[BUG][Java][RestClient][jackson3] restclient.messageConverters unset for ApiClient with default construction

[chumbalum]

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

• when constructing an ApiClient with the default constructor, the internal generated RestClient will not have any messageConverters set, resulting in errors like

Could not extract response: no suitable HttpMessageConverter found for response type [class ag.xyz.model.SomethingDto] and content type [application/json]

• with the pull request to support jackson3, logic for adding JacksonJsonHttpMessageConverter to the rest client was added (which will be used, if useJackson3 is true
  • https://github.com/OpenAPITools/openapi-generator/pull/23023/changes#top (see ApiClient.mustache)
• to achieve it, the method .withJsonConverter was used
  • https://github.com/spring-projects/spring-framework/blob/3be51b1c77617e2e199958ca4fc96fc82b4bd50b/spring-web/src/main/java/org/springframework/http/converter/DefaultHttpMessageConverters.java#L411
• the build method DefaultHttpMessageConverters will only pick up the jsonConverter as part of getCoreConverters(), if registerDefaults is set (by .registerDefaults())
  • https://github.com/spring-projects/spring-framework/blob/3be51b1c77617e2e199958ca4fc96fc82b4bd50b/spring-web/src/main/java/org/springframework/http/converter/DefaultHttpMessageConverters.java#L482
  • the build method is called via initMessageConverters in DefaultRestClientBuilder
    • https://github.com/spring-projects/spring-framework/blob/3be51b1c77617e2e199958ca4fc96fc82b4bd50b/spring-web/src/main/java/org/springframework/web/client/DefaultRestClientBuilder.java#L508

openapi-generator version

• openapi-generator-maven-plugin 7.21.0
• spring framework 7.0.6

additional params used

<useSpringBoot4>true</useSpringBoot4>
<useJackson3>true</useJackson3>
<library>restclient</library>
<openApiNullable>false</openApiNullable>
<useJakartaEe>true</useJakartaEe>
<generateBuilders>true</generateBuilders>
<useSingleRequestParameter>static</useSingleRequestParameter>
<useAbstractionForFiles>true</useAbstractionForFiles>

Steps to reproduce

• create ApiClient with default constructor
• apiClient.restClient.messageConverters will be empty

this.apiClient = new ApiClient();

• create ApiClient with construction like in mustache template
• apiClient.restClient.messageConverters will be empty

this.apiClient = new ApiClient(RestClient.builder()
		.configureMessageConverters(c -> c
				.withJsonConverter(new JacksonJsonHttpMessageConverter())
		)
		.build()
);

• create ApiClient with construction like in mustache template, but use registerDefaults
• apiClient.restClient.messageConverters will have entries (5), including the JacksonJsonHttpMessageConverter

this.apiClient = new ApiClient(RestClient.builder()
		.configureMessageConverters(c -> c
				.registerDefaults()
				.withJsonConverter(new JacksonJsonHttpMessageConverter())
		)
		.build()
);

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/pull/23023/changes#top (see ApiClient.mustache)

Suggest a fix

• adding registerDefaults() beforehand withJsonConverter(...) will lead to existing messageConverters, nevertheless I'm not too deep into it, to see the full consequences