I have a problem with sending multipart/form-data requests from my Java service (using WebClient but same history with RestTemplate) to Flask service. It seems like a sent body is just empty as I get empty Immutablemultidict([]). I dont think that's revelant but I host java app locally on my machine and Flask app is run in docker container. Does anyone see what can be an issue?
Controller:
@RequestMapping("/api/biometric")
@RequiredArgsConstructor
public class BiometricController {
private final BiometricClient biometricClient;
@PostMapping("/register")
public ResponseEntity<String> registerUser(
@RequestParam("user_id") String userId,
@RequestParam("image") MultipartFile image) throws IOException {
String response = biometricClient.register(userId, image);
return ResponseEntity.ok(response);
}
}
WebClient:
@Component
public class BiometricClient {
private final WebClient webClient;
public BiometricClient() {
this.webClient = WebClient.builder()
.baseUrl("http://localhost:5001")
.build();
}
public String register(String userId, MultipartFile image) {
try {
Resource file = new org.springframework.core.io.ByteArrayResource(image.getBytes()) {
@Override
public String getFilename() {
return image.getOriginalFilename();
}
};
MultipartBodyBuilder builder = new MultipartBodyBuilder();
builder.part("user_id", userId); // string part
builder.part("image", file);
MultiValueMap<String, org.springframework.http.HttpEntity<?>> multipartData = builder.build();
String response = webClient.post()
.uri("/register")
.contentType(MediaType.MULTIPART_FORM_DATA)
.body(BodyInserters.fromMultipartData(multipartData))
.retrieve()
.bodyToMono(String.class)
.block();
return response;
} catch (WebClientResponseException e) {
throw e;
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
Flask endpoint:
@app.route("/register", methods=["POST"])
def register_user():
"""Receive image, extract embedding, save to DB"""
# Extract data
file = request.files.get("image")
user_id = request.form.get("user_id")
if not file or not user_id:
print("❌ Missing image or user_id")
return jsonify({"error": "Missing image or user_id"}), 400
# Example: process file
# embedding = engine.get_embedding(file.read())
# save_embedding(db_session, user_id, embedding)
return jsonify({"message": "Embedding registered"}), 200
Curl request to java service:
curl -F "user_id=123" -F "[email protected]" http://localhost:8080/api/biometric/register
Log:
❌ Missing image or user_id
chunked encoding error while discarding request body. client=172.23.0.1 request="POST /register HTTP/1.1" error="invalid literal for int() with base 16: b''"
172.23.0.1 - - [12/Nov/2025 00:14:47] "POST /register HTTP/1.1" 400 159 0.084201
Also, when trying to hardcode filename in webclient service (not pass from controller), it doesnt work or when trying to sent just user_id.
Curl request directly to Flask service:
curl -F "[email protected]" -F "user_id=123" http://localhost:5001/register
It returns then 200.
