[BUG] [java] nullable oneOf deserialization incorrect for isNullable

[taviLaies]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• [] 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 using a nullable oneOf property, deserialization does not preserve the isNullable flag.

openapi-generator version

All versions since 7.5.0.
7.4.0 works correctly.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: API
  description: Hello
  version: v1
servers:
  - url: http://localhost:8080/
    description: server url
tags:
  - name: "Hello"
    description: "The Hello resource"
paths:
  /hello:
    post:
      tags:
        - Hello
        - FIDO
      operationId: hello
      summary: Returns a hello message.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/HelloRequest'
      responses:
        '200': # status code
          description: Succeeded
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HelloResponse'

components:
  schemas:
    HelloRequest:
      type: object
      properties:
        message:
          type: string
          minLength: 1
          example: "friends"
        optionalField:
          type: integer
          nullable: true
        polymorphicProperties:
          type: array
          items:
            $ref: '#/components/schemas/ParentProperty'
        propertyOption:
          $ref: '#/components/schemas/PropertyOption'
      required:
      - message

    HelloResponse:
      type: object
      properties:
        message:
          type: string
          example: "Hello friends"

    ParentProperty:
      type: object
      discriminator:
        propertyName: "propertyType"
      properties:
        parentValue:
          type: string

    ChildProperty1:
      allOf:
        - $ref: '#/components/schemas/ParentProperty'
        - type: object
          properties:
            stringValue:
              type: string
            propertyType:
              type: string
              default: "ChildProperty1"
          required:
            - stringValue

    ChildProperty2:
      type: object
      allOf:
        - $ref: '#/components/schemas/ParentProperty'
        - type: object
          properties:
            intValue:
              type: integer
            propertyType:
              type: string
              default: "ChildProperty2"
          required:
            - intValue

    PropertyOption:
      nullable: true
      oneOf:
        - $ref: '#/components/schemas/PropertyOption1'
        - $ref: '#/components/schemas/PropertyOption2'

    PropertyOption1:
      type: object
      properties:
        stringValue:
          type: string
      required:
        - stringValue

    PropertyOption2:
      type: object
      properties:
        intValue:
          type: integer
      required:
        - intValue

Generation Details

I'm Using gradle plugin:

openApiGenerate {
    generatorName = 'java'
    additionalProperties = [
            sourceFolder : "",
            "annotationLibrary": "none",
            "library" : "jersey2",
            "performBeanValidation" : "true",
            "useBeanValidation" : "true",
            "useRuntimeException" : "true",
            "hideGenerationTimestamp" : "true",
            "useJakartaEe": "true"
    ]
}

Steps to reproduce

Generate Java models and perform a JSON round-trip of the DTO:

@Test
void request() throws JsonProcessingException {
    HelloRequest initial = new HelloRequest()
        .message("message")
        .optionalField(15)
        .polymorphicProperties(List.of(
                new ChildProperty1()
                        .stringValue("stringValue"),
                new ChildProperty2()
                        .intValue(10)))
        .propertyOption(
                new PropertyOption(
                        new PropertyOption2()
                                .intValue(15)));

    String json = mapper.writeValueAsString(initial);

    HelloRequest fromJson = mapper.readValue(json, HelloRequest.class);

    assertThat(fromJson).isEqualTo(initial); // Fails because propertyOption.isNullable is true in fromJson
}

Related issues/PRs

Suggest a fix

The deserialization logic constructs an empty PropertyOption (using the default constructor which initializes isNullable to true) and then calls setActualInstance. So options I can see is either change isNullable in setActualnstance or change deserialization logic to use the constructor that takes the instance as an argument.