[BUG] ApiClient fails to encode query paramaters

[Albertwaninge-2]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I ran into an issue with a generated client, which sends an offsetdatetime parameter which contains a + character, e.g. at=2025-11-12T11:00:00+01:00. The server (Tomcat, which is the default server in any Spring Boot project.) interprets the + character as a encoded space and, hence, decodes it to a space. Subsequently, the server tries to parse an OffsetDateTime from "2025-11-12T11:00:00 01:00", which throws a parse exception because it is invalid.

The generated ApiClient in the Kotlin generator with the jvm-spring-restclient template does not encode query params as it should. This is not only problematic for the somewhat ambiguous + character. Also the &, = and ? characters will definitely cause problems.

openapi-generator version

7.15.0 using the kotlin generator and the jvm-spring-restclient library.

Steps to reproduce

Generate a client for an API which has an endpoint which takes a parameter of type string($date-time) (in Java that would be ZonedDateTime, OffsetDateTime or Instant).
The service exposing the API must be hosted on Tomcat server (Spring Boot will do) or any other server that inteprets a + as en encoded space.
Use this client to call that endpoint with a datetime value for a timezone east of GMT (i.e. the offset should have a + character).
Now, the server will throw a parse exception, as it tries to parse a datetime string containing a space rather than a +.

Related issues/PRs

I did not find any related issues.

Suggest a fix

I am no expert on Spring Boots RestClient nor URI or URL encoding. But Spring Boots manual has a nice page on URI encoding: https://docs.spring.io/spring-framework/reference/web/webmvc/mvc-uri-building.html#uri-encoding

Based on that manual, I guess the solution would be something of the following:

Replace the function in the generated ApiClient

private fun <I> RestClient.RequestBodyUriSpec.uri(requestConfig: RequestConfig<I>) =
        uri(requestConfig.path) { builder ->
            builder
                .queryParams(LinkedMultiValueMap(requestConfig.query))
                .build(requestConfig.params)
        }

with something like

private fun <I> RestClient.RequestBodyUriSpec.uri(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
        val uriComponentsBuilder  = UriComponentsBuilder.fromPath(requestConfig.path)
        requestConfig.query.forEach { key, values ->
            uriComponentsBuilder.queryParam(key, "{$key}")
        }
        return uri(uriComponentsBuilder.encode().buildAndExpand(requestConfig.query + requestConfig.params).toUri())
    }

I have tried the above solution for myself and it worked. But I did not exhaustively test for exotic sets of params. And I wonder whether this functions properly when parameters have collections.

[wing328]

can you please open a PR with the suggested fix to start with? we will review accordingly