[BUG][Java] nullable not considered in Enum.fromValue on OpenAPI 3.1 specs

[david0]

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

We try to update from OpenAPI 3.0 to 3.1 and face the issue that enums fromValue method behave different now

the enum type has nullable: true set.
With openapi 3.0.3 this is generated:

  @JsonCreator
  public static StatusEnum fromValue(String value) {
    for (StatusEnum b : StatusEnum.values()) {
      if (b.value.equals(value)) {
        return b;
      }
    }
    return null;
  }

when changing to 3.1.0 the null is not supported anymore:

  @JsonCreator
  public static StatusEnum fromValue(String value) {
    for (StatusEnum b : StatusEnum.values()) {
      if (b.value.equals(value)) {
        return b;
      }
    }
    throw new IllegalArgumentException("Unexpected value '" + value + "'");
  }

I'm not aware of any way to change that.

openapi-generator version

7.11.0

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: Minimal API Example
  version: 1.0.0
paths:
  /items:
    get:
      summary: Get items
      responses:
        '200':
          description: A list of items
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Item'
components:
  schemas:
    Item:
      type: object
      properties:
        name:
          type: string
        status:
          $ref: '#/components/schemas/StatusEnum'

    StatusEnum:
      type: string
      nullable: true
      enum:
        - available
        - out_of_stock
        - discontinued

Generation Details

openapi-generator generate -g java --library webclient -i example.yaml

Steps to reproduce

1. run command above.
2. grep fromValue -A10 ./src/main/java/org/openapitools/client/model/StatusEnum.java

Related issues/PRs

Suggest a fix

[david0]

I found out that nullable is not a thing anymore and hence dropped when parsing with swagger-parser.

The alternative is pointed out here:
OAI/OpenAPI-Specification#2244 (comment)

Instead of nullable:true, change the type type: [string, 'null']