问在生成唯一序列号(Objectify4)的测试中，XG事务隔离失败

EN

public void saveInvoice(final Invoice invoice) throws BusinessRuleException {

    final Objectify ofy = ObjectifyService.factory().begin().cache(true);
    ofy.transact(new Work<Void>() {

        @Override
        public Void run() {
            CustomerSettings customerSettings = ofy.load()
                    .key(Key.create(CustomerSettings.class, CustomerSettings.ID)).safeGet();
            Contact contact = ofy.load().key(createContactKey(invoice.getContactId()).safeGet();
            contact.setContactType(ContactType.CLIENT);
            ofy.save().entity(contact).now();
            String invoiceNumber = generateSequence(ofy, customerSettings);
            invoice.setInvoiceNumber(invoiceNumber);
            ofy.save().entity(invoice).now();
            return null;
        }
    });
}

private synchronized String generateSequence(Objectify ofy, CustomerSettings settings) {
    String ret = "";
    int sequence = settings.getNextSequence();
    settings.setNextSequence(sequence + 1);
    ofy.save().entity(settings).now();
    ret = "" + sequence;
    return ret;
}

private void test(final int threadCount) throws InterruptedException, ExecutionException {
    final Environment currentEnvironment = ApiProxy.getCurrentEnvironment();
    Callable<String> task = new Callable<String>() {
        @Override
        public String call() {
            ApiProxy.setEnvironmentForCurrentThread(currentEnvironment);
            return generateInvoiceNumber();
        }
    };
    List<Callable<String>> tasks = Collections.nCopies(threadCount, task);
    ExecutorService executorService = Executors.newFixedThreadPool(threadCount);
    List<Future<String>> futures = executorService.invokeAll(tasks);
    List<String> resultList = new ArrayList<String>(futures.size());
    // Check for exceptions
    for (Future<String> future : futures) {
        // Throws an exception if an exception was thrown by the task.
        resultList.add(future.get());
    }
    // Validate the IDs
    Assert.assertEquals(futures.size(), threadCount);
    List<String> expectedList = new ArrayList<String>(threadCount);
    for (long i = 1; i <= threadCount; i++) {
        expectedList.add("" + i);
    }
    Collections.sort(resultList);
    Assert.assertEquals(expectedList, resultList);
}

@SuppressWarnings("unchecked")
private String generateInvoiceNumber() {
    InvoiceDAO invoiceDAO = new InvoiceDAO();
    Invoice invoice = ... create a valid invoice
    invoiceDAO.saveInvoice(invoice);
    log.info("generated invoice number : " + invoice.getInvoiceNumber());
    return invoice.getInvoiceNumber();
}

@Test
public void test32() throws InterruptedException, ExecutionException {
    test(32);
}