3.1@initBinder和@ModelAttribute的作用原理

@initBinder和@ModelAttribute都是请求过程中的处理，我们知道springMvc通过HandlerApapter定位到具体的方法进行请求处理，因此查看HandlerHaper的实现类，发现RequestMappingHandlerAdapter比较符合我们的目标

点进去RequestMappingHandlerAdapter后发现里面的一个方法如下

@Override
    public void afterPropertiesSet() {
        // Do this first, it may add ResponseBody advice beans
// 这里会添加ResponseBody advice beans
        initControllerAdviceCache();

        if (this.argumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
            this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.initBinderArgumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
            this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.returnValueHandlers == null) {
            List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
            this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
        }
    }
// 这里找到contollerAdvice注解的类，缓存里面的方法
private void initControllerAdviceCache() {
        if (getApplicationContext() == null) {
            return;
        }
// 找到@ControllerAdvice注解标注的类
        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);
            }
// 找到所有ModelAttribute标注的方法进行缓存，就可以使用了
            Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
            if (!attrMethods.isEmpty()) {
                this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
            }
// 找到所有initBinder注解标注的方法进行缓存，就可以使用了
            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);
        }
// ......日志处理
    }

3.2@ExceptionHandler注解的作用原理

相同的思路，@ExceptionHandler是响应时的处理，因此需要找到对应的Resolver，进入initHandlerExceptionResolvers(context)方法,

属性填充后会进行afterPropertiesSet方法，这个方法可以用在一些特殊情况中，也就是某个对象的某个属性需要经过外界得到，比如说查询数据库等方式，这时候可以用到spring的该特性，只需要实现InitializingBean。