Serialisation: extensions in maps like paths are not written

[jmini]

When I run this code:

OpenAPI openAPI = new OpenAPI();

final Info info = new Info();
info.setDescription("API under test");
info.setVersion("1.0.7");
info.setTitle("My title");
openAPI.setInfo(info);
openAPI.servers(Collections.singletonList(new Server().url("http://localhost:9999/api")));

openAPI.path("/foo/bar",
        new PathItem().get(new Operation().responses(new ApiResponses().addApiResponse("200",
                new ApiResponse().description("it works!").content(
                        new Content().addMediaType("application/json", new MediaType().schema(new ObjectSchema()
                                .title("inline_response_200").addProperties("name", new StringSchema()))))))));
openAPI.getPaths().addExtension("x-test", "TEST ME"); // <= IMPORTANT LINE

String string = Yaml.mapper().writerWithDefaultPrettyPrinter().writeValueAsString(openAPI);
System.out.println(string);

I get this output:

openapi: 3.0.1
info:
  title: My title
  description: API under test
  version: 1.0.7
servers:
- url: http://localhost:9999/api
paths:
  /foo/bar:
    get:
      responses:
        200:
          description: it works!
          content:
            application/json:
              schema:
                title: inline_response_200
                type: object
                properties:
                  name:
                    type: string

The line for the extension x-test: TEST ME under paths: is missing.

I think that other maps with extensions might be concerned as well.

---

Expected output:

openapi: 3.0.1
info:
  title: Test extensions
  description: API under test
  version: 1.0.7
paths:
  /foo/bar:
    get:
      responses:
        200:
          description: it works!
          content:
            application/json:
              schema:
                title: inline_response_200
                type: object
                properties:
                  name:
                    type: string
  x-test: TEST ME

Which is correctly parsed by Swagger-Parser:

OpenAPIParser openApiParser = new OpenAPIParser();
OpenAPI openAPI = openApiParser.readLocation(inputSpec, null, new ParseOptions()).getOpenAPI();

System.out.println(openAPI.getPaths().getExtensions().get("x-test"));

[frantuma]

Thanks for reporting this! fixed in #2931