I have a @RestController with the following endpoint

@PutMapping
public void updateEvent(
    @Valid @RequestPart EventRequest request,
    @RequestPart(required = false) MultipartFile file) {
    // body omitted
}

EventRequest is a standard DTO with constraints such as

@NotNull(message = "{event.endDate.empty}")
private LocalDateTime endDate;

If the request object violates any of these constraints a MethodArgumentNotValidException is thrown, which gets converted to a list of validation errors in the response body.

Previously, any type of file could be uploaded, but now I want to restrict the types, so I made these changes:

@PutMapping
public void updateEvent(
    @Valid @RequestPart EventRequest request,
    @Valid @ValidFileType @RequestPart(required = false) MultipartFile file) {
    // body omitted
}

ValidFileType is a validation annotation like this one, which is mapped to a ConstraintValidator implementation like this one.

After I’ve added these annotations, if validation of EventRequest fails, a different exception HandlerMethodValidationException is thrown, which doesn’t get converted to a list of validation errors.

Why does a different exception get thrown when I add validation to the file request part, and is it possible to preserve the previous behaviour?

I’m not sure if this is relevant, but neither the endpoint method nor the @RestController class are annotated with @Validated.