public class PropertiesHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver {
      
      
    @Override
    public boolean supportsParameter(MethodParameter parameter) {
      
      
        return Properties.class.isAssignableFrom(parameter.getParameterType());
    }
    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
      
      
        ServletWebRequest servletWebRequest = (ServletWebRequest)webRequest ;
        HttpServletRequest request = servletWebRequest.getRequest();
        String contentType = request.getHeader("Content-Type") ;
        MediaType mediaType = MediaType.parseMediaType(contentType);
        Charset charset = mediaType.getCharset();
        charset = charset == null ? Charset.forName("UTF-8") : charset ;
        // 请求输入字节流
        ServletInputStream inputStream = request.getInputStream();
        InputStreamReader reader = new InputStreamReader(inputStream,charset);
        Properties properties = new Properties() ;
        // 加载字符流成为Properties对象
        properties.load(reader);
        return properties;
    }
}

@Configuration
public class RestWebMvcConfigurer implements WebMvcConfigurer {
      
      
    @Autowired
    private RequestMappingHandlerAdapter requestMappingHandlerAdapter ;
    @PostConstruct
    public void init(){
      
      
        // 获取当前RequestMappingHandlerAdapter中所有的Resolver对象
        //HandlerMethodArgumentResolverComposite.getResolvers()获取集合不可变，所以这里需要新建List
        List<HandlerMethodArgumentResolver> resolvers =
                requestMappingHandlerAdapter.getArgumentResolvers();
        List<HandlerMethodArgumentResolver> newResolvers = new ArrayList<>(resolvers.size() + 1) ;
        newResolvers.add(new PropertiesHandlerMethodArgumentResolver());
        newResolvers.addAll(resolvers) ;
        // 重置Resolver对象
        requestMappingHandlerAdapter.setArgumentResolvers(newResolvers);
    }
    @Override
    public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
      
      
        // 自定义HandlerMethodArgumentResolver优先级低于内建HandlerMethodArgumentResolver，
        // 详情见:
        // RequestMappingHandlerAdapter.afterPropertiesSet() => {
      
      
        //    List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        //    new HandlerMethodArgumentResolverComposite().addResolvers(resolvers)
        // }
        // 所以这里添加了也没用，参数将被MapMethodProcessor处理，因为Properties extends Hashtable
        // 这里调整到@PostConstruct注解的init()中处理
        //resolvers.add(new PropertiesHandlerMethodArgumentResolver());
    }
}

@Slf4j
@RestController
public class PropertiesRestController {
      
      
    @PostMapping(value = "/add/props",
        consumes = "text/properties;charset=UTF-8")
    public Properties addProp(Properties properties){
      
      
        log.info("====> prop : {}" , properties);
        return properties ;
    }
}

4.1 header内容
    Content-Type设置为text/properties
4.1 body内容
    username=yicj
    addr=北京