[JAVA][SPRING] Serialization of generated model shows duplicated properties

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

The serialization of generated model object contains duplicated properties.

openapi-generator version

5.3.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: test
  version: 0.0.1

paths:
  /dummy:
    description: dummy

components:
  schemas:
    Dummy:
      properties:
        FOO:
          type: string
        BAR:
          type: string

Generation Details

docker run --rm --user=$(id -u) -v $PWD:/local openapitools/openapi-generator-cli generate -i /local/openapi.yaml -g spring  -o /local/out

Steps to reproduce

1. Run the command above to generate a Maven project that contains the model class Dummy.java
2. Use ObjectMapper to serialize an instnace of that class:

package org.openapitools.model;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class DummyTest {
    

    @Test
    void test() throws JsonProcessingException {

        ObjectMapper om = new ObjectMapper();

        Dummy d = new Dummy();

        String json = om.writeValueAsString(d);

        System.out.println(json);
    }
}

Result Output:

{
    "bar": null,
    "foo": null,
    "FOO": null,
    "BAR": null
}

Expected Output:

{
    "FOO": null,
    "BAR": null
}

Related issues/PRs

The comments in the following issues suggest that this serialization behavior is actually by design:

https://github.com/FasterXML/jackson-databind/issues/1701
https://github.com/FasterXML/jackson-databind/issues/1609

This post suggests the right way to strucutre the model class to avoid this issue behavior:

https://stackoverflow.com/questions/13262662/duplicate-json-property-when-converting-java-object-to-json-string-using-jackson

Suggest a fix

The post suggests that an annotation should be placed on top of the class to tell the library what to serialize and what not:

@JsonAutoDetect(fieldVisibility = Visibility.ANY, getterVisibility = Visibility.NONE, setterVisibility = Visibility.NONE)

Thus there should be an option in the Java/Spring generator to specify such annotation.

As we heavily depends on OpenAPI Generator (thanks for the hard work!), this is blocking us from releasing our API endpoint to customer.

Appreciated.

Start by running the provided Docker generation command with the YAML spec, then inspect the generated Dummy.java model and reproduce serialization with the ObjectMapper test shown. Compare the output with the expected JSON and trace how the Spring generator defines model properties and annotations. Done means generated models serialize each property only once, with the requested annotation behavior or configuration clearly verified.