根据Date类型的时间段【2020-11-20 09:09:09,2021-10-09 10:10:10】先转换为【2020-11-20 00:00:00,2021-10-09 59:59:59】 再根据转换时间把里面的每一天的开始时间和结束时间获取出来 java

可以通过Java中的日期时间类来实现。具体步骤如下：

1. 将Date类型的时间段转换为LocalDateTime类型，并将时间段的开始时间和结束时间分别设置为当天的0时0分0秒和23时59分59秒。

Date startTime = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2020-11-20 09:09:09");
Date endTime = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2021-10-09 10:10:10");

LocalDateTime startDateTime = LocalDateTime.ofInstant(startTime.toInstant(), ZoneId.systemDefault()).with(LocalTime.MIN);
LocalDateTime endDateTime = LocalDateTime.ofInstant(endTime.toInstant(), ZoneId.systemDefault()).with(LocalTime.MAX);

2. 循环遍历时间段内的每一天，并获取每一天的开始时间和结束时间。

LocalDateTime currentDateTime = startDateTime;
while (!currentDateTime.isAfter(endDateTime)) {
    LocalDateTime dayStartDateTime = currentDateTime.with(LocalTime.MIN);
    LocalDateTime dayEndDateTime = currentDateTime.with(LocalTime.MAX);
    System.out.println("Day " + currentDateTime.toLocalDate() + " start time: " + dayStartDateTime);
    System.out.println("Day " + currentDateTime.toLocalDate() + " end time: " + dayEndDateTime);
    currentDateTime = currentDateTime.plusDays(1);
}

完整代码如下：

import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.ZoneId;
import java.util.Date;

public class DateUtils {

    public static void main(String[] args) throws Exception {
        Date startTime = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2020-11-20 09:09:09");
        Date endTime = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2021-10-09 10:10:10");

        LocalDateTime startDateTime = LocalDateTime.ofInstant(startTime.toInstant(), ZoneId.systemDefault()).with(LocalTime.MIN);
        LocalDateTime endDateTime = LocalDateTime.ofInstant(endTime.toInstant(), ZoneId.systemDefault()).with(LocalTime.MAX);

        LocalDateTime currentDateTime = startDateTime;
        while (!currentDateTime.isAfter(endDateTime)) {
            LocalDateTime dayStartDateTime = currentDateTime.with(LocalTime.MIN);
            LocalDateTime dayEndDateTime = currentDateTime.with(LocalTime.MAX);
            System.out.println("Day " + currentDateTime.toLocalDate() + " start time: " + dayStartDateTime);
            System.out.println("Day " + currentDateTime.toLocalDate() + " end time: " + dayEndDateTime);
            currentDateTime = currentDateTime.plusDays(1);
        }
    }
}