[BUG][JavaSpring] Multipart request regression in 6.x

Created by: cc-ju

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?
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Description

It seems the PR #11449 breaks with some multipart requests.

With an endpoint:

Consuming multipart/form-data
File as parameter
A second parameter consisting of an object

and the generator settings:

skipFormModel set to false to use the second parameter
interfaceOnly set to true to only generate models & interfaces

The generated API causes an error while spring tries to convert the request part to the object:

Resolved [org.springframework.web.method.annotation.MethodArgumentConversionNotSupportedException: Failed to convert value of type 'java.lang.String' to required type 'com.example.demo.api.model.TestObjectPart'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type 'com.example.demo.api.model.TestObjectPart': no matching editors or conversion strategy found]

openapi-generator version

It's a regression introduced with openapi-generator 6.x. With the 5.x version the generated code behaves as expected.

OpenAPI declaration file content or url

Or see the reproduction repo: https://github.com/cc-ju/openapi-genrator-spring-error-reproduction/blob/main/openapi.yaml

openapi: 3.0.2
info:
  version: 1.0.0
  title: test

paths:
  /test:
    post:
      requestBody:
        content:
          multipart/form-data:
            encoding:
              file:
                contentType: "application/octet-stream"
              content:
                contentType: "application/json"
            schema:
              type: object
              required:
                - file
                - content
              properties:
                file:
                  type: string
                  format: binary
                content:
                  $ref: "#/components/schemas/testObjectPart"
      responses:
        200:
          description: OK

components:
  schemas:
    testObjectPart:
      type: object
      required:
        - foo
      properties:
        foo:
          type: string
        bar:
          type: number

Generation Details

Actual output

    default ResponseEntity<Void> testPost(
        @Parameter(name = "file", description = "", required = true) @RequestPart(value = "file", required = true) MultipartFile file,
        @Parameter(name = "content", description = "", required = true) @Valid @RequestParam(value = "content", required = true) TestObjectPart content
    ) {
        return new ResponseEntity<>(HttpStatus.NOT_IMPLEMENTED);

    }

Expected output

Note that this would re-introduce the problem #11449 tried to fix

    default ResponseEntity<Void> testPost(
        @Parameter(name = "file", description = "", required = true) @RequestPart(value = "file", required = true) MultipartFile file,
        @Parameter(name = "content", description = "", required = true) @Valid @RequestPart(value = "content", required = true) TestObjectPart content
    ) {
        return new ResponseEntity<>(HttpStatus.NOT_IMPLEMENTED);

    }

Steps to reproduce

check out the reproduction repo: https://github.com/cc-ju/openapi-genrator-spring-error-reproduction

run ./gradlew check --info

To switch to the older generator just change:

Related issues/PRs

As mentioned above this regression is caused by #11449

Suggest a fix

Obviously #11449 resolved another problem. But broke this use case. Currently I don't have a solution which does not break either this use case or the use case from the PR.