原始碼剖析Springboot自定義例外

　　博主看到新服務是封裝的自定義例外，準備入手剖析一下，自定義的例外是如何進行抓住我們請求的方法的例外，并進行封裝回傳到，廢話不多說，先看看如何才能實作封裝例外，先來一個示例：

 1 @ControllerAdvice
 2 public class TstExceptionHandle{
 3 
 4     @ExceptionHandler(Exception.class)
 5     public void myExceptionHandle(HttpServletResponse response){
 6         response.setStatus(403);
 7         System.out.println("做封裝處理");
 8     }
 9 
10 }

　　博主只做了簡單的配置示例，主要的是進行原始碼剖析Springboot是如何獲取自定義例外并進行回傳的，來吧！

　　第一步：肯定是在Springboot啟動的程序中進行的例外處理初始化，于是就找到了handlerExceptionResolver類，在創建該類的時候，會進行添加我們自定義例外，

 1     public HandlerExceptionResolver handlerExceptionResolver(
 2             @Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager) {
 3         List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<>();
 4         //不用管這個方法，這個方法主要進行的是呼叫實作了WebMvcConfigurer介面bean的configureHandlerExceptionResolvers方法，系統的都是空方法
 5         configureHandlerExceptionResolvers(exceptionResolvers);
 6         if (exceptionResolvers.isEmpty()) {
 7             //我們的在這里才添加，我們看看這個方法
 8             addDefaultHandlerExceptionResolvers(exceptionResolvers, contentNegotiationManager);
 9         }
10         extendHandlerExceptionResolvers(exceptionResolvers);
11         HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();
12         composite.setOrder(0);
13         composite.setExceptionResolvers(exceptionResolvers);
14         return composite;
15     }

 1     protected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers,
 2             ContentNegotiationManager mvcContentNegotiationManager) {
 3 
 4         ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();
 5         exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager);
 6         exceptionHandlerResolver.setMessageConverters(getMessageConverters());
 7         exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());
 8         exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());
 9         if (jackson2Present) {
10             exceptionHandlerResolver.setResponseBodyAdvice(
11                     Collections.singletonList(new JsonViewResponseBodyAdvice()));
12         }
13         if (this.applicationContext != null) {
14             exceptionHandlerResolver.setApplicationContext(this.applicationContext);
15         }
16         //上面的 都是設定的屬性，跟我們沒啥大關系，主要在這里進行的添加自定義例外處理
17         exceptionHandlerResolver.afterPropertiesSet();
18         exceptionResolvers.add(exceptionHandlerResolver);
19 
20         ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();
21         responseStatusResolver.setMessageSource(this.applicationContext);
22         exceptionResolvers.add(responseStatusResolver);
23 
24         exceptionResolvers.add(new DefaultHandlerExceptionResolver());
25     }

 　　 最主要的初始化程序在這里，從這些代碼中就可以看到為什么我們自定義例外需要進行使用@ControllerAdvice，并且方法使用@ExceptionHandler(Exception.class)注解了

 1     @Override
 2     public void afterPropertiesSet() {
 3         // Do this first, it may add ResponseBodyAdvice beans
 4         //走這里初始化，添加
 5         initExceptionHandlerAdviceCache();
 6 
 7         if (this.argumentResolvers == null) {
 8             List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
 9             this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
10         }
11         if (this.returnValueHandlers == null) {
12             List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
13             this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
14         }
15     }
16 
17 
18     org/springframework/web/servlet/mvc/method/annotation/ExceptionHandlerExceptionResolver.java
19     private void initExceptionHandlerAdviceCache() {
20         if (getApplicationContext() == null) {
21             return;
22         }
23         //看到這里基本就知道啥意思了，找出帶有@ControllerAdvice的注解bean
24         List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
25         for (ControllerAdviceBean adviceBean : adviceBeans) {
26             Class<?> beanType = adviceBean.getBeanType();
27             if (beanType == null) {
28                 throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
29             }
30             //找出當前bean的例外處理方法
31             ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
32             if (resolver.hasExceptionMappings()) {
33                 this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
34             }
35             if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
36                 this.responseBodyAdvice.add(adviceBean);
37             }
38         }
39 
40         if (logger.isDebugEnabled()) {
41             int handlerSize = this.exceptionHandlerAdviceCache.size();
42             int adviceSize = this.responseBodyAdvice.size();
43             if (handlerSize == 0 && adviceSize == 0) {
44                 logger.debug("ControllerAdvice beans: none");
45             }
46             else {
47                 logger.debug("ControllerAdvice beans: " +
48                         handlerSize + " @ExceptionHandler, " + adviceSize + " ResponseBodyAdvice");
49             }
50         }
51     }

　　找到類后，是如何找到方法的呢？主要看如何創建ExceptionHandlerMethodResolver的程序，

 1 public ExceptionHandlerMethodResolver(Class<?> handlerType) {
 2     //EXCEPTION_HANDLER_METHODS的定義：
 3     //public static final MethodFilter EXCEPTION_HANDLER_METHODS = method ->
 4     //            AnnotatedElementUtils.hasAnnotation(method, ExceptionHandler.class);
 5     //所以他會尋找帶有ExceptionHandler注解的方法
 6         for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
 7             //尋找方法注解上配置的捕獲的例外類，并添加，如果有兩個方法都對一個例外進行自定義處理了，怎么辦呢，
 8             for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
 9                 //他會出例外的，不過前提是同一個類里，不同類對同一個例外進行自定義的話，誰在前面就有誰來處理
10                 addExceptionMapping(exceptionType, method);
11             }
12         }
13     }

　　添加自定義例外的時候拋例外是在這里

1     private void addExceptionMapping(Class<? extends Throwable> exceptionType, Method method) {
2         Method oldMethod = this.mappedMethods.put(exceptionType, method);
3         //在這里，已經顯示出來了，博主就不試了
4         if (oldMethod != null && !oldMethod.equals(method)) {
5             throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" +
6                     exceptionType + "]: {" + oldMethod + ", " + method + "}");
7         }
8     }

　　好了，所有例外添加完畢了，我們來測驗一下例外來的時候，Springboot是如何選擇自定義例外并回傳的，我們上面所有的操作都是在創建HandlerExceptionResolver時進行的，為什么要添加到HandlerExceptionResolver這里呢？看一下代碼：

 1 //第一次請求進來時，會先查找是否有自定義例外，如果有的話添加，沒有記錄日志就完了
 2     private void initHandlerExceptionResolvers(ApplicationContext context) {
 3         this.handlerExceptionResolvers = null;
 4 
 5         if (this.detectAllHandlerExceptionResolvers) {
 6             // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
 7                         //這里會在beanfactroy中查找到HandlerExceptionResolver類，剛才初始化的時候，我們所有的自定義例外都在里面 
 8             Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
 9                     .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
10             if (!matchingBeans.isEmpty()) {
11                 this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
12                 // We keep HandlerExceptionResolvers in sorted order.
13                 AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
14             }
15         }
16         else {
17             try {
18                 HandlerExceptionResolver her =
19                         context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
20                 this.handlerExceptionResolvers = Collections.singletonList(her);
21             }
22             catch (NoSuchBeanDefinitionException ex) {
23                 // Ignore, no HandlerExceptionResolver is fine too.
24             }
25         }
26 
27         // Ensure we have at least some HandlerExceptionResolvers, by registering
28         // default HandlerExceptionResolvers if no other resolvers are found.
29         if (this.handlerExceptionResolvers == null) {
30             this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
31             if (logger.isTraceEnabled()) {
32                 logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +
33                         "': using default strategies from DispatcherServlet.properties");
34             }
35         }
36     }            

　　走完初始化，經過過濾器，攔截器終于到了我們的請求方法，我們的方法還報錯了，所以會走到例外中，我們DispatcherServlet會進行抓住例外，然后回呼叫我們的processDispatchResult方法，大家可以自己看一下org/springframework/web/servlet/DispatcherServlet.java的原始碼，然后我們來分析一下這個方法都干啥了吧

 1     private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
 2             @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
 3             @Nullable Exception exception) throws Exception {
 4 
 5         boolean errorView = false;
 6 
 7         if (exception != null) {
 8             if (exception instanceof ModelAndViewDefiningException) {
 9                 logger.debug("ModelAndViewDefiningException encountered", exception);
10                 mv = ((ModelAndViewDefiningException) exception).getModelAndView();
11             }
12             else {
13                 Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
14                 //如果請求方法有例外，則進行處理，并回傳ModelAndView
15                 mv = processHandlerException(request, response, handler, exception);
16                 errorView = (mv != null);
17             }
18         }
19     .........
20     }

　　那Springboot是如何選擇哪一個是符合條件的自定義例外處理呢？如果我們定義了兩個處理類，都對同一個例外進行捕獲并回傳不一樣的資訊咋辦呢？看原始碼吧

 1 //這里會選擇符合條件的自定義例外
 2     protected ServletInvocableHandlerMethod getExceptionHandlerMethod(
 3             @Nullable HandlerMethod handlerMethod, Exception exception) {
 4 
 5         Class<?> handlerType = null;
 6 
 7         if (handlerMethod != null) {
 8             // Local exception handler methods on the controller class itself.
 9             // To be invoked through the proxy, even in case of an interface-based proxy.
10             handlerType = handlerMethod.getBeanType();
11             ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
12             if (resolver == null) {
13                 resolver = new ExceptionHandlerMethodResolver(handlerType);
14                 this.exceptionHandlerCache.put(handlerType, resolver);
15             }
16             Method method = resolver.resolveMethod(exception);
17             if (method != null) {
18                 return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
19             }
20             // For advice applicability check below (involving base packages, assignable types
21             // and annotation presence), use target class instead of interface-based proxy.
22             if (Proxy.isProxyClass(handlerType)) {
23                 handlerType = AopUtils.getTargetClass(handlerMethod.getBean());
24             }
25         }
26         //exceptionHandlerAdviceCache這個map是我們添加 的自定義例外
27         for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
28             ControllerAdviceBean advice = entry.getKey();
29             //這個判斷條件是查看是否有符合條件的自定義例外，如果有兩個的話，
30             if (advice.isApplicableToBeanType(handlerType)) {
31                 ExceptionHandlerMethodResolver resolver = entry.getValue();
32                 Method method = resolver.resolveMethod(exception);
33                 if (method != null) {
34                     return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
35                 }
36             }
37         }
38 
39         return null;
40     }

　　邏輯基本是上面的，但是真正處理是否符合是在這里的一個方法中：

 1 public boolean isApplicableToBeanType(@Nullable Class<?> beanType) {
 2         return this.beanTypePredicate.test(beanType);
 3     }
 4     public boolean test(Class<?> controllerType) {
 5          ///默認不配的其他屬性的時候是回傳true的，就是對所有包下的例外都適用
 6         if (!hasSelectors()) {
 7             return true;
 8         }
 9         else if (controllerType != null) {
10             //我們的@ControllerAdvice注解是有basePackages屬性的，只有匹配成功才會回傳，否則就算自定義例外想要捕獲，不在捕獲包范圍下不管該例外
11             for (String basePackage : this.basePackages) {
12                 if (controllerType.getName().startsWith(basePackage)) {
13                     return true;
14                 }
15             }
16             for (Class<?> clazz : this.assignableTypes) {
17                 if (ClassUtils.isAssignable(clazz, controllerType)) {
18                     return true;
19                 }
20             }
21             for (Class<? extends Annotation> annotationClass : this.annotations) {
22                 if (AnnotationUtils.findAnnotation(controllerType, annotationClass) != null) {
23                     return true;
24                 }
25             }
26         }
27         return false;
28     }

　　到這里基本如何寫自定義例外、以及為什么這么寫、底層做了哪些判斷都已經講解完了，自定義例外在作業中還是非常常用的一種手段，因為我們不可能暴露出我們內部的錯誤資訊直接回傳給用戶，不僅用戶體驗不好，并且安全性也極其差，

　　原創不易，轉載請說明出處：https://www.cnblogs.com/guoxiaoyu/p/13489565.html

標籤：Java

上一篇：關于Mybatis使用原生介面開發的心得以及示例代碼

下一篇：Spring 解決回圈依賴的 3 種方式！