test.java

    @Test
    void shouldCreateOrder() throws InterruptedException {
        Product iPhone12 = new Product("iPhone12", 1);
        productRepository.saveAndFlush(iPhone12); // Insert product with count 1 into DB

        ExecutorService threadPool = Executors.newFixedThreadPool(10); // Create a threadpool with 10 threads
        List<Callable<Optional<Order>>> orderRequests = getOrderRequests(iPhone12.getId(), 20); // Create 20 tasks for placing 20 orders
        List<Future<Optional<Order>>> results = threadPool.invokeAll(orderRequests); // Place those 20 orders simulteanously 

        long successCount = results.stream() // Calculate how many orders got placed successfully
                .filter(Future::isDone)
                .map(result -> {
                    try {
                        return result.get();
                    } catch (InterruptedException | ExecutionException e) {
                        e.printStackTrace();
                        return Optional.empty();
                    }
                })
                .filter(Optional::isPresent)
                .count();
      
        assertEquals(1, successCount); // Assert that only one order was successfull since we had only one item
    }

    private List<Callable<Optional<Order>>> getOrderRequests(Long productId, int count) {
        return LongStream.range(0, count) // Generate tasks for placing orders
                .mapToObj(index -> {
                    Callable<Optional<Order>> task = () -> orderService.placeOrder(productId, index); // Single task for placing order
                    return task;
                }).collect(Collectors.toList());
    }