How to get the message of the Bad request in API response with openAPI generated controllers in gradle spring

Question

I have a Spring boot application (2.6.9) with gradle 7.6.

I'm using the open api generator gradle plugin to generate request controllers in my application, with the delegate pattern.

    id 'org.openapi.generator' version '6.3.0'

Here are my open api generate config options,

        "dateLibrary": "java8",
        "hideGenerationTimestamp": "true",
        "delegatePattern": "true",
        "library": "spring-boot",
        "serializableModel": "true",
        "useBeanValidation": "true",
        "useTags": "true",
        "implicitHeaders": "true",
        "openApiNullable": "false"

In the open API specification, I have an endpoint with a required parameter.

/resource/getByDate:
  get:
    tags:
      - resource
    summary: summary
    description: desc
    operationId: operation
    parameters:
      - name: fromDate
        in: query
        description: date
        required: true
        schema:
          type: string

Module compiles successfully and initializes as expected. Once a request is sent without the required 'fromDate' parameter the response is,

"timestamp": "2023-03-02T13:59:52.112+00:00",
"status": 400,
"error": "Bad Request",
"path": "/resource/getByDate"

In the application logs the following message is logged,

 Resolved [org.springframework.web.bind.MissingServletRequestParameterException: Required request parameter 'fromDate' for method parameter type String is not present

How to include this message in the response body with the code generator support?

Answer 1

As part of your responses, include a response for a 400 status. Something like this:

  /resource/getByDate:
    get:
      tags:
        - resource
      summary: summary
      description: desc
      operationId: operation
      parameters:
        - name: fromDate
          in: query
          description: date
          required: true
          schema:
            type: string
      responses:
        '400':
          description: Invalid Request
          headers:
            x-whatever:
              description: If you want to include headers in your response
              schema:
                type: string
          content:
            application/json:
              schema:
                properties: 
                  title:
                    type: string
                  message:
                    type: string
                  statusCode:
                    type: string
                  method:
                    type: string

This will generate a class that looks something similar to this:

public class Operation400Response implements Serializable {

  private static final long serialVersionUID = 1L;

  @JsonProperty("title")
  private String title;

  @JsonProperty("message")
  private String message;

  @JsonProperty("statusCode")
  private String statusCode;

  @JsonProperty("method")
  private String method;

Now you can create a ControllerAdvice class to handle the MissingServletRequestParameterException like this:

@ControllerAdvice
public class MyExceptionHandler extends ResponseEntityExceptionHandler
{
  @Override
  public ResponseEntity<Operation400Response> handleMissingServletRequestParameter(MissingServletRequestParameterException ex, HttpHeaders headers, HttpStatus status, WebRequest request)
  {
    var response = new Operation400Response().title("Missing required parameter").message(ex.getMessage()).statusCode(status);
    return new ResponseEntity(response, HttpStatus.BAD_REQUEST);
  }
}