在编程中，日期格式化是一个常见的任务。使用不同的格式化选项可能会导致一些意外的结果。最近遇到一个问题，就是使用YYYY格式化选项会导致时间异常。
public static void main(String[] args) throws ParseException {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
SimpleDateFormat sdf2 = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
String time = "2023-12-31 23:59:59";
System.out.printf("yyyy-MM-dd HH:mm:ss: %s\n",sdf.format(sdf.parse(time)));
System.out.printf("YYYY-MM-dd HH:mm:ss: %s\n",sdf2.format(sdf2.parse(time)));
System.out.println("======================================");
String time2 = "2023-11-31 23:59:59"; //11月没有31号
System.out.printf("yyyy-MM-dd HH:mm:ss: %s\n",sdf.format(sdf.parse(time2)));
System.out.printf("YYYY-MM-dd HH:mm:ss: %s\n",sdf2.format(sdf2.parse(time2)));
System.out.println("======================================");
String time3 = "2023-10-31 23:59:59";
System.out.printf("yyyy-MM-dd HH:mm:ss: %s\n",sdf.format(sdf.parse(time3)));
System.out.printf("YYYY-MM-dd HH:mm:ss: %s\n",sdf2.format(sdf2.parse(time3)));
}
//输出结果
yyyy-MM-dd HH:mm:ss: 2023-12-31 23:59:59
YYYY-MM-dd HH:mm:ss: 2023-01-01 23:59:59
======================================
yyyy-MM-dd HH:mm:ss: 2023-12-01 23:59:59
YYYY-MM-dd HH:mm:ss: 2023-01-01 23:59:59
======================================
yyyy-MM-dd HH:mm:ss: 2023-10-31 23:59:59
YYYY-MM-dd HH:mm:ss: 2023-01-01 23:59:59
网上查阅说大写的YYYY表示一个基于周的年份，它是根据周计算的年份，而不是基于日历的年份。通常情况下，两者的结果是相同的，但在跨年的第一周或最后一周可能会有差异。但这里发现无论怎么调整时间，YYYY-MM-dd HH:mm:ss格式化后一直是2023-01-01 23:59:59，不仅仅是在跨年的时候出现问题，具体原因尚不清楚
如何避免
定义通用的格式类，所有使用日期格式的地方都引用这个类，这个类中就定义好yyyy-MM-dd格式即可，这样就不会出现有人手误给大家埋雷了。
public class LocalDateUtils {
/**
* 显示年月日时分秒，例如 2023-10-31 09:51:53.
*/
public static final String DATETIME_PATTERN = "yyyy-MM-dd HH:mm:ss";
/**
* 仅显示年月日，例如 2023-10-31.
*/
public static final String DATE_PATTERN = "yyyy-MM-dd";
private static final String YEAR = "year";
private static final String MONTH = "month";
private static final String WEEK = "week";
private static final String DAY = "day";
/**
* 将日期转换为字符串，格式为：yyyy-MM-dd HH:mm:ss
*/
public static String getLocalDateTimeStr(LocalDateTime localDateTime) {
return format(localDateTime, DATETIME_PATTERN);
}
/**
* 将日期转换为字符串，格式为：yyyy-MM-dd
*/
public static String getLocalDateStr(LocalDateTime localDateTime) {
return format(localDateTime, DATE_PATTERN);
}
/**
* 将字符串转换为日期，格式为：yyyy-MM-dd HH:mm:ss
*/
public static LocalDateTime parseLocalDateTime(String localDateTimeStr) {
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(DATETIME_PATTERN);
return LocalDateTime.parse(localDateTimeStr, dateTimeFormatter);
}
/**
* 将字符串转换为日期，格式为：yyyy-MM-dd
*/
public static LocalDate parseLocalDate(String localDateStr) {
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(DATE_PATTERN);
return LocalDate.parse(localDateStr, dateTimeFormatter);
}
/**
* 将字符串转日期成Long类型的时间戳
*/
public static Long convertLocalDateTimeToLong(String time) {
LocalDateTime parse = parse(time, DATETIME_PATTERN);
return LocalDateTime.from(parse).atZone(ZoneId.systemDefault()).toInstant().toEpochMilli();
}
/**
* 将字符串转日期成Long类型的时间戳
*/
public static Long convertLocalDateToLong(String time) {
LocalDateTime parse = parse(time, DATE_PATTERN);
return LocalDateTime.from(parse).atZone(ZoneId.systemDefault()).toInstant().toEpochMilli();
}
public static LocalDateTime parse(String time, String pattern) {
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(pattern);
return LocalDateTime.parse(time, dateTimeFormatter);
}
/**
* 将Long类型的时间戳转换成String 类型的时间格式
*/
public static String getLocalDateTimeStr(Long time) {
return format(LocalDateTime.ofInstant(Instant.ofEpochMilli(time), ZoneId.systemDefault()), DATETIME_PATTERN);
}
/**
* 将Long类型的时间戳转换成String 类型的时间格式，时间格式为：yyyy-MM-dd
*/
public static String getLocalDateStr(Long time) {
return format(LocalDateTime.ofInstant(Instant.ofEpochMilli(time), ZoneId.systemDefault()), DATE_PATTERN);
}
/**
* 获取日期时间字符串
*/
public static String format(TemporalAccessor temporal, String pattern) {
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(pattern);
return dateTimeFormatter.format(temporal);
}
/**
* 取本月第一天
*/
public static LocalDate firstDayOfThisMonth() {
LocalDate today = LocalDate.now();
return today.with(TemporalAdjusters.firstDayOfMonth());
}
/**
* 取本月第N天
*/
public static LocalDate dayOfThisMonth(int n) {
LocalDate today = LocalDate.now();
return today.withDayOfMonth(n);
}
/**
* 取本月最后一天
*/
public static LocalDate lastDayOfThisMonth() {
LocalDate today = LocalDate.now();
return today.with(TemporalAdjusters.lastDayOfMonth());
}
/**
* 获取指定日期时间加上指定数量日期时间单位之后的日期时间.
*/
public static LocalDateTime plus(LocalDateTime localDateTime, int num, ChronoUnit chronoUnit) {
return localDateTime.plus(num, chronoUnit);
}
/**
* 获取指定日期时间减去指定数量日期时间单位之后的日期时间.
*/
public static LocalDateTime minus(LocalDateTime localDateTime, int num, ChronoUnit chronoUnit) {
return localDateTime.minus(num, chronoUnit);
}
/**
* 根据ChronoUnit计算两个日期时间之间相隔日期时间
*/
public static long getChronoUnitBetween(LocalDateTime start, LocalDateTime end, ChronoUnit chronoUnit) {
return Math.abs(start.until(end, chronoUnit));
}
/**
* 根据ChronoUnit计算两个日期之间相隔年数或月数或天数
*/
public static long getChronoUnitBetween(LocalDate start, LocalDate end, ChronoUnit chronoUnit) {
return Math.abs(start.until(end, chronoUnit));
}
/**
* 切割日期。按照周期切割成小段日期段。例如：
*
* @param startDate 开始日期（yyyy-MM-dd）
* @param endDate 结束日期（yyyy-MM-dd）
* @param period 周期（天，周，月，年）
* @return 切割之后的日期集合
* - startDate="2023-10-27",endDate="2023-10-31",period="day"
* - 结果为：[2023-10-27, 2023-10-28, 2023-10-29, 2023-10-03,2023-10-31]
* - startDate="2023-10-27",endDate="2023-10-31",period="week"
* - 结果为：[2023-10-27,2023-10-31]
* - startDate="2023-10-27",endDate="2023-10-31",period="month"
* - 结果为：[2023-10-27,2023-10-31]
* - startDate="2023-10-27",endDate="2023-10-31",period="year"
* - 结果为：[2023-10-27,2023-10-31]
*/
public static List<String> listDateStrs(String startDate, String endDate, String period) {
List<String> result = new ArrayList<>();
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(DATE_PATTERN);
LocalDate end = LocalDate.parse(endDate, dateTimeFormatter);
LocalDate start = LocalDate.parse(startDate, dateTimeFormatter);
LocalDate tmp = start;
switch (period) {
case DAY:
while (start.isBefore(end) || start.isEqual(end)) {
result.add(start.toString());
start = start.plusDays(1);
}
break;
case WEEK:
while (tmp.isBefore(end) || tmp.isEqual(end)) {
if (tmp.plusDays(6).isAfter(end)) {
result.add(tmp.toString() + "," + end);
} else {
result.add(tmp.toString() + "," + tmp.plusDays(6));
}
tmp = tmp.plusDays(7);
}
break;
case MONTH:
while (tmp.isBefore(end) || tmp.isEqual(end)) {
LocalDate lastDayOfMonth = tmp.with(TemporalAdjusters.lastDayOfMonth());
if (lastDayOfMonth.isAfter(end)) {
result.add(tmp.toString() + "," + end);
} else {
result.add(tmp.toString() + "," + lastDayOfMonth);
}
tmp = lastDayOfMonth.plusDays(1);
}
break;
case YEAR:
while (tmp.isBefore(end) || tmp.isEqual(end)) {
LocalDate lastDayOfYear = tmp.with(TemporalAdjusters.lastDayOfYear());
if (lastDayOfYear.isAfter(end)) {
result.add(tmp.toString() + "," + end);
} else {
result.add(tmp.toString() + "," + lastDayOfYear);
}
tmp = lastDayOfYear.plusDays(1);
}
break;
default:
break;
}
return result;
}
}