patch request spring boot

Patch request changes only the data that I pass to it, the rest become null , who will tell

@PatchMapping(UPDATE_ACCOUNT)
    public ResponseEntity<AccountEntity> partialUpdate(@PathVariable("accountId") Long accountId,
                                                       @RequestBody AccountDTO accountDTO) {
        AccountEntity entitySaved = accountRepository.findById(accountId)
                .orElseThrow(() -> new NotFoundException("Нет такого ID"));
        entitySaved.setIik(accountDTO.getIik());
        entitySaved.setBik(accountDTO.getBik());
        entitySaved.setBank_name(accountDTO.getBank_name());
        entitySaved.setCurrency(accountDTO.getCurrency());
        final AccountEntity updateAccount = accountRepository.save(entitySaved);
        return ResponseEntity.ok(updateAccount);
    }