[BUG] [kotlin-spring] readOnly properties nullable

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

In #14280 a "bug" was reported, that required readOnly values are generated as nullable. This "bug" was "fixed" in #14509.

However, this was not a bug. Like @zaenk pointed out this is necessary to use the resource as request and as response body. (see this comment)

Now the generated code leads to deserialization errors in a request because the readOnly properties are not given.

[http-nio-80-ex] DefaultHandlerExcept WARN  - Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Instantiation of [simple type, class org.openapitools.model.Example] value failed for JSON property id due to missing (therefore NULL) value for creator parameter id which is a non-nullable type] [mvc.support.DefaultHandlerExceptionResolver]

openapi-generator version

since 6.5.0

Generated code for 6.4.0

data class Example(

    @Schema(example = "null", required = true, readOnly = true, description = "")
    @get:JsonProperty("id", required = true) val id: kotlin.String? = null,

    @Schema(example = "null", required = true, description = "")
    @get:JsonProperty("name", required = true) val name: kotlin.String
)

Generated code since 6.5.0

data class Example(

    @Schema(example = "null", required = true, readOnly = true, description = "")
    @get:JsonProperty("id", required = true) val id: kotlin.String,

    @Schema(example = "null", required = true, description = "")
    @get:JsonProperty("name", required = true) val name: kotlin.String
)

--> the id is no longer nullable with default value null.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
    version: 1.0.0
    title: Example
paths:
  /example:
    post:
      requestBody:
        content:
            application/json:
                schema:
                    $ref: '#/components/schemas/example'
      responses:
        200:
          description: OK
          content:
            application/json:
                schema:
                    $ref: '#/components/schemas/example'
components:
    schemas:
        example:
          required:
            - id
            - name
          properties:
            id:
              type: string
              readOnly: true
            name:
              type: string

Generation Details

Gradle Config:

openApiGenerate {
    generatorName.set("kotlin-spring")
    additionalProperties.put("useSpringBoot3", "true")

    inputSpec.set(openApiDefinitionFile)
    outputDir.set(openApiGeneratedSourcesDir)
}

Steps to reproduce

1. Use the OpenAPI example from above as input file for the generator
2. Start the Server
3. Make a Post-Request to /example that contains only the necessary properties:

   {
     "name": "John"
   }
   

4. The request will fail.

Related issues/PRs

• #14280
• #14509

Suggest a fix

revert #14509

Start by reviewing the kotlin-spring generator change in #14509 and the OpenAPI declaration in this issue. Reproduce generation with the provided schema and send a request containing only name. Done means the request succeeds without a missing non-null id error while the generated response still represents the readOnly property correctly.