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。

　　原创不易，转载请说明出处：https://www.cnblogs.com/guoxiaoyu/p/13489565.html

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