前言
本文素材的起源自业务部门技术负责人一次代码走查引发的故事，技术负责人在某次走查成员的代码时，发现他们的业务管制层大量充斥着如下的代码
@PostMapping("add") public User add(@RequestBody User user, HttpServletRequest request){ String tenantId = request.getHeader("x-tenantid"); String appId = request.getHeader("x-appid"); user.setAppId(appId); user.setTenantId(tenantId); return user; }他们的tenantId和appId是作为元数据放在申请头，而业务model又须要tenantId和appId，于是他们团队的成员就写出了形如上的代码，尽管这样的代码是能满足业务要求，然而大面积如上的写法，都是重复性的代码，很不优雅。前面这个技术负责人项通过自定义HandlerMethodArgumentResolver的形式来优雅解决这问题，他的代码形如下
@Datapublic class MetaInfo { private String tenantId; private String appId; }public class MetaInfoHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver { private RequestResponseBodyMethodProcessor handlerMethodArgumentResolver; public MetaInfoHandlerMethodArgumentResolver(RequestResponseBodyMethodProcessor handlerMethodArgumentResolver) { this.handlerMethodArgumentResolver = handlerMethodArgumentResolver; } @Override public boolean supportsParameter(MethodParameter parameter) { return parameter.hasParameterAnnotation(RequestBody.class) && MetaInfo.class.isAssignableFrom(parameter.getParameterType()); } @Override public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception { MetaInfo metaInfo = (MetaInfo) handlerMethodArgumentResolver.resolveArgument(parameter,mavContainer,webRequest,binderFactory); HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class); metaInfo.setAppId(request.getHeader("x-appid")); metaInfo.setTenantId(request.getHeader("x-tenantid")); return metaInfo; }}@Configurationpublic class WebConfig implements WebMvcConfigurer { @Autowired private MetaInfoHandlerMethodArgumentResolver metaInfoHandlerMethodArgumentResolver; @Bean @ConditionalOnMissingBean public MetaInfoHandlerMethodArgumentResolver metaInfoHandlerMethodArgumentResolver(List<HttpMessageConverter<?>> httpMessageConverters){ RequestResponseBodyMethodProcessor handlerMethodArgumentResolver = new RequestResponseBodyMethodProcessor(httpMessageConverters); return new MetaInfoHandlerMethodArgumentResolver(handlerMethodArgumentResolver); } @Override public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) { resolvers.add(metaInfoHandlerMethodArgumentResolver); }}当他写下如下代码时，按他的想法应该是没问题才对，然而事实上这个HandlerMethodArgumentResolver却无奈失效，他排查了很久，没啥脉络，于是就找我探讨了一下。本文就来聊一下该自定义HandlerMethodArgumentResolver不失效起因
为何自定义的HandlerMethodArgumentResolver不失效
看过springmvc的源码或者背过springmvc相干八股文的敌人，可能会晓得springmvc执行HandlerMethodArgumentResolver，次要是通过HandlerMethodArgumentResolverComposite这个聚合器来进行执行。而HandlerMethodArgumentResolverComposite这个聚合器是如何获取要执行的HandlerMethodArgumentResolver呢？咱们能够间接查看源码
@Nullable private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) { HandlerMethodArgumentResolver result = (HandlerMethodArgumentResolver)this.argumentResolverCache.get(parameter); if (result == null) { Iterator var3 = this.argumentResolvers.iterator(); while(var3.hasNext()) { HandlerMethodArgumentResolver resolver = (HandlerMethodArgumentResolver)var3.next(); if (resolver.supportsParameter(parameter)) { result = resolver; this.argumentResolverCache.put(parameter, resolver); break; } } } return result; }看到这个源码，我想老司机应该会有点脉络，HandlerMethodArgumentResolverComposite外部是会保护一个key为MethodParameter，值为HandlerMethodArgumentResolver的本地缓存，因而要取HandlerMethodArgumentResolver，就会通过MethodParameter来取。
接着咱们在来思考一个问题，源码中的this.argumentResolvers的是什么时候放进去的，咱们持续跟踪源码会发现，他是通过
public HandlerMethodArgumentResolverComposite addResolvers(@Nullable List<? extends HandlerMethodArgumentResolver> resolvers) { if (resolvers != null) { this.argumentResolvers.addAll(resolvers); } return this; }这个办法进行增加。而addResolvers又是什么时候被调用的，咱们持续跟踪源码，会发现addResolvers，他是会RequestMappingHandlerAdapter的afterPropertiesSet办法中的被调用
@Override public void afterPropertiesSet() { 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); } 从这个代码片段，咱们能够看到HandlerMethodArgumentResolverComposite初始会增加一些默认的HandlerMethodArgumentResolver
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers(); this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);而getDefaultArgumentResolvers这办法点开
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() { List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(); // Annotation-based argument resolution resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false)); resolvers.add(new RequestParamMapMethodArgumentResolver()); resolvers.add(new PathVariableMethodArgumentResolver()); resolvers.add(new PathVariableMapMethodArgumentResolver()); resolvers.add(new MatrixVariableMethodArgumentResolver()); resolvers.add(new MatrixVariableMapMethodArgumentResolver()); resolvers.add(new ServletModelAttributeMethodProcessor(false)); resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice)); resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice)); resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory())); resolvers.add(new RequestHeaderMapMethodArgumentResolver()); resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory())); resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory())); resolvers.add(new SessionAttributeMethodArgumentResolver()); resolvers.add(new RequestAttributeMethodArgumentResolver()); // Type-based argument resolution resolvers.add(new ServletRequestMethodArgumentResolver()); resolvers.add(new ServletResponseMethodArgumentResolver()); resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice)); resolvers.add(new RedirectAttributesMethodArgumentResolver()); resolvers.add(new ModelMethodProcessor()); resolvers.add(new MapMethodProcessor()); resolvers.add(new ErrorsMethodArgumentResolver()); resolvers.add(new SessionStatusMethodArgumentResolver()); resolvers.add(new UriComponentsBuilderMethodArgumentResolver()); // Custom arguments if (getCustomArgumentResolvers() != null) { resolvers.addAll(getCustomArgumentResolvers()); } // Catch-all resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true)); resolvers.add(new ServletModelAttributeMethodProcessor(true)); return resolvers; }就会发现一堆默认的HandlerMethodArgumentResolver，有教训的老司机看到这里，应该就晓得为啥自定义HandlerMethodArgumentResolver会生效了吧。
自定义HandlerMethodArgumentResolver会生效的起因是当咱们办法中有引入@RequestBody时，他的用到的HandlerMethodArgumentResolver是RequestResponseBodyMethodProcessor，而咱们自定义的
HandlerMethodArgumentResolver是通过setCustomArgumentResolvers塞进去，而从源码咱们能够看出，咱们自定义的HandlerMethodArgumentResolver是放在默认的HandlerMethodArgumentResolver之后
当咱们办法中同时存在@RequestBody和自定义HandlerMethodArgumentResolver，因为他们的Method雷同，即MethodParameter一样，因而argumentResolverCache的key是一样的，从一开始的源码咱们就能够得悉，当key曾经找到值时，它就间接返回了，因而当它找到@RequestBody的HandlerMethodArgumentResolver，它就不会再找自定义的HandlerMethodArgumentResolver，这就会导致咱们自定义的HandlerMethodArgumentResolver不失效
HandlerMethodArgumentResolver不失效的解法
1、办法一：间接去掉办法中的@RequestBody
去掉办法中的@RequestBody，此时办法就不存在解析@RequestBody的HandlerMethodArgumentResolver，因而就只剩咱们自定义的HandlerMethodArgumentResolver必然会执行
2、办法二：进步咱们自定义HandlerMethodArgumentResolver的执行程序
具体做法如下
@Configurationpublic class HandlerMethodArgumentResolverAutoConfiguration implements InitializingBean{ @Autowired private RequestMappingHandlerAdapter requestMappingHandlerAdapter; @Override public void afterPropertiesSet() throws Exception { List<HandlerMethodArgumentResolver> argumentResolvers = requestMappingHandlerAdapter.getArgumentResolvers(); List<HandlerMethodArgumentResolver> customArgumentResolvers = new ArrayList<>(); for (HandlerMethodArgumentResolver argumentResolver : argumentResolvers) { if(argumentResolver instanceof RequestResponseBodyMethodProcessor){ customArgumentResolvers.add(new MetaInfoHandlerMethodArgumentResolver(argumentResolver)); } customArgumentResolvers.add(argumentResolver); } requestMappingHandlerAdapter.setArgumentResolvers(customArgumentResolvers); }}将自定义的HandlerMethodArgumentResolver放在解析@RequestBody的HandlerMethodArgumentResolver之前。调整后，咱们测试一下
此时会发现曾经有值填充进去了
总结
本文次要解说自定义HandlerMethodArgumentResolver不失效起因与解法，咱们能够思考一个问题批改或者填充申请参数，除了利用HandlerMethodArgumentResolver之外，还有没有其余实现形式？下篇文章揭晓答案