Polymorphic typed arrays are not supported in java code generation

Bug Report Checklist

• [x ] Have you provided a full/minimal spec to reproduce the issue?
• [ x] Have you validated the input using an OpenAPI validator (example)?
• [x ] What's the version of OpenAPI Generator used?
• [x ] Have you search for related issues/PRs?
• [x ] What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Generated code does not compile becuse of this construct:

    FmlOutputList:
      type: array
      items:
        oneOf:
          - $ref: '#/components/schemas/FmlTjanst'
          - $ref: '#/components/schemas/FmlXml'
          - $ref: '#/components/schemas/FmlAntal'

Compiler output:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project fmf-formansformedling-app: Compilation failure: Compilation failure:
[ERROR] PrefixFmlOutput.java:[26,57] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: package model
[ERROR] PrefixFmlOutput.java:[38,16] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: class model.PrefixFmlOutput
[ERROR] PrefixFmlOutput.java:[41,64] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: class model.PrefixFmlOutput
[ERROR] PrefixFmlOutput.java:[47,66] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: class model.PrefixFmlOutput
[ERROR] PrefixFmlOutput.java:[64,15] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: class model.PrefixFmlOutput
[ERROR] PrefixFmlOutput.java:[69,42] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXmlFmlAntal
[ERROR]   location: class model.PrefixFmlOutput
[ERROR] PrefixFmlInput.java:[26,57] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: package model
[ERROR] PrefixFmlInput.java:[38,16] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: class model.PrefixFmlInput
[ERROR] PrefixFmlInput.java:[41,52] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: class model.PrefixFmlInput
[ERROR] PrefixFmlInput.java:[47,54] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: class model.PrefixFmlInput
[ERROR] PrefixFmlInput.java:[64,15] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: class model.PrefixFmlInput
[ERROR] PrefixFmlInput.java:[69,31] cannot find symbol
[ERROR]   symbol:   class PrefixOneOfFmlTjanstFmlXml
[ERROR]   location: class model.PrefixFmlInput

This construct works:

    FmlOutputList:
      type: array
      items:
        type: object

openapi-generator version

4.2.2

OpenAPI declaration file content or url

openapi: 3.0.2
info:
  title: example
  description:  >-
    Generated by rose-link-swaggerator.groovy
  version: 1.0.0
  contact: {}
paths:
  /something/request:
    post:
      operationId: post
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/FmlInput'
      parameters:
        - name: example
          description: example
          in: header
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/FmlOutput'
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'
security: []
servers: []
components:
  links: {}
  callbacks: {}
  schemas: 
    Error:
      type: object
      properties:
        error:
          type: string
    FmlInput:
      type: object
      properties:
        payload:
          $ref: '#/components/schemas/FmlInputList'
    FmlOutput:
      type: object
      properties:
        serviceReturnValue:
          $ref: '#/components/schemas/FmlOutputList'
    FmlInputList:
      type: array
      items:
        oneOf:
          - $ref: '#/components/schemas/FmlTjanst'
          - $ref: '#/components/schemas/FmlXml'
    FmlOutputList:
      type: array
      items:
        oneOf:
          - $ref: '#/components/schemas/FmlTjanst'
          - $ref: '#/components/schemas/FmlXml'
          - $ref: '#/components/schemas/FmlAntal'
    FmlTjanst:
      type: object
      properties:
        FML_TJANST:
          type: string
    FmlXml:
      type: object
      properties:
        FML_XML:
          type: string
    FmlAntal:
      type: object
      properties:
        FML_ANTAL:
          type: integer

Command line used for generation

        <plugin>
            <groupId>org.openapitools</groupId>
            <artifactId>openapi-generator-maven-plugin</artifactId>
            <version>4.2.2</version>
            <executions>
               <execution>
                  <goals>
                     <goal>generate</goal>
                  </goals>
                  <configuration>
                     <inputSpec>spec.yaml</inputSpec>
                     <generatorName>java</generatorName>
                     <modelPackage>model</modelPackage>
                     <modelNamePrefix>Prefix</modelNamePrefix>
                     <configHelp>false</configHelp>
                     <configOptions>
                        <java8>true</java8>
                        <dateLibrary>java8</dateLibrary>
                     </configOptions>
                     <library>jersey2</library>
                  </configuration>
               </execution>
            </executions>
         </plugin>

Steps to reproduce

mvn clean install

Related issues/PRs

Suggest a fix

Should work

Start with the provided spec.yaml and the Java generator output, especially PrefixFmlInput.java and PrefixFmlOutput.java, then run mvn clean install to reproduce the missing polymorphic classes. Compare the generated references with the schemas using oneOf in the array items; done means the generated Java project compiles for this specification.