[BUG][Java][okhttp-gson] Cannot serialize oneOf array of strings #25541

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?

Description

The Java okhttp-gson client throws IllegalStateException: Not a JSON Primitive: ["alpha","beta"] when serializing a oneOf value containing an array of strings. The expected result is the JSON array ["alpha","beta"].

openapi-generator version

Reproduced on master 57edb417ea3fb15de7bc523a133c9cc016acab19 (7.28.0-SNAPSHOT).

OpenAPI declaration file content

openapi: 3.0.3
info:
  title: oneOf primitive array
  version: '1'
paths: {}
components:
  schemas:
    Union:
      oneOf:
        - $ref: '#/components/schemas/Variant'
        - type: array
          items:
            type: string
    Variant:
      type: object
      required: [kind]
      properties:
        kind:
          type: string

Generation Details

java -jar openapi-generator-cli.jar generate -g java --library okhttp-gson -i spec.yaml -o out

Steps to reproduce

Using the generated client and its dependencies:

Union value = JSON.deserialize("[\"alpha\",\"beta\"]", Union.class);
System.out.println(JSON.serialize(value));

The deserialization succeeds but serialization throws IllegalStateException: Not a JSON Primitive. A separate pre-existing limitation causes empty arrays to be rejected by setActualInstance; the reproduction above uses a nonempty array.

Related issues/PRs

#25539 also concerns Java Gson oneOf generation, but this issue reproduces on master without that PR.

Suggest a fix

In the generated Gson adapter, write array-valued primitive schemas as JsonArray instead of converting them to JsonPrimitive.