说明: 项目中出现了一个BUG, 本来日期是1651766400(2022-05-06), 但是使用simpleDateFormat转换之后变成了2022-05-05的日期，首先代码是 "多线程" 的. 下面我模拟了项目中的代码写法

package com.home.free.thread;

import java.io.Serializable;
import java.text.SimpleDateFormat;

public class MultiThread {
    public static void main(String[] args) {

        for (int i = 0; i< 100; i++) {
            new Thread(()->{
                Demo demo = new Demo();
                demo.printDemo(1651766400);
            }).start();

            new Thread(()->{
                Demo demo = new Demo();
                demo.printDemo(1651852800);
            }).start();
        }
    }
}

class Demo {
    private static SimpleDateFormat simpleDateFormat =new SimpleDateFormat("yyyyMMdd");

    public void printDemo(Integer deliveryDate) {
        DemoDto demoDto = new DemoDto();
        demoDto.setDeliveryDate(deliveryDate);
        String dateStr = simpleDateFormat.format(demoDto.getDeliveryDate()*1000L);
        System.out.println(deliveryDate + "-" + dateStr);
    }
}

class DemoDto implements Serializable {
    private Integer deliveryDate;

    public Integer getDeliveryDate() {
        return deliveryDate;
    }

    public void setDeliveryDate(Integer deliveryDate) {
        this.deliveryDate = deliveryDate;
    }
}

结果 : 
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651766400-20220507
                1651852800-20220506
1651766400-20220506
1651766400-20220507
1651852800-20220507
1651766400-20220506
                1651852800-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
                1651852800-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651766400-20220506
1651766400-20220507
1651766400-20220506
1651766400-20220507
              1651852800-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220507
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220506
            1651852800-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
            1651852800-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220506
1651766400-20220507
1651766400-20220507
1651852800-20220506
1651852800-20220507
1651852800-20220507
1651852800-20220507
1651852800-20220507
1651852800-20220506
1651766400-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220506
1651852800-20220506
1651852800-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651852800-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651766400-20220506
1651766400-20220506
1651852800-20220506
1651852800-20220507
1651852800-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651852800-20220506
1651766400-20220506
1651852800-20220506
1651852800-20220507
1651766400-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651852800-20220506
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220507
1651766400-20220507
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220506
1651766400-20220506
1651766400-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220506
1651766400-20220507
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651852800-20220507
1651852800-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220506
1651852800-20220506
1651766400-20220506
1651766400-20220506
1651852800-20220506
1651852800-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220507
1651766400-20220506
1651852800-20220507
1651766400-20220507
1651852800-20220507
1651766400-20220506
1651852800-20220506
1651852800-20220507
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651766400-20220506
            1651852800-20220506
1651766400-20220506
1651852800-20220506
1651766400-20220506
1651766400-20220506
1651852800-20220507
1651852800-20220507
1651766400-20220506

结论: 发现多线程情况下，有很多数据是转换错误的。因为simpleDateFormat是多线程的，而创建实体是在类类上，会共享这个数据。

解决：只需将simpleDateFormat的创建对象在自己的线程中创建即可

public void printDemo(Integer deliveryDate) {
        DemoDto demoDto = new DemoDto();
        demoDto.setDeliveryDate(deliveryDate);
        SimpleDateFormat simpleDateFormat =new SimpleDateFormat("yyyyMMdd");
        String dateStr = simpleDateFormat.format(demoDto.getDeliveryDate()*1000L);
        System.out.println(deliveryDate + "-" + dateStr);
    }