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

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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

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