Taking the code below into consideration:

@RestControllerAdvice
public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {
    @Resource
    private MessageSource messageSource;
    private HttpHeaders headers(){
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);
        return headers;
    }
    private ResponseError responseError(String message,HttpStatus statusCode){
        ResponseError responseError = new ResponseError();
        responseError.setStatus("error");
        responseError.setError(message);
        responseError.setStatusCode(statusCode.value());
        return responseError;
    }
    @ExceptionHandler(Exception.class)
    private ResponseEntity<Object> handleGeneral(Exception e, WebRequest request) {
        if (e.getClass().isAssignableFrom(UndeclaredThrowableException.class)) {
            UndeclaredThrowableException exception = (UndeclaredThrowableException) e;
            return handleBusinessException((BusinessException) exception.getUndeclaredThrowable(), request);
        } else {
            String message = messageSource.getMessage("error.server", new Object[]{e.getMessage()}, null);
            ResponseError error = responseError(message,HttpStatus.INTERNAL_SERVER_ERROR);
            return handleExceptionInternal(e, error, headers(), HttpStatus.INTERNAL_SERVER_ERROR, request);
        }
    }
    @ExceptionHandler({BusinessException.class})
    private ResponseEntity<Object> handleBusinessException(BusinessException e, WebRequest request) {
        ResponseError error = responseError(e.getMessage(),HttpStatus.CONFLICT);
        return handleExceptionInternal(e, error, headers(), HttpStatus.CONFLICT, request);
    }
}

What would be the biggest difference between replacing

if (e.getClass().isAssignableFrom(UndeclaredThrowableException.class)) {

with

if (e instanceof UndeclaredThrowableException) {

Line 19.

I found some explanations but none detailed enough.I wanted to understand the situations in which I should use isAssignableFrom or instanceof.