MethodArgumentNotValidException ☕ Java

MethodArgumentNotValidException: Validation failed for argument [0]

Bean Validation (@Valid with @NotNull, @Email, @Size…) rejected the request body.

Seen on: Java

Meaning

Spring returns 400 by default. The exception lists each field error — expose them to clients instead of a generic error.

Common causes

  • Missing required fields
  • Constraint violations (format, size)
  • Client/server field name mismatch

⚡ Quick fix

  1. Return field errors from an @ControllerAdvice
  2. Align DTO constraints with the client
  3. Check the defaultMessage per field

Detailed fix by platform

Java

  1. @ExceptionHandler(MethodArgumentNotValidException.class) ResponseEntity<?> handle(MethodArgumentNotValidException e) { return ResponseEntity.badRequest().body(e.getBindingResult().getFieldErrors()); }

How to diagnose

  1. Fields — Which field errors are listed?
  2. Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.

🧠 Still stuck? Analyze your error

Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.