[BUG][Java] Content mediatype is hardcoded in api.mustache

[p-daniil]

Hi!
I'm trying to return String value.
My OAS looks like this:

openapi: 3.0.0
info:
  title: API
  description: API
  version: LATEST
tags:
  - name: helloWorld
    description: Hello World Api
paths:
  /helloWorld:
    get:
      tags:
        - helloWorld
      responses:
        '200':
          description: OK
          content:
            'text/plain':
              schema:
                type: string
      parameters:
        - name: name
          in: query
          schema:
            type: string

But it results in this method:

/**
     * GET /helloWorld
     *
     * @param name  (optional)
     * @return OK (status code 200)
     */
    @Operation(summary = "", tags={ "helloWorld", }, responses = {  @ApiResponse(responseCode = "200", description = "OK", content = @Content(mediaType = "application/json", schema = @Schema(implementation =  String.class))) })
        @RequestMapping(
        method = RequestMethod.GET,
        value = "/helloWorld",
        produces = { "text/plain" }
    )
    default ResponseEntity<String> helloWorldGet(@Parameter(name = "name", description = "") @Valid @RequestParam(value = "name", required = false) String name

) {
        return getDelegate().helloWorldGet(name);
    }

As you can see, @Content(mediaType = "application/json"...
Root of the problem is in this line, where mediatype is hardcoded as application/json:

openapi-generator/modules/openapi-generator/src/main/resources/JavaSpring/api.mustache

Line 140 in dc1df25

@ApiResponse(responseCode = "{{{code}}}", description = "{{{message}}}"{{#baseType}}, content = @Content(mediaType = "application/json", schema = @Schema(implementation = {{{baseType}}}.class)){{/baseType}}){{^-last}},{{/-last}}

As a temporary solution, it can be replaced:
mediaType = "application/json"
on
mediaType = {{#produces}}"{{{mediaType}}}"{{/produces}}{{^produces}}"application/json"{{/produces}}

But mediaType can contain multiple values, separated with comma, because it used in @RequestMapping(produces={...}).
True way to support multiple content types will be generate @Content annotation in array, I suppose, because in @ApiResponse annotation content is type of array:

/**
     * An array containing descriptions of potential response payloads, for different media types.
     *
     * @return array of content
     **/
    Content[] content() default {};

[mattdarwin]

I think the PR introduces a new bug.

Consider the following spec extract:

  responses:
    '200':
      description: OK
      content:
        'text/plain':
          schema:
            type: string
    '403':
      description: NOTOK
      content:
        'application/json':
          schema:
            $ref: '#/components/schema/SomeObject' 

The resulting code should have a single content type for each response, like this:

@ApiResponse(responseCode = "200", 
description = "OK", content = @Content(mediaType = "text/plain", schema = @Schema(implementation =  String.class))) ,
@ApiResponse(responseCode = "403", 
description = "NOTOK", content = @Content(mediaType = "application/json", schema = @Schema(implementation =  SomeObject.class)))
})

But in fact it gives each ApiResponse all the content types:

@ApiResponse(responseCode = "200", 
description = "OK", content = { @Content(mediaType = "text/plain", schema = @Schema(implementation =  String.class))
                                @Content(mediaType = "application/json", schema = @Schema(implementation =  SomeObject.class))}) ,
@ApiResponse(responseCode = "403", 
description = "NOTOK", content = { @Content(mediaType = "text/plain", schema = @Schema(implementation =  String.class))
                                   @Content(mediaType = "application/json", schema = @Schema(implementation =  SomeObject.class))}) 
})