关于springboot:记一次springboot项目自定义HandlerMethodArgumentResolver不生效原因与解法

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

@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，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

@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，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