[Java] Skip wildcard media types when selecting request Content-Type (#24118)

[seonwooj0810]

Fixes #24118

Root cause

ApiClient.selectHeaderContentType(String[]) could hand back a wildcard media type. isJsonMime() treats wildcards such as application/* and */* as JSON-compatible, so the JSON branch returned the wildcard unchanged; and when no JSON type was present the method returned contentTypes[0] verbatim. When that value is applied as a request Content-Type header, Spring throws IllegalArgumentException: Content-Type cannot contain wildcard type '*', so any spec declaring a wildcard request body (e.g. application/*) produced clients that fail at runtime.

Change

• JSON-compatible wildcards now fall back to the concrete application/json.
• The no-JSON path returns the first non-wildcard media type, and application/json only as a last resort when every candidate is a wildcard.
• Applied to the restclient, resttemplate and webclient ApiClient.mustache templates; all affected Java samples regenerated.

Tests

Added ApiClientTest to the restclient and resttemplate petstore samples. selectHeaderContentTypeFallsBackToJsonForJsonCompatibleWildcard fails before this change (the old code returned the wildcard) and passes after; companion cases cover JSON preference, first-concrete selection, and skipping non-JSON wildcards. Both run green (Tests run: 4, Failures: 0).

Verification done

• Checks 1-5: no in-flight/own PR for this root cause (gh pr list by symbol selectHeaderContentType/wildcard content-type + cross-check of my open PRs); no claim in the thread; fix is code (.mustache + generated .java); confirmed the old pattern still present on the current head; issue is standalone (no parent epic).
• Rebuilt the CLI (JDK21) and regenerated all java restclient/resttemplate/webclient configs; git diff confirms every changed sample is exactly the selectHeaderContentType change and matches the templates, so the samples-up-to-date CI check will pass.

---

Summary by cubic

Skip wildcard media types when selecting request Content-Type in Java clients to avoid Spring errors. Ensures a concrete media type is always set and updates restclient, resttemplate, and webclient generators. Fixes #24118.

• Bug Fixes
  • JSON-compatible wildcards (e.g., application/*, */*) now fall back to application/json.
  • If no JSON is present, pick the first non-wildcard type; if all are wildcards, use application/json.
  • Applied in modules/openapi-generator/src/main/resources/Java/libraries/{restclient,resttemplate,webclient}/ApiClient.mustache; regenerated affected samples and added ApiClientTest in petstore restclient and resttemplate.

^{Written for commit 472db60. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 38 files

_{Re-trigger cubic}

[wing328]

thanks for the pr

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)