[BUG] [JavaSpring] Discriminator property gets serialized twice

openapi-generator version

3.3.4

Command line used for generation

I'm working with a JHipster API-First project, which uses openapi-generator-maven-plugin (v3.3.4).

Description

By following the OpenAPI 3.x documentation/examples, it seems that the property used as discriminator must also be specified in the properties list of the schema.

Example:

TicketEvent:
  type: object
  description: A generic event
  discriminator:
    propertyName: type
  required:
    - id
    - sequenceNumber
    - timestamp
    - type
  properties:
    id:
      type: integer
      format: int64
      readOnly: true
    sequenceNumber:
      type: integer
      readOnly: true
    timestamp:
      type: string
      format: date-time
      readOnly: true
    type:
      type: string
      readOnly: true
TicketMovedEvent:
  description: A ticket move event
  allOf:
    - $ref: '#/components/schemas/Event'
    - type: object
      required:
        - source
        - target
      properties:
        source:
          $ref: '#/components/schemas/Queue'
        target:
          $ref: '#/components/schemas/Queue'

The generated Java class for the code above seems to differ from the Jackson guidelines for polymorphic type handling with annotations, where the property used as discriminator must not be present in the class.
The generated code also includes this property as a class attribute with getter/setter.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type", visible = true)
@JsonSubTypes({
  @JsonSubTypes.Type(value = TicketMovedEvent.class, name = "TicketMovedEvent")
})

public class TicketEvent   {
   ...

   @JsonProperty("type")
   private String type;

This causes the serialization output to include the property twice, as shown below.

{
    ...
    "type": "TicketMovedEvent",
    "type": null,
    ...
}

I've also tried to remove the property from the OpenAPI properties list, leaving intact the discriminator part; this way the generated code corresponds to the Jackson's guidelines and the serialization works just fine. On the other hand, I get the following error during the deserialization process.

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "type" (class it.blutec.bludesk.web.api.model.TicketMovedEvent), not marked as ignorable ...

To fix this, i've configured the Jackson ObjectMapper object for ignoring this kind of situations.

ObjectMapper om = new ObjectMapper()
				.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)

By the way, this looks like a workaround to me beacuse my OpenAPI file doesn't (strictly) follows the spec.

Suggest a fix

The generator should not include an additional attribute (with getter/setter) for a discriminator property.

Start at the JavaSpring generator's model-generation path and reproduce the example with openapi-generator-maven-plugin 3.3.4. Trace how discriminator properties become Jackson annotations and Java fields, then verify that serialization emits the discriminator once and deserialization still accepts it for the generated TicketEvent hierarchy.