[BUG] [spring][server] oneOf Polymorphism support for spring boot

Created by: dkirrane

Description

Currently, openapi-generator doesn't support oneOf. It currently generates the following broken code, outputting UNKNOWN_BASE_TYPE rather than Pet for the example below. The code is broken with or without the discriminator in the Spec.

    default ResponseEntity<Void> _petsPost(@ApiParam(value = ""  )  @Valid @RequestBody(required = false) UNKNOWN_BASE_TYPE UNKNOWN_BASE_TYPE) {
        return petsPost(UNKNOWN_BASE_TYPE);
    }

openapi-generator version

pom.xml

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>4.2.3</version>
    <executions>
        <execution>
            <id>generate</id>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${project.basedir}/src/main/resources/api.yaml</inputSpec>
                <generatorName>spring</generatorName>
                <generateSupportingFiles>true</generateSupportingFiles>
                <configOptions>
                    <sourceFolder>src/gen/java/main</sourceFolder>
                    <library>spring-boot</library>
                    <java8>true</java8>
                    <interfaceOnly>true</interfaceOnly>
                    <skipDefaultInterface>false</skipDefaultInterface>
                    <delegatePattern>true</delegatePattern>
                </configOptions>
            </configuration>
        </execution>
    </executions>
</plugin>

OpenAPI Spec

api.yaml

openapi: "3.0.2"
info:
  version: 1.0.0
  title: Polymorphism
paths:
  /pets:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/Cat'
                - $ref: '#/components/schemas/Dog'
              discriminator:
                propertyName: pet_type
      responses:
        '200':
          description: created
components:
  schemas:
    # Parent
    Pet:
      type: object
      required:
        - pet_type      
      properties:
        pet_type:
          type: string
        eats:
          type: string
      discriminator:
        propertyName: pet_type
    # Child 1
    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Dog`
          properties:
            bark:
              type: boolean
    # Child 2
    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Cat`
          properties:
            climbs:
              type: boolean

Command line used for generation

Steps to reproduce

• create a pom.xml with openapi-generator-maven-plugin 4.2.3
• create api.yaml file above
• mvn clean compile
• see broken code under target directory \target\generated-sources\openapi\src\gen\java\main\org\openapitools\api\PetsApi.java

Related issues/PRs

This bug is related to the Server side Java code (SpringBoot) @bkabrda has already merged PR for client Jackson Client side: https://github.com/OpenAPITools/openapi-generator/pull/5120

Suggest a fix

Try apply client side fix from https://github.com/OpenAPITools/openapi-generator/pull/5120 to server side.