[java] Combining "properties" and "additionalProperties" in one class generates broken Jackson mapping.

Created by: r-alukhanov

Description

If using both "properties" and "additionalProperties", the generated class extends HashMap and contains plain properties. Like this:

public class Example extends HashMap<...>  {
  @JsonProperty("a")
  private String a;
  ...
}

Such a class cannot be meaningfully deserialised using Jackson. Jackson puts all the fields as key/value pairs into the HashMap ignoring the plain fields. In provided example getA() would return "null", while get("a") contains the value.

openapi-generator version

Used version: openapi-generator-maven-plugin : 3.3.3

OpenAPI declaration file

Example:
  type: object
  properties:
    a:
    type: string
  additionalProperties: true

Suggest a fix/enhancement

I would suggest to generate such a class (which has both "properties" and "additionalProperites") as:

public class Example {
  private String a;
  private HashMap<> otherProperties;
  ...
}

Similar suggestion in "swagger-codegen" project: https://github.com/swagger-api/swagger-codegen/issues/5187