@ExceptionHandler Method Guide

@ExceptionHandler Annotation Guide

The @ExceptionHandler annotation marks a method inside a @RestControllerAdvice or @Controller class as the official handler for a specific Exception class.

Handling MethodArgumentNotValidException (DTO Validation Errors)

When input DTO validation fails (e.g. `@NotBlank` or `@Email`), Spring throws a MethodArgumentNotValidException. Here is how to format field-level validation error messages for frontend forms:

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity<Map<String, Object>> handleValidationExceptions(MethodArgumentNotValidException ex) {
    Map<String, String> fieldErrors = new HashMap<>();
    
    ex.getBindingResult().getFieldErrors().forEach(error -> {
        fieldErrors.put(error.getField(), error.getDefaultMessage());
    });

    Map<String, Object> response = new HashMap<>();
    response.put("timestamp", LocalDateTime.now());
    response.put("status", 400);
    response.put("error", "Validation Failed");
    response.put("errors", fieldErrors);

    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(response);
}

Sample JSON Output Returned to Frontend:

{
  "timestamp": "2026-08-09T00:15:00",
  "status": 400,
  "error": "Validation Failed",
  "errors": {
    "email": "Please provide a valid email address",
    "fullName": "Name cannot be empty"
  }
}