RequestBodyAdvice ResponseBodyAdvice增强类
RequestBodyAdvice类和ResponseBodyAdvice是spring提供的接口，它可以在请求参数解析前，和响应输出前对controller的返回值
进行拦截，替换。
先看RequestBodyAdvice的Api，它可以在从流中读取参数前，读取参数后进行拦截
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public interface RequestBodyAdvice {
boolean supports(MethodParameter methodParameter, Type targetType,
Class<? extends HttpMessageConverter<?>> converterType);
HttpInputMessage beforeBodyRead(HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType, Class<? extends HttpMessageConverter<?>> converterType) throws IOException;
Object afterBodyRead(Object body, HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
@Nullable
Object handleEmptyBody(@Nullable Object body, HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
}
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再看ResponseBodyAdvice的Api,它可以在输出返回值前，对响应进行拦截
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| public interface ResponseBodyAdvice<T> {
boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType);
@Nullable
T beforeBodyWrite(@Nullable T body, MethodParameter returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType,
ServerHttpRequest request, ServerHttpResponse response);
}
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他的加载是在下面这个方法里面实现的
初始化方法
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#afterPropertiesSet
被调用
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#initControllerAdviceCache
他在扫描@controllerAdvice类时，如果发现@controllerAdvice的类同时也是RequestBodyAdvice或ResponseBodyAdvice
则将其加入集合中。
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| private void initControllerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
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);
}
Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
if (!attrMethods.isEmpty()) {
this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
}
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);
}
}
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加入集合存储起来，并且在创建RequestResponseBodyMethodProcessor时，将其传递过去。
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| private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);
handlers.add(new ModelAndViewMethodReturnValueHandler());
handlers.add(new ModelMethodProcessor());
handlers.add(new ViewMethodReturnValueHandler());
handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),
this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));
handlers.add(new StreamingResponseBodyReturnValueHandler());
handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
this.contentNegotiationManager, this.requestResponseBodyAdvice));
handlers.add(new HttpHeadersReturnValueHandler());
handlers.add(new CallableMethodReturnValueHandler());
handlers.add(new DeferredResultMethodReturnValueHandler());
handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
handlers.add(new ModelAttributeMethodProcessor(false));
handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
this.contentNegotiationManager, this.requestResponseBodyAdvice));
handlers.add(new ViewNameMethodReturnValueHandler());
handlers.add(new MapMethodProcessor());
if (getCustomReturnValueHandlers() != null) {
handlers.addAll(getCustomReturnValueHandlers());
}
if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
}
else {
handlers.add(new ModelAttributeMethodProcessor(true));
}
return handlers;
}
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RequestResponseBodyMethodProcessor类就是处理ResponseBody响应的返回值处理器，他在将返回值转换成json返回前调用ResponseBodyAdvice
，我们可以在此修改或替换返回值的内容
同理RequestBodyAdvice是在RequestResponseBodyMethodProcessor解析参数时发挥作用的。
总结一下就是，
- 这两个类一个是在解析参数时发挥作用，一个是在输出返回值前发挥作用
- 这两个类的加载时机是在加载@controllerAdvice类时顺便判断的，所以要将这两个类上标注@controllerAdvice才能生效
- 输出响应必须使用RequestResponseBodyMethodProcessor处理器或HttpEntityMethodProcessor处理器，这两个Advice才能生效
一般我用它输出请求响应的值记录到日志里，似乎也没有其他用处非得用这两个不可。