Describe the bug
The FormFilter incorrectly handles POST requests with URL-encoded parameters in the query string, causing double encoding and parameter duplication.

Reproduction Steps

1. Start a gateway server with the following route configuration:

@SpringBootApplication
public class GatewayServer {

    public static void main(String[] args) {
        SpringApplication.run(GatewayServer.class, args);
    }

    @Bean
    public RouterFunction<ServerResponse> getRoute() {
        return route()
                .POST("/anything", http("https://httpbin.org"))
                .build();
    }
}

2. Send a POST request to the gateway server:

curl -X POST "http://localhost:8080/anything?foo=%7B%7D" \
  -H "Content-Type: application/x-www-form-urlencoded; charset=UTF-8"

Expected vs. Actual Behavior

Expected request sent to upstream:

POST /anything?foo=%7B%7D HTTP/1.1
Content-Type: application/x-www-form-urlencoded; charset=UTF-8

<empty body>

Actual request sent to upstream:

POST /anything?foo=%257B%257D HTTP/1.1
Content-Type: application/x-www-form-urlencoded; charset=UTF-8

foo=%7B%7D

Root Cause

The issue stems from a mismatch in parameter handling:

• HttpServletRequest#getParameterMap() returns parameters that have already been URL-decoded by the servlet container
• The existing queryParams contains raw (non-decoded) parameters
• This causes the filter to incorrectly process the parameters, resulting in double encoding (%7B%7D becomes %257B%257D) and duplication of parameters in the request body

Solution

I will submit a pull request to address this parameter handling inconsistency in the FormFilter.