`@RequestParam`的文档里有说明
在 Spring WebFlux 中`@RequestParam`只映射查询参数(URL 后面跟着的)，而 x-www-form-urlencoded 是请求体的,自然不会解析

https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestParam.html

> In Spring MVC, "request parameters" map to query parameters, form data, and parts in multipart requests. This is because the Servlet API combines query parameters and form data into a single map called "parameters", and that includes automatic parsing of the request body.
In Spring WebFlux, "request parameters" map to query parameters only. To work with all 3, query, form data, and multipart data, you can use data binding to a command object annotated with ModelAttribute.