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最近在做系统升级的时候，引发了一个BUG，原本系统是有一个异常处理器A，引入了某个底包中也带了一个异常处理器B，最终走了底包的异常处理器B。 A对于异常的时候会返回HTTP状态码为500，B对于异常处理器返回的HTTP状态码为200，前端基于HTTP状态码进行提示的，就出了问题

本篇文章我们就来讨论一下在JavaWeb中的全局异常处理器是何时何地如何执行的。

在进行学习之前需要先知道：HTTP执行流程，SpringMVC执行流程

一、全局异常处理器的类型

全局异常处理器的父接口是 HandlerExceptionResolver，简单来说就是实现或间接实现它的类就叫全局异常处理器。

package org.springframework.web.servlet;import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.lang.Nullable;public interface HandlerExceptionResolver {@NullableModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);
}

HandlerExceptionResolver 的继承关系图

1-1、实现方式一

SpringBoot项目最大的特点就是注解，在SpringBoot项目中全局异常拦截的注解是@ControllerAdvice （@RestControllerAdvice = @ControllerAdvice + @ResponseBody）

使用 @ControllerAdvice的类最终会生成 ExceptionHandlerExceptionResolver

1-2、实现方式二

重写 doResolveHandlerMethodException 方法，然后注册当前的bean

public class ExceptionHandler extends AbstractHandlerMethodExceptionResolver {private final static Logger logger = LoggerFactory.getLogger(ExceptionHandler.class);@Overrideprotected ModelAndView doResolveHandlerMethodException(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod, Exception ex) {return new ModelAndView();}
}

二、全局异常拦截点

2-1、入口

org.springframework.web.servlet.DispatcherServlet#doDispatch 这个方法就是SpringMVC的执行流程的核心代码了，下面是简化代码

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {// ...mv = ha.handle(processedRequest, response, mappedHandler.getHandler());// ....}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {dispatchException = new NestedServletException("Handler dispatch failed", err);}// 异常处理的入口processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {// ....}catch (Throwable err) {// ....}finally {// ...       }
}

org.springframework.web.servlet.DispatcherServlet#processDispatchResult

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,@Nullable Exception exception) throws Exception {boolean errorView = false;if (exception != null) {Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);// 异常处理mv = processHandlerException(request, response, handler, exception);errorView = (mv != null);}// ...
}

org.springframework.web.servlet.DispatcherServlet#processHandlerException

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,@Nullable Object handler, Exception ex) throws Exception {// Success and error responses may use different content typesrequest.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);// Check registered HandlerExceptionResolvers...ModelAndView exMv = null;if (this.handlerExceptionResolvers != null) {// 遍历循环所有的拦截器来尝试处理这个异常（拦截器已经按照 order 排好序了）for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {exMv = resolver.resolveException(request, response, handler, ex);// 只有返回了 ModelAndView 才结束，不然一直往下走if (exMv != null) {break;}}}// ...// 如果没有全局异常处理器 可以处理这个异常 就继续抛出去throw ex;
}

2-2、全局异常拦截器是如何注入到 DispatcherServlet 的

上面看到是从 handlerExceptionResolvers 从获取所有的异常处理器，它是一个list

@Nullable
private List<HandlerExceptionResolver> handlerExceptionResolvers;

在DispatcherServlet里面有一个onRefresh方法，它是重写的父类FrameworkServlet的，在初始化ServletBean的时候会被调用一次，它里面会做很多初始化的操作，其中一个就是获取容器里面的全局异常拦截器

一层层看上去其实是 Servlet接口的 init方法触发的

@Override
protected void onRefresh(ApplicationContext context) {initStrategies(context);
}protected void initStrategies(ApplicationContext context) {// ...initHandlerExceptionResolvers(context);// ...
}

找到bean容器里面的所有异常拦截器，把它存在 handlerExceptionResolvers 里面，并排序

private void initHandlerExceptionResolvers(ApplicationContext context) {this.handlerExceptionResolvers = null;if (this.detectAllHandlerExceptionResolvers) {// 从bean容器里面找到所有的 HandlerExceptionResolverMap<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());// 排序AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);}}// ...
}

三、ControllerAdvice 如何解析、执行

3-1、解析

在springframework 中有这样一个类 ExceptionHandlerExceptionResolver

package org.springframework.web.servlet.mvc.method.annotation;public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolverimplements ApplicationContextAware, InitializingBean {// ...
}

⚠️：可以回到【全局异常处理器的类型】的图看看，ExceptionHandlerExceptionResolver其实就是全局异常处理器HandlerExceptionResolver的子类

它实现了 InitializingBean，重写了afterPropertiesSet（这个方法会在bean初始化完之后执行）

@Override
public void afterPropertiesSet() {// Do this first, it may add ResponseBodyAdvice beansinitExceptionHandlerAdviceCache();// ...
}

initExceptionHandlerAdviceCache 会把所有使用了@ControllerAdvice 的bean找到并把它存在自己的参数里面

private final Map<ControllerAdviceBean, ExceptionHandlerMethodResolver> exceptionHandlerAdviceCache = new LinkedHashMap<>();private void initExceptionHandlerAdviceCache() {if (getApplicationContext() == null) {return;}// 找到所有使用了 @ControllerAdvice 的beanList<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());for (ControllerAdviceBean adviceBean : adviceBeans) {Class<?> beanType = adviceBean.getBeanType();if (beanType == null) {throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);}// 解析全部的 ExceptionHandler 注解ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);if (resolver.hasExceptionMappings()) {// 存入当前的类参数里面this.exceptionHandlerAdviceCache.put(adviceBean, resolver);}if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {this.responseBodyAdvice.add(adviceBean);}}// ...
}

org.springframework.web.method.ControllerAdviceBean#findAnnotatedBeans

public static List<ControllerAdviceBean> findAnnotatedBeans(ApplicationContext context) {ListableBeanFactory beanFactory = context;if (context instanceof ConfigurableApplicationContext) {// Use internal BeanFactory for potential downcast to ConfigurableBeanFactory abovebeanFactory = ((ConfigurableApplicationContext) context).getBeanFactory();}List<ControllerAdviceBean> adviceBeans = new ArrayList<>();// 遍历所有的beanfor (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, Object.class)) {if (!ScopedProxyUtils.isScopedTarget(name)) {// 找到符合的beanControllerAdvice controllerAdvice = beanFactory.findAnnotationOnBean(name, ControllerAdvice.class);if (controllerAdvice != null) {// 存起来adviceBeans.add(new ControllerAdviceBean(name, beanFactory, controllerAdvice));}}}// 排序OrderComparator.sort(adviceBeans);return adviceBeans;
}

配合@ControllerAdvice 注解的通常是 @ExceptionHandler 它用来制定具体的异常，把所有的 ExceptionHandler都存入了 mappedMethods 中org.springframework.web.method.annotation.ExceptionHandlerMethodResolver#ExceptionHandlerMethodResolver

public ExceptionHandlerMethodResolver(Class<?> handlerType) {for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {addExceptionMapping(exceptionType, method);}}
}
private void addExceptionMapping(Class<? extends Throwable> 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 + "}");}
}

至此@ControllerAdvice的解析完成

1. 生成了一个ExceptionHandlerExceptionResolver，它通过多级实现了 HandlerExceptionResolver
2. 所有使用@ControllerAdvice的类都存在了 exceptionHandlerAdviceCache 中
3. 所有使用 @ExceptionHandler 的方法否存在了mappedMethods 中

3-2、执行

1. 从【2-1】得知，执行异常处理器的时候是执行 HandlerExceptionResolver.resolveException方法（它只有这一个方法）
2. 从【3-1】得知，所有使用 @ControllerAdvice 注解的类都被存在了ExceptionHandlerExceptionResolver 中
3. 从【1】得知，ExceptionHandlerExceptionResolver的继承关系如下图
   

一层层去看调用关系，最终会执行的是 （这个很简单直接去看即可）org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver#doResolveHandlerMethodException

执行过程就是循环exceptionHandlerAdviceCache中的每一个全局拦截器，再循环每个拦截器里面的mappedMethods看哪个可以匹配上，就执行哪个

四、其它

4-1、设置HTTP状态码

大多数情况下我们会自定义返回值code，比如未鉴权，返回给前端HTTP状态码是200，code为401，但在某些情况下也会直接返回HTTP状态码401，可以使用 @ResponseStatus

@ResponseStatus(HttpStatus.UNAUTHORIZED)
@ExceptionHandler(Exception.class)
public ResultObj bizExceptionHandler(Exception e) {log.info("全局异常拦截", e);return ResultObj.success();
}

4-2、异常处理器排序

springframework 里面提供了一个Ordered 接口，实现它重写里面 getOrder 方法就可以进行排序了

4-3、所谓全局异常

并不是系统任何异常都会被它所拦截，因为我们已经知道它的执行点是在MVC的流程中，所以就只有HTTP异常才会被拦截处理