[BUG] Java Jersey2 client throw exception when post multipart/form-data body

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I tried to generate Java Jersey 2 client based on json file (attached below), when I test operation "/stbCheckout" that POST multipart/form-data, I caught this error:
client.ApiException: multipart/form-data not yet supported for serializeToString (http signature authentication)
I found this error cause by method "ApiClient.invokeAPI", it called:

// update different parameters (e.g. headers) for authentication
    updateParamsForAuth(
        authNames,
        queryParams,
        allHeaderParams,
        cookieParams,
        serializeToString(body, formParams, contentType, isBodyNullable),
        method,
        target.getUri());

and serializeToString method throw exception when contentType == multipart/form-data:

public String serializeToString(Object obj, Map<String, Object> formParams, String contentType, boolean isBodyNullable) throws ApiException {
    try {
      if (contentType.startsWith("multipart/form-data")) {
        throw new ApiException("multipart/form-data not yet supported for serializeToString (http signature authentication)");
      } else if (contentType.startsWith("application/x-www-form-urlencoded")) {
        String formString = "";
        for (Entry<String, Object> param : formParams.entrySet()) {
          formString = param.getKey() + "=" + URLEncoder.encode(parameterToString(param.getValue()), "UTF-8") + "&";
        }

openapi-generator version

OpenAPI declaration file content or url

{
  "openapi": "3.0.2",
  "info": {
    "title": "Sacombank Subscription Gateway",
    "version": "3.0"
  },
  "servers": [
    {
      "url": "https://cardtest.sacombank.com.vn:9448/checkout"
    }
  ],
  "components": {
    "schemas": {
      "CheckoutRequestBody": {
        "type": "object",
        "required": [
          "ProfileID",
          "AccessKey",
          "TransactionID",
          "TransactionDateTime",
          "Language",
          "IsTokenRequest",
          "Currency",
          "FirstName",
          "LastName",
          "Gender",
          "Address",
          "District",
          "City",
          "Country",
          "Email",
          "Mobile",
          "ReturnUrl",
          "CancelUrl",
          "Signature"
        ],
        "properties": {
          "ProfileID": {
            "type": "string",
            "description": "Merchant's profile id",
            "minLength": 1,
            "maxLength": 32
          },
          "AccessKey": {
            "type": "string",
            "minLength": 1,
            "maxLength": 32
          },
          "TransactionID": {
            "type": "string",
            "minLength": 1,
            "maxLength": 32
          },
          "TransactionDateTime": {
            "type": "string",
            "format": "date-time",
            "minLength": 1,
            "maxLength": 32
          },
          "Language": {
            "type": "string",
            "minLength": 2,
            "maxLength": 2
          },
          "SubscribeOnly": {
            "type": "boolean"
          },
          "IsTokenRequest": {
            "type": "boolean"
          },
          "Description": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "TotalAmount": {
            "type": "number",
            "minLength": 1,
            "maxLength": 15
          },
          "DomesticFee": {
            "type": "number",
            "minLength": 1,
            "maxLength": 15
          },
          "InternationalFee": {
            "type": "number",
            "minLength": 1,
            "maxLength": 15
          },
          "SSN": {
            "type": "string"
          },
          "Currency": {
            "type": "string",
            "minLength": 3,
            "maxLength": 3
          },
          "CorporateCust": {
            "type": "boolean"
          },
          "TaxIdentificationNumber": {
            "type": "string",
            "maxLength": 20
          },
          "CompanyName": {
            "type": "string",
            "maxLength": 50
          },
          "FirstName": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "LastName": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "Gender": {
            "type": "string",
            "minLength": 1,
            "maxLength": 1
          },
          "Address": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "District": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "City": {
            "type": "string",
            "minLength": 1,
            "maxLength": 60
          },
          "PostalCode": {
            "type": "string",
            "minLength": 1,
            "maxLength": 15
          },
          "Country": {
            "type": "string"
          },
          "Email": {
            "type": "string",
            "minLength": 1,
            "maxLength": 255
          },
          "Mobile": {
            "type": "string",
            "minLength": 1,
            "maxLength": 15
          },
          "ReturnUrl": {
            "type": "string"
          },
          "CancelUrl": {
            "type": "string"
          },
          "Signature": {
            "type": "string"
          }
        }
      }
    }
  },
  "paths": {
    "/stbCheckout": {
      "post": {
        "tags": [
          "Sacombank Subscription Service"
        ],
        "operationId": "stbCheckout",
        "description": "Checkout",
        "requestBody": {
          "description": "Request Body",
          "required": true,
          "content": {
            "multipart/form-data": {
              "schema": {
                "$ref": "#/components/schemas/CheckoutRequestBody"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }
    },
    "/stbCheckoutJSON": {
      "post": {
        "tags": [
          "Sacombank Subscription Service"
        ],
        "operationId": "stbCheckoutJSON",
        "description": "Checkout",
        "requestBody": {
          "description": "Request Body",
          "required": true,
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/CheckoutRequestBody"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }
    }
  }
}

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

I can temporary fix it by create a derived class of ApiClient that overload serializeToString to allow multipart/form-data content type, but I think should not throw exception in this method

Start with the generated Java Jersey2 ApiClient.invokeAPI entry point and its serializeToString method, using the supplied OpenAPI declaration with the /stbCheckout multipart/form-data operation. Reproduce the HTTP signature authentication path and trace how multipart parameters are handled; done means the operation no longer fails with the reported serializeToString exception.