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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
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Description

The java-spring generator allows users to generate Spring Boot controllers & service interfaces for both reactive (Spring WebFlux) or non-reactive (Spring MVC) servers.

In PR #6993 @bit-factor addressed an issue with the parameter type used when generating code to use multipart-form-data upload for Spring WebFlux. However, it looks like the solution, or the current state of the support, is incorrectly generating the wrong parameter type for OpenAPI specs are uploading a single file vs multiple files.

For single file uploads the java-spring generator is using the parameter type Flux<Part>, which is ok but may be technically incorrect as this supports uploading more than a single file. However, for multiple file uploads the generator is using the parameter type of List<Flux<Part>> which is incorrect.

Actually using an endpoint with the List<Flux<Part>> parameter type generates a 415 UNSUPPORTED_MEDIA_TYPE HTTP status when attempting to upload a file

{
    "timestamp": "2025-07-14T21:03:19.192+00:00",
    "path": "/upload",
    "status": 415,
    "error": "Unsupported Media Type",
    "requestId": "83f6588a-1"
}

Additionally the generated code is counter to some of the example articles referenced in the original issue which state:

From https://medium.com/@eaimanshoshi/step-by-step-procedure-of-spring-webflux-multipart-file-upload-and-read-each-line-without-saving-it-6a12be64f6ee

N.B: Remember that,

1. To upload multiple file you must have to use Flux<FilePart> .
2. To upload single file you have to use Mono<FilePart> or FilePart .
3. Mono<MultiValueMap<String, Part>> can be used for both case. But in that case you have to find out the
   FilePart(s) from the map by key.

openapi-generator version

version 7.14.0

OpenAPI declaration file content or url

For reference there are two different example specs for uploading a single file with multipart-form-data versus uploading multiple files

The main difference is that the schema for single file upload looks like

          fileName:
            type: string
            format: binary

while multiple file upload looks like

          fileName:
            type: array
            items:
              type: string
              format: binary

Generation Details

java -jar openapi-generator-cli.jar generate -g spring --library spring-boot --additional-properties=reactive=true -i openapi-spec.yaml -o src

Steps to reproduce

1. Generate a Java Spring service using the following openapi-spec:

openapi: 3.0.1

info:
  title: webflux-file-upload
  description: Test service for multiple file upload with webflux
  version: 0.0.1

paths:
  /upload:
    post:
      summary: uploads a file
      operationId: uploadFile
      tags:
        - upload
      description: upload a file
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                filename:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        201:
          description: upload was successful

use the command line java -jar openapi-generator-cli.jar generate -g spring --library spring-boot --additional-properties=reactive=true -i openapi-spec.yaml -o src

2. Compile the project generated in the src directory
3. Run the web service
4. Attempt to upload a file or files using the /upload endpoint:
   curl -X POST http://localhost:8080/upload -H 'Content-Type: multipart/form-data' -F filename=@"pom.xml"

Related issues/PRs

#6993

Suggest a fix

The relevant part of the templates which are including List<Flux<Part>> look like:

{{#isFile}}{{#isArray}}List<{{/isArray}}{{#reactive}}Flux<Part>{{/reactive}}{{^reactive}}{{#isFormParam}}MultipartFile{{/isFormParam}}{{^isFormParam}}{{>optionalDataType}}{{/isFormParam}}{{/reactive}}{{#isArray}}>{{/isArray}}{{/isFile}}

I think that they should look more like:

{{#isFile}}{{#reactive}}{{#isArray}}Flux<{{/isArray}}Part{{#isArray}}>{{/isArray}}{{/reactive}}{{^reactive}}{{#isArray}}List<{{/isArray}}{{#isFormParam}}MultipartFile{{/isFormParam}}{{^isFormParam}}{{>optionalDataType}}{{/isFormParam}}{{#isArray}}>{{/isArray}}{{/reactive}}{{/isFile}}

This would use Flux<Part> when the file parameters is reactive and is an array, otherwise it would use Part. For non-reactive parameters the template would continue to use List for arrays.