[BUG][JAVA][restclient] useJackson3: generated RestClient is missing Spring's default message converters (follow-up to #23354)

[anatoly-tamarack]

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

With useJackson3=true, buildRestClientBuilder never calls registerDefaults(), so the generated
RestClient carries 2 of Spring's 5 default converters. ByteArray, String and Resource are
missing, and any operation the generator emits with one of those return types fails on read. The
useJackson3=false branch of the same template is unaffected — it hands its converter to an API
that starts from the defaults:

{{#useJackson3}}
builder.addCustomConverter(new JacksonJsonHttpMessageConverter(mapper));   // starts empty
return RestClient.builder().configureMessageConverters(messageConverters);

{{^useJackson3}}
converters.add(0, new MappingJackson2HttpMessageConverter(mapper));        // starts with defaults
return RestClient.builder().messageConverters(messageConverters);

Follow-up to #23354, which reported the stronger symptom (no converters at all, JSON included) and
was closed pending spring-projects/spring-framework#36579, fixed in Spring 7.0.7. #23588 separately
switched withJsonConverter → addCustomConverter, which also made JSON work. JSON is fixed twice
over; the non-JSON defaults were never restored.

Actual vs expected output

Converters on the built client, spring-web 7.0.8 (Spring fix in place):

addCustomConverter only              (master today) -> 2 [JacksonJson, Form]
registerDefaults + addCustomConverter                -> 6 [JacksonJson, ByteArray, String, Resource, Form, JacksonJson]
registerDefaults + withJsonConverter    (expected)   -> 5 [ByteArray, String, Resource, Form, JacksonJson]
plain RestClient.builder().build()                   -> 5 [ByteArray, String, Resource, Form, JacksonJson]

Calling the generated CSV operation against a server responding text/csv:

org.springframework.web.client.UnknownContentTypeException: Could not extract response:
  no suitable HttpMessageConverter found for response type
  [interface org.springframework.core.io.Resource] and content type [text/csv]
    at DefaultRestClient.readWithMessageConverters(DefaultRestClient.java:263)
    at com.example.client.api.ReportControllerApi.getReportCsv(ReportControllerApi.java:496)

The generator emits a method returning Resource, then builds a client that cannot read one.

openapi-generator version

7.23.0, and still present in ApiClient.mustache on master (verified by reading the template, not by
building master). Regression relative to useJackson3=false; root cause unchanged since #23023.

OpenAPI declaration file content or url

openapi: 3.0.3
info: { title: repro, version: "1.0.0" }
paths:
  /reports/{id}/csv:
    get:
      operationId: getReportCsv
      parameters:
        - { name: id, in: path, required: true, schema: { type: string } }
      responses:
        "200":
          description: CSV export
          content:
            text/csv:
              schema: { type: string, format: binary }

Generation Details

generatorName: java
library: restclient
additionalProperties:
  useSpringBoot4: "true"
  useJackson3: "true"

Spring Framework 7.0.8 / Spring Boot 4, Jackson 3.1.5, Java 25.

Steps to reproduce

1. Generate the spec above with the config above.
2. ApiClient client = new ApiClient(ApiClient.buildRestClientBuilder().build()).setBasePath(...);
3. getReportCsv("x") against a server responding text/csv → UnknownContentTypeException.
4. Regenerate with useJackson3=false → the same call succeeds.

Related issues/PRs

• [BUG][Java][RestClient][jackson3] restclient.messageConverters unset for ApiClient with default construction #23354 — same root cause, closed 2026-04-10 pending the Spring fix. Its symptom is resolved; the
  missing defaults are not.
• Converter overrides in HttpMessageConverters only apply when defaults are registered spring-projects/spring-framework#36579 — fixed in 7.0.7. Confirmed on 7.0.8: withJsonConverter
  now applies without registerDefaults(), but registerDefaults() is still needed for the rest.
• fix(java-restclient): Spring 7 wrong serializers setup #23588 — switched to addCustomConverter; fixed the JSON symptom only.
• feature(jackson3) add jackson3 support for Java Client generator - Restclient #23023 — the original useJackson3 restclient PR. [BUG][SPRING] Deprecated MappingJackson2HttpMessageConverter for RestClient generator #22985 — the thread that introduced the flags.
• [BUG] [JAVA] [RESTCLIENT] [MAVEN] Maven plugin 7.22.0 produces invalid client code to get application/octet-stream #23885 (open) — same exception, but that reproducer doesn't set useJackson3, so it takes the
  defaults-registered branch and fails on java.io.File having no converter. Different cause;
  linking so the two aren't conflated.
• Companion issue, createDefaultMapper(null) dropping the default DateFormat:

Suggest a fix

Register the defaults, and — now that spring-framework#36579 is fixed — use withJsonConverter so
the generated mapper replaces the default JSON converter rather than duplicating it:

Consumer<HttpMessageConverters.ClientBuilder> messageConverters = builder -> {
    builder.registerDefaults()
           .withJsonConverter(new JacksonJsonHttpMessageConverter(mapper));
    {{#withXml}}
    builder.withXmlConverter(new JacksonXmlHttpMessageConverter(xmlMapper));
    {{/withXml}}
};

That gives the same 5 converters as RestClient.builder().build() with the generated mapper
substituted in. It implies a Spring 7.0.7 floor; if that's not acceptable,
registerDefaults().addCustomConverter(...) also works, at the cost of a duplicate JSON converter.

[adityaanikam]

Confirmed against current master — same file, buildRestClientBuilder()'s Jackson3 branch:

Consumer<HttpMessageConverters.ClientBuilder> messageConverters = builder -> {
    builder.addCustomConverter(new JacksonJsonHttpMessageConverter(mapper));
    ...
};
return RestClient.builder().configureMessageConverters(messageConverters);

No registerDefaults() call anywhere in this branch — addCustomConverter is called on a builder that starts empty, so only the converters explicitly added here exist. Matches your converter-count breakdown exactly.

Your proposed shape (registerDefaults().withJsonConverter(...)) is the cleaner fix and gets the mapper substituted into the same slot Spring's own JSON converter would occupy, rather than adding a duplicate alongside it. Given it implies a Spring 7.0.7 floor, worth checking what minimum Spring version this template family already targets before committing to that path over the addCustomConverter fallback.