源码剖析Springboot自定义异常

博主看到新服务是封装的自定义异常，准备入手剖析一下，自定义的异常是如何进行抓住我们请求的方法的异常，并进行封装返回到。废话不多说，先看看如何才能实现封装异常，先来一个示例：

@ControllerAdvice
public class TstExceptionHandle{

    @ExceptionHandler(Exception.class)
    public void myExceptionHandle(HttpServletResponse response){
        response.setStatus(403);
        System.out.println("做封装处理");
    }

}

博主只做了简单的配置示例，主要的是进行源码剖析Springboot是如何获取自定义异常并进行返回的。来吧！

第一步：肯定是在Springboot启动的过程中进行的异常处理初始化，于是就找到了handlerExceptionResolver类，在创建该类的时候，会进行添加我们自定义异常。

     public HandlerExceptionResolver handlerExceptionResolver(
              @Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager) {
          List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<>();
          //不用管这个方法，这个方法主要进行的是调用实现了WebMvcConfigurer接口bean的configureHandlerExceptionResolvers方法，系统的都是空方法
          configureHandlerExceptionResolvers(exceptionResolvers);
          if (exceptionResolvers.isEmpty()) {
              //我们的在这里才添加，我们看看这个方法
              addDefaultHandlerExceptionResolvers(exceptionResolvers, contentNegotiationManager);
          }
         extendHandlerExceptionResolvers(exceptionResolvers);
         HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();
         composite.setOrder(0);
         composite.setExceptionResolvers(exceptionResolvers);
         return composite;
     }

     protected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers,
              ContentNegotiationManager mvcContentNegotiationManager) {
  
          ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();
          exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager);
          exceptionHandlerResolver.setMessageConverters(getMessageConverters());
          exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());
          exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());
          if (jackson2Present) {
             exceptionHandlerResolver.setResponseBodyAdvice(
                     Collections.singletonList(new JsonViewResponseBodyAdvice()));
         }
         if (this.applicationContext != null) {
             exceptionHandlerResolver.setApplicationContext(this.applicationContext);
         }
         //上面的 都是设置的属性，跟我们没啥大关系，主要在这里进行的添加自定义异常处理
         exceptionHandlerResolver.afterPropertiesSet();
         exceptionResolvers.add(exceptionHandlerResolver);
 
         ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();
         responseStatusResolver.setMessageSource(this.applicationContext);
         exceptionResolvers.add(responseStatusResolver);
 
         exceptionResolvers.add(new DefaultHandlerExceptionResolver());
     }

最主要的初始化过程在这里，从这些代码中就可以看到为什么我们自定义异常需要进行使用@ControllerAdvice，并且方法使用@ExceptionHandler(Exception.class)注解了

     @Override
      public void afterPropertiesSet() {
          // Do this first, it may add ResponseBodyAdvice beans
          //走这里初始化，添加
          initExceptionHandlerAdviceCache();
  
          if (this.argumentResolvers == null) {
              List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
              this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
         }
         if (this.returnValueHandlers == null) {
             List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
             this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
         }
     }
 
 
     org/springframework/web/servlet/mvc/method/annotation/ExceptionHandlerExceptionResolver.java
     private void initExceptionHandlerAdviceCache() {
         if (getApplicationContext() == null) {
             return;
         }
         //看到这里基本就知道啥意思了，找出带有@ControllerAdvice的注解bean
         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);
             }
             //找出当前bean的异常处理方法
             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");
             }
         }
     }

找到类后，是如何找到方法的呢？主要看如何创建ExceptionHandlerMethodResolver的过程。

 public ExceptionHandlerMethodResolver(Class<?> handlerType) {
      //EXCEPTION_HANDLER_METHODS的定义：
      //public static final MethodFilter EXCEPTION_HANDLER_METHODS = method ->
      //            AnnotatedElementUtils.hasAnnotation(method, ExceptionHandler.class);
      //所以他会寻找带有ExceptionHandler注解的方法
          for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
              //寻找方法注解上配置的捕获的异常类，并添加，如果有两个方法都对一个异常进行自定义处理了，怎么办呢。
              for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
                  //他会出异常的。不过前提是同一个类里，不同类对同一个异常进行自定义的话，谁在前面就有谁来处理
                 addExceptionMapping(exceptionType, method);
             }
         }
     }

添加自定义异常的时候抛异常是在这里

     private void addExceptionMapping(Class<? extends Throwable> exceptionType, Method method) {
         Method oldMethod = this.mappedMethods.put(exceptionType, method);
         //在这里，已经显示出来了，博主就不试了
         if (oldMethod != null && !oldMethod.equals(method)) {
throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" +
                     exceptionType + "]: {" + oldMethod + ", " + method + "}");
         }
     }

好了。所有异常添加完毕了，我们来测试一下异常来的时候，Springboot是如何选择自定义异常并返回的，我们上面所有的操作都是在创建HandlerExceptionResolver时进行的，为什么要添加到HandlerExceptionResolver这里呢？看一下代码：

 //第一次请求进来时，会先查找是否有自定义异常，如果有的话添加，没有记录日志就完了
      private void initHandlerExceptionResolvers(ApplicationContext context) {
          this.handlerExceptionResolvers = null;
  
          if (this.detectAllHandlerExceptionResolvers) {
              // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
                          //这里会在beanfactroy中查找到HandlerExceptionResolver类，刚才初始化的时候，我们所有的自定义异常都在里面 
              Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
                     .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
             if (!matchingBeans.isEmpty()) {
                 this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
                 // We keep HandlerExceptionResolvers in sorted order.
                 AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
             }
         }
         else {
             try {
                 HandlerExceptionResolver her =
                         context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
                 this.handlerExceptionResolvers = Collections.singletonList(her);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, no HandlerExceptionResolver is fine too.
             }
         }
 
         // Ensure we have at least some HandlerExceptionResolvers, by registering
         // default HandlerExceptionResolvers if no other resolvers are found.
         if (this.handlerExceptionResolvers == null) {
             this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
             if (logger.isTraceEnabled()) {
                 logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +
                         "': using default strategies from DispatcherServlet.properties");
             }
         }
     }            

走完初始化，经过过滤器，拦截器终于到了我们的请求方法，我们的方法还报错了，所以会走到异常中，我们DispatcherServlet会进行抓住异常，然后回调用我们的processDispatchResult方法，大家可以自己看一下org/springframework/web/servlet/DispatcherServlet.java的源码，然后我们来分析一下这个方法都干啥了吧

     private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
              @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
              @Nullable Exception exception) throws Exception {
  
          boolean errorView = false;
  
          if (exception != null) {
              if (exception instanceof ModelAndViewDefiningException) {
                  logger.debug("ModelAndViewDefiningException encountered", exception);
                 mv = ((ModelAndViewDefiningException) exception).getModelAndView();
             }
             else {
                 Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                 //如果请求方法有异常，则进行处理，并返回ModelAndView
                 mv = processHandlerException(request, response, handler, exception);
                 errorView = (mv != null);
             }
         }
     .........
     }

那Springboot是如何选择哪一个是符合条件的自定义异常处理呢？如果我们定义了两个处理类，都对同一个异常进行捕获并返回不一样的信息咋办呢？看源码吧

 //这里会选择符合条件的自定义异常
      protected ServletInvocableHandlerMethod getExceptionHandlerMethod(
              @Nullable HandlerMethod handlerMethod, Exception exception) {
  
          Class<?> handlerType = null;
  
          if (handlerMethod != null) {
              // Local exception handler methods on the controller class itself.
              // To be invoked through the proxy, even in case of an interface-based proxy.
             handlerType = handlerMethod.getBeanType();
             ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
             if (resolver == null) {
                 resolver = new ExceptionHandlerMethodResolver(handlerType);
                 this.exceptionHandlerCache.put(handlerType, resolver);
             }
             Method method = resolver.resolveMethod(exception);
             if (method != null) {
                 return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
             }
             // For advice applicability check below (involving base packages, assignable types
             // and annotation presence), use target class instead of interface-based proxy.
             if (Proxy.isProxyClass(handlerType)) {
                 handlerType = AopUtils.getTargetClass(handlerMethod.getBean());
             }
         }
         //exceptionHandlerAdviceCache这个map是我们添加 的自定义异常
         for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
             ControllerAdviceBean advice = entry.getKey();
             //这个判断条件是查看是否有符合条件的自定义异常，如果有两个的话，
             if (advice.isApplicableToBeanType(handlerType)) {
                 ExceptionHandlerMethodResolver resolver = entry.getValue();
                 Method method = resolver.resolveMethod(exception);
                 if (method != null) {
                    return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
                 }
             }
         }
 
         return null;
     }

逻辑基本是上面的，但是真正处理是否符合是在这里的一个方法中：

 public boolean isApplicableToBeanType(@Nullable Class<?> beanType) {
          return this.beanTypePredicate.test(beanType);
      }
      public boolean test(Class<?> controllerType) {
           ///默认不配的其他属性的时候是返回true的，就是对所有包下的异常都适用
          if (!hasSelectors()) {
              return true;
          }
          else if (controllerType != null) {
             //我们的@ControllerAdvice注解是有basePackages属性的，只有匹配成功才会返回，否则就算自定义异常想要捕获，不在捕获包范围下不管该异常
             for (String basePackage : this.basePackages) {
                 if (controllerType.getName().startsWith(basePackage)) {
                     return true;
                 }
             }
             for (Class<?> clazz : this.assignableTypes) {
                 if (ClassUtils.isAssignable(clazz, controllerType)) {
                     return true;
                 }
             }
             for (Class<? extends Annotation> annotationClass : this.annotations) {
                 if (AnnotationUtils.findAnnotation(controllerType, annotationClass) != null) {
                     return true;
                 }
             }
         }
         return false;
     }

到这里基本如何写自定义异常、以及为什么这么写、底层做了哪些判断都已经讲解完了，自定义异常在工作中还是非常常用的一种手段，因为我们不可能暴露出我们内部的错误信息直接返回给用户，不仅用户体验不好，并且安全性也极其差。