隨機生成唯一編號

@Component
public class OrderNumber implements Serializable {
    private static final long serialVersionUID = -8249764603117125748L;

    private final ReentrantLock prepareLock = new ReentrantLock();
    //最大值
    private static final int max = 9999;
    //最小值
    private static final int min = 1000;
    private static volatile OrderNumber instance = null;

    public static OrderNumber getInstance() {
        if (instance == null) {
            synchronized (OrderNumber.class) {
                if (instance == null) {
                    instance = new OrderNumber();
                }
            }
        }
        return instance;
    }

    /**
     * 生成訂單號
     * 年月日+隨機數
     *
     * @return 返回編號
     */
    public String generateOrderNumber() {
        prepareLock.lock();
        String orderNo = null;
        try {
            SimpleDateFormat format = new SimpleDateFormat("yyMMdd");
            String time = format.format(new Date());
            int number = new Random().nextInt(max) % (max - min + 1) + min;
            orderNo = time + number;
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            prepareLock.unlock();
        }
        return orderNo;
    }

}