@Test
void concurrency_stock_decrease() throws InterruptedException {
    int threadCnt = 100;
    ExecutorService executorService = Executors.newFixedThreadPool(32);
    CountDownLatch latch = new CountDownLatch(threadCnt);

    for (int i = 0; i < threadCnt; i++) {
        executorService.submit(() -> {
            try {
                stockService.decrease(1L, 1L);
            } finally {
                latch.countDown();
            }
        });
    }

    latch.await();

    Stock stock = stockRepository.findById(1L).orElseThrow();

    assertEquals(0L, stock.getQuantity());
}

@Transactional
public synchronized void decrease(Long id,Long quantity){
        Stock stock=stockRepository.findById(id).orElseThrow();

        stock.decrease(quantity);
        stockRepository.saveAndFlush(stock);
        }

public class TransactionStockService {

    private final StockService stockService;

    public TransactionStockService(StockService stockService) {
        this.stockService = stockService;
    }

    public void decrease(Long id, Long quantity) {
        startTransaction(); // begin

        stockService.decrease(id, quantity);

        endTransaction(); // commit
    }
}