所示:
List-4
public interface HandlerExceptionResolver { @Nullable ModelAndView resolveException( HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);}AbstractHandlerExceptionResolver.resolveException->AbstractHandlerMethodExceptionResolver.doResolveException->ExceptionHandlerExceptionResolver.doResolveHandlerMethodException。
接下来，来看Springmvc中是如何处理我们的ControllerAdvice的。
DispatcherServlet中，doDispatch()->processDispatchResult()->processHandlerException()，如下List-5所示，会遍历HandlerExceptionResovler来处理。
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,@Nullable Object handler, Exception ex) throws Exception { ModelAndView exMv = null; if (this.handlerExceptionResolvers != null) { for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) { exMv = handlerExceptionResolver.resolveException(request, response, handler, ex); if (exMv != null) { break; } }}ExceptionHandlerExceptionResolver是如何加入到Springmvc中handlerExceptionResolvers的，是因为DispatcherServlet.properties中HandlerExceptionResolver的值有ExceptionHandlerExceptionResolver，所以会被Spring自动加入进去。
Spring通过上面的方式，将捕获到的异常交给ExceptionHandlerExceptionResolver.doResolveHandlerMethodException来处理，通过多次转换，最终调用我们设置带有ExceptionHandler注解的方法。
通过源码分析，带有ControllerAdvice和ExceptionHandler注解的拦截处理的执行先于HandlerInterceptor的afterCompletion。
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