前言

本文素材的来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，发现他们的业务控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是作为元数据放在请求头，而业务model又需要tenantId和appId，于是他们团队的成员就写出了形如上的代码，虽然这样的代码是能满足业务要求，但是大面积如上的写法，都是重复性的代码，很不优雅。后面这个技术负责人项通过自定义HandlerMethodArgumentResolver的方式来优雅解决这问题，他的代码形如下

@Data
public 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;
    }
}

@Configuration
public 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，但是事实上这个HandlerMethodArgumentResolver却无法生效，他排查了很久，没啥头绪，于是就找我探讨了一下。本文就来聊一下该自定义HandlerMethodArgumentResolver不生效原因

为何自定义的HandlerMethodArgumentResolver不生效

看过springmvc的源码或者背过springmvc相关八股文的朋友，可能会知道springmvc执行HandlerMethodArgumentResolver，主要是通过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？我们可以直接查看源码

  @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头绪，HandlerMethodArgumentResolverComposite内部是会维护一个key为MethodParameter，值为HandlerMethodArgumentResolver的本地缓存，因此要取HandlerMethodArgumentResolver，就会通过MethodParameter来取。

接着我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，我们继续跟踪源码会发现，他是通过

  public HandlerMethodArgumentResolverComposite addResolvers(@Nullable List<? extends HandlerMethodArgumentResolver> resolvers) {
        if (resolvers != null) {
            this.argumentResolvers.addAll(resolvers);
        }

        return this;
    }

这个方法进行添加。而addResolvers又是什么时候被调用的，我们继续跟踪源码，会发现addResolvers，他是会RequestMappingHandlerAdapter的afterPropertiesSet方法中的被调用

@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，我们可以看到HandlerMethodArgumentResolverComposite初始会添加一些默认的HandlerMethodArgumentResolver

	List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);

而getDefaultArgumentResolvers这方法点开

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，有经验的老司机看到这里，应该就知道为啥自定义HandlerMethodArgumentResolver会失效了吧。

自定义HandlerMethodArgumentResolver会失效的原因是当我们方法中有引入@RequestBody时，他的用到的HandlerMethodArgumentResolver是RequestResponseBodyMethodProcessor，而我们自定义的
HandlerMethodArgumentResolver是通过setCustomArgumentResolvers塞进去，而从源码我们可以看出，我们自定义的HandlerMethodArgumentResolver是放在默认的HandlerMethodArgumentResolver之后

当我们方法中同时存在@RequestBody和自定义HandlerMethodArgumentResolver，因为他们的Method相同，即MethodParameter一样，因此argumentResolverCache的key是一样的，从一开始的源码我们就可以得知，当key已经找到值时，它就直接返回了，因此当它找到@RequestBody的HandlerMethodArgumentResolver，它就不会再找自定义的HandlerMethodArgumentResolver，这就会导致我们自定义的HandlerMethodArgumentResolver不生效

HandlerMethodArgumentResolver不生效的解法

1、方法一：直接去掉方法中的@RequestBody

去掉方法中的@RequestBody，此时方法就不存在解析@RequestBody的HandlerMethodArgumentResolver，因此就只剩我们自定义的HandlerMethodArgumentResolver必然会执行

2、方法二：提高我们自定义HandlerMethodArgumentResolver的执行顺序

具体做法如下

@Configuration
public 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，我们测试一下

此时会发现已经有值填充进去了

总结

本文主要讲解自定义HandlerMethodArgumentResolver不生效原因与解法，我们可以思考一个问题修改或者填充请求参数，除了利用HandlerMethodArgumentResolver之外，还有没有其他实现方式？下篇文章揭晓答案