title: Spring Boot with Webflux: No 'Access-Control-Allow-Origin' header is present on the requested resource
body: <p>I've tried to build a Spring Boot backend and connect my React frontend with it. In my local deployment, my API requests fail because CrossOrigin is not supported by localhost:3000.
  I've inserted a "CORSGlobalConfig" to my Spring Boot:</p>
  
  <pre><code>@Configuration
  @EnableWebFlux
  public class CorsGlobalConfig implements WebFluxConfigurer {
      @Override
      public void addCorsMappings(CorsRegistry corsRegistry) {
          corsRegistry.addMapping("/**")
                  .allowedOrigins("http://localhost:3000")
                  .allowedMethods("PUT","POST","GET");
      }
  }
  </code></pre>
  
  <p>In my opinion, this should work fine but something is not right. Do I need to implement my configuration somewhere?
  The error looks like the following:</p>
  
  <pre><code>Access to XMLHttpRequest at 'http://localhost:8080/REQUESTPATH'
  from origin 'http://localhost:3000' has been blocked by CORS policy:
  Response to preflight request doesn't pass access control check:
  No 'Access-Control-Allow-Origin' header is present on the requested resource.
  </code></pre>
  
  <p>Thanks for your help!</p>