同事让组个字符串，给一个插件用，用了一个小时拼了一下，就当练习了

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.log4j.Log4j2;

import java.io.IOException;
import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

import static java.time.temporal.ChronoField.MONTH_OF_YEAR;


@Log4j2
class TestGenArray {


    public static void main(String args[]) {

        String dateArray = genDateArray(1996, 2);
        log.debug(dateArray.length());

    }

    private static String genDateArray(int startYear, int howManyYears) {

        LocalDate today = LocalDate.now();

        List<Map> yearsResult = new ArrayList();

        List<String> daysList = new ArrayList();


        for (int yearCount = 0; yearCount < howManyYears; yearCount++) {
            Map<String, Map> yearMap = new TreeMap<>();
            for (int everyYear = startYear; everyYear < startYear + howManyYears; everyYear++) {
                Map<String, List> monthMap = new TreeMap<>();
                log.debug(everyYear);
                for (int monthsValue = (int)MONTH_OF_YEAR.range().getMinimum(); monthsValue <= MONTH_OF_YEAR.range().getMaximum(); monthsValue++) {
                    daysList.clear();
                    log.debug(monthsValue);
                    log.debug(today.withYear(everyYear).withMonth(monthsValue).lengthOfMonth());
                    for (int dayOfMonth = 1; dayOfMonth <= today.withYear(everyYear).withMonth(monthsValue).lengthOfMonth(); dayOfMonth++) {
                        daysList.add(String.valueOf(dayOfMonth));
                    }
                    monthMap.put(String.valueOf(monthsValue), daysList);
                }

                yearMap.put(String.valueOf(everyYear), monthMap);
                log.debug(yearMap.keySet());
            }
            yearsResult.add(yearMap);
            log.debug(yearsResult.size());
        }




        ObjectMapper mapperObj = new ObjectMapper();

        String jsonStr = "";
        try {
            // get Employee object as a json string
            jsonStr = mapperObj.writeValueAsString(yearsResult);
        } catch (IOException e) {
            log.error(e);
        }


        return jsonStr;
    }


}