java不重复订单号生成规则

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

public class OrderNo{
    private static int sn = 0;
      
    public static String NextOrderNo(){
        if(sn == 999999999)
            sn = 0;
        else
            sn++;
        DateFormat df = new SimpleDateFormat("yyyyMMddHHmmss");
        return df.format(new Date()) + padRight(String.valueOf(sn),9, '0');
    }
    public static String padLeft(String src, int len, char ch) {
        int diff = len - src.length();
        if (diff <= 0) {
            return src;
        }

        char[] charr = new char[len];
        System.arraycopy(src.toCharArray(), 0, charr, 0, src.length());
        for (int i = src.length(); i < len; i++) {
            charr[i] = ch;
        }
        return new String(charr);
    }
    public static String padRight(String src, int len, char ch) {
        int diff = len - src.length();
        if (diff <= 0) {
            return src;
        }

        char[] charr = new char[len];
        System.arraycopy(src.toCharArray(), 0, charr, diff, src.length());
        for (int i = 0; i < diff; i++) {
            charr[i] = ch;
        }
        return new String(charr);
    }
    // 防止创建类的实例
    private OrderNo(){}
    
    public static void main(String[] args) {
        Thread t1 = new Thread(){
            @Override
            public void run() {
                for (int i = 0; i < 5000; i++) {
                    try {
                        sleep(300);
                        System.out.println(OrderNo.NextOrderNo());
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        Thread t2 = new Thread(){
            @Override
            public void run() {
                for (int i = 0; i < 5000; i++) {
                    try {
                        sleep(200);
                        System.out.println(OrderNo.NextOrderNo());
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        Thread t3 = new Thread(){
            @Override
            public void run() {
                for (int i = 0; i < 5000; i++) {
                    try {
                        sleep(50);
                        System.out.println(OrderNo.NextOrderNo());
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        t1.run();
        t2.run();
        t3.run();
    }
}