Unexpected behavior: GET converted to POST without body when @RequestParam is missing in OpenFeign

[dkswnkk]

Description

When using OpenFeign, I originally intended to send a query parameter with a GET request.

For example:

@FeignClient(name = "test-client")
public interface TestClient {

    @GetMapping("/test")
    DataRs getData(String query); // `query` has a non-null value
}

My goal was to send the request like:

GET /test?query=value

What actually happened

• Because the query parameter was not annotated with @RequestParam, OpenFeign treated it as a request body.
• However, since GET requests should not have a body, the internal HTTP client (HttpURLConnection) automatically changed the method to POST at runtime:

// https://github.com/JetBrains/jdk8u_jdk/blob/master/src/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L1326
if (method.equals("GET")) {
    method = "POST"; // Backward compatibility
}

• Even though the HTTP method was changed to POST, the body was empty (Content-Length: 0).

• So:

  • The request became POST,
  • But no request body was actually sent,
  • And there was no error or warning during Feign request generation.

Important note

• When I explicitly add @RequestParam, everything works correctly:

@FeignClient(name = "test-client")
public interface TestClient {

    @GetMapping("/test")
    DataRs getData(@RequestParam String query); // ✅ Works as intended: GET with query string
}

• So I fully understand that @RequestParam is needed when sending query parameters.

Suggestion

Given this behavior, I would like to ask:

Wouldn't it be more appropriate for OpenFeign to throw a warning or an error early — for example, during startup or client creation — if it detects a GET method that has parameters but is missing annotations like @RequestParam or @SpringQueryMap?

In many cases, when a developer defines a GET method with parameters, the expected behavior is to send those as query parameters, not as a body.
If the annotation is missing, and OpenFeign ends up silently treating the request as a POST with an empty body, it can easily cause confusion, unexpected behavior, or even runtime issues under strict API gateways or HTTP method validation rules.

Additionally, when I checked the client-side logs, it still showed the request being sent as a GET,
but when I confirmed with the receiving server, the request was actually received as a POST.
This discrepancy made it very difficult to quickly identify and debug the issue.

In my local logging, using the following RequestInterceptor:

@Slf4j
@Configuration
public class FeignLoggingInterceptorConfig {

    @Bean
    public RequestInterceptor requestLoggingInterceptor() {
        return template -> log.info("[Feign Request] method={}, url={}, body={}",
                                    template.method(), template.url(), template.body() != null ? new String(template.body()) : "null");
    }
}

the request was logged as being sent with the GET method.
However, on the server side that received the API request, it was actually received as a POST request.

I fully understand that @RequestParam is necessary when intending to send query parameters.
However, it seems like providing an early warning in cases where a GET method's parameters would be treated incorrectly could greatly help developers catch such issues before runtime.

If there is a particular reason why OpenFeign currently does not enforce this behavior — for example, to preserve flexibility or for backward compatibility — I would be happy to understand that as well.