🍃前言
在前面的项目开发中，我们使用了统一数据返回和统一异常的功能
它们都是基于 @ControllerAdvice 注解来实现的,接下来我们将通过分析
@ControllerAdvice 的源码,来了解他们的执行流程.
🎄@ControllerAdvice源码分析
首先我们点击@ControllerAdvice，查看其实现源码
Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Component public @interface ControllerAdvice { @AliasFor("basePackages") String[] value() default {}; @AliasFor("value") String[] basePackages() default {}; Class<?>[] basePackageClasses() default {}; Class<?>[] assignableTypes() default {}; Class<? extends Annotation>[] annotations() default {}; }
从上述源码可以看出 @ControllerAdvice 派⽣于@Component组件,这也就是为什么没有五⼤注解,ControllerAdvice 就⽣效的原因.
下⾯我们看看Spring是怎么实现着两项功能的,我们还是从 DispatcherServlet 的代码开始分析.
DispatcherServlet 对象在创建时会初始化⼀系列的对象
public class DispatcherServlet extends FrameworkServlet { //... @Override protected void onRefresh(ApplicationContext context) { initStrategies(context); } /** * Initialize the strategy objects that this servlet uses. * <p>May be overridden in subclasses in order to initialize further strategy objects. */ protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); } //... }
对于 @ControllerAdvice 注解，我们重点关注initHandlerAdapters(context) 和initHandlerExceptionResolvers(context) 这两个⽅法.
🚩initHandlerAdapters(context)
initHandlerAdapters(context) 方法会取得所有实现了 HandlerAdapter 接⼝的bean并保存起来，其中有⼀个类型为 RequestMappingHandlerAdapter 的bean，这个bean就是@RequestMapping 注解能起作⽤的关键，这个bean在应⽤启动过程中会获取所有被@ControllerAdvice 注解标注的bean对象,并做进⼀步处理，关键代码如下
public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter implements BeanFactoryAware, InitializingBean { //... /** * 添加ControllerAdvice bean的处理 */ private void initControllerAdviceCache() { if (getApplicationContext() == null) { return; } //获取所有所有被 @ControllerAdvice 注解标注的bean对象 List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext()); List<Object> requestResponseBodyAdviceBeans = new ArrayList<>(); for (ControllerAdviceBean adviceBean : adviceBeans) { Class<?> beanType = adviceBean.getBeanType(); if (beanType == null) { throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean); } Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS); if (!attrMethods.isEmpty()) { this.modelAttributeAdviceCache.put(adviceBean, attrMethods); } Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS); if (!binderMethods.isEmpty()) { this.initBinderAdviceCache.put(adviceBean, binderMethods); } if (RequestBodyAdvice.class.isAssignableFrom(beanType) || ResponseBodyAdvice.class.isAssignableFrom(beanType)) { requestResponseBodyAdviceBeans.add(adviceBean); } } if (!requestResponseBodyAdviceBeans.isEmpty()) { this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans); } if (logger.isDebugEnabled()) { int modelSize = this.modelAttributeAdviceCache.size(); int binderSize = this.initBinderAdviceCache.size(); int reqCount = getBodyAdviceCount(RequestBodyAdvice.class); int resCount = getBodyAdviceCount(ResponseBodyAdvice.class); if (modelSize == 0 && binderSize == 0 && reqCount == 0 && resCount == 0) { logger.debug("ControllerAdvice beans: none"); } else { logger.debug("ControllerAdvice beans: " + modelSize + " @ModelAttribute, " + binderSize + " @InitBinder, " + reqCount + " RequestBodyAdvice, " + resCount + " ResponseBodyAdvice"); } } } //... }
这个方法在执行时会查找使用所有的 @ControllerAdvice 类，把 ResponseBodyAdvice 类放在容器中，当发⽣某个事件时，调用相应的Advice⽅法，就比如如返回数据前调用统⼀数据封装
🚩initHandlerExceptionResolvers(context)
DispatcherServlet 的 initHandlerExceptionResolvers(context) ⽅法，这个⽅法会取得所有实现了 HandlerExceptionResolver 接⼝的bean并保存起来，其中就有⼀个类型为 ExceptionHandlerExceptionResolver 的bean，这个bean在应⽤启动过程中会获取所有被 @ControllerAdvice 注解标注的bean对象做进⼀步处理,代码如下：
public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolver implements ApplicationContextAware, InitializingBean { //... private void initExceptionHandlerAdviceCache() { if (getApplicationContext() == null) { return; } // 获取所有所有被 @ControllerAdvice 注解标注的bean对象 List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext()); 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 (ResponseBodyAdvice.class.isAssignableFrom(beanType)) { this.responseBodyAdvice.add(adviceBean); } } if (logger.isDebugEnabled()) { int handlerSize = this.exceptionHandlerAdviceCache.size(); int adviceSize = this.responseBodyAdvice.size(); if (handlerSize == 0 && adviceSize == 0) { logger.debug("ControllerAdvice beans: none"); } else { logger.debug("ControllerAdvice beans: " + handlerSize + " @ExceptionHandler, " + adviceSize + " ResponseBodyAdvice"); } } } //... }
当Controller抛出异常时， DispatcherServlet 通过ExceptionHandlerExceptionResolver 来解析异常，⽽ExceptionHandlerExceptionResolver 又通过ExceptionHandlerMethodResolver来解析异常，ExceptionHandlerMethodResolver最终解析异常找到适⽤的@ExceptionHandler标注的⽅法是这⾥，如下代码所示：
public class ExceptionHandlerMethodResolver { //... private Method getMappedMethod(Class<? extends Throwable> exceptionType) { List<Class<? extends Throwable>> matches = new ArrayList(); //根据异常类型, 查找匹配的异常处理⽅法 //⽐如NullPointerException会匹配两个异常处理⽅法: //handler(Exception e) 和 handler(NullPointerException e) for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) { if (mappedException.isAssignableFrom(exceptionType)) { matches.add(mappedException); } } //如果查找到多个匹配, 就进⾏排序, 找到最使⽤的⽅法. 排序的规则依据抛出异常相对于声明异常的深度 //⽐如抛出的是NullPointerException(继承于RuntimeException,RuntimeException⼜继承于Exception) //相对于handler(NullPointerException e) 声明的NullPointerException深度为0, //相对于handler(Exception e) 声明的Exception 深度 为2 //所以 handler(NullPointerException e)标注的⽅法会排在前⾯ if (!matches.isEmpty()) { if (matches.size() > 1) { matches.sort(new ExceptionDepthComparator(exceptionType)); } return this.mappedMethods.get(matches.get(0)); } else { return NO_MATCHING_EXCEPTION_HANDLER_METHOD; } } //... }
⭕总结
关于《【JavaEE进阶】 @ControllerAdvice源码分析》就讲解到这儿，感谢大家的支持，欢迎各位留言交流以及批评指正，如果文章对您有帮助或者觉得作者写的还不错可以点一下关注，点赞，收藏支持一下！
