Support custom Spring type converters

[florensie]

Is your feature request related to a problem? Please describe.
Custom Spring type converters can be added as such:

@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(new UserConverter());
    }
}

public class UserConverter implements Converter<String, User> {

    @Override
    public User convert(String userId) {
        // Fetch from repository
        User user = new User();
        user.setId(userId);
        return user;
    }
}

With the above configuration, and the following controller:

@Log4j2
@RestController
@RequestMapping("/user")
public class UserController {

    @GetMapping("/{userId}")
    public ResponseEntity<Void> doSomething(@PathVariable("userId") User user) {
        LOGGER.info("user = " + user);
        return ResponseEntity.ok().build();
    }
}

We get this openapi output:

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenAPI definition",
    "version": "v0"
  },
  "servers": [
    {
      "url": "http://localhost:8080",
      "description": "Generated server url"
    }
  ],
  "paths": {
    "/user/{userId}": {
      "get": {
        "tags": [
          "user-controller"
        ],
        "operationId": "doSomething",
        "parameters": [
          {
            "name": "userId",
            "in": "path",
            "required": true,
            "schema": {
              "$ref": "#/components/schemas/User"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "User": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string"
          },
          "firstName": {
            "type": "string"
          },
          "lastName": {
            "type": "string"
          }
        }
      }
    }
  }
}

As you can see, the userId parameter is a ref to User, while in reality, we expect a string in this API.

Describe the solution you'd like
Springdoc could get the list for all active type converters, and base the schema off of the source type instead of the resulting type.

This is the expected openapi definition:

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenAPI definition",
    "version": "v0"
  },
  "servers": [
    {
      "url": "http://localhost:8080",
      "description": "Generated server url"
    }
  ],
  "paths": {
    "/user/{userId}": {
      "get": {
        "tags": [
          "user-controller"
        ],
        "operationId": "doSomething",
        "parameters": [
          {
            "name": "userId",
            "in": "path",
            "required": true,
            "schema": {
              "type": "string"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }
    }
  },
  "components": {}
}

Describe alternatives you've considered
I haven't looked into it much but I think a custom ModelConverter might be able to provide a temporary workaround.

Additional context

• This appears to have worked correctly in Springfox
• Baeldung article on Spring type conversion: https://www.baeldung.com/spring-type-conversions

[florensie]

Thank you!