Disclaimer: This article is a blogger original article, shall not be reproduced without the bloggers allowed. https://blog.csdn.net/u011033906/article/details/90489803
0. SimpleDateFormat API Introduction
/**
* SimpleDateFormat
* 一个与语言环境相关的格式化日期和分析日期的工具类。
* 利用该类可以将日期转换成文本，或者将文本转换成日期。
*
* 在使用SimpleDateFormat时需要指定一个需要的格式(pattern)来格式日期(Date).
* 在此请注意几个字母大小写的差异:
*
* 大写的H为24小时制表示一天中的小时数(0-23)
* 小写的h为12小时制表示一天中的小时数(1-12)
*
* 大写的M表示年中的月份
* 小写的m表示小时中的分钟数
*
* 大写的S表示毫秒数
* 小写的s表示秒数
*
* 所以最常用的24小时制的具体日期的pattern为:
* yyyy-MM-dd HH:mm:ss
*
*
* SimpleDateFormat中format()方法小结:
* 1 format()方法的作用是将日期(Date)转换为文本
* 2 format()方法的输入参数是一个Date
*
*
* SimpleDateFormat中parse()方法小结:
* 1 parse()方法的作用是将文本转换为日期(Date)
* 2 parse()方法的输入参数是一个文本,比如String
*/
1. convert the date to text
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;
Date date = new Date();
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
String time = simpleDateFormat.format(date);
System.out.println("----> 格式化后的日期为: "+time);
System.out.println("----------------------------------");
Output:
System.out: ----> 格式化后的日期为: 2019-05-23 22:28:01
System.out: ----------------------------------
2. Convert text to date
/**
* 请注意:
*
* 文本的格式应该与 SimpleDateFormat 中的 pattern 保持一致,否则导致异常
* 比如:
* 2008年08月18日 20:07:33 对应于yyyy年MM月dd日 HH:mm:ss
* 2008-08-18 20:07:33 对应于yyyy-MM-dd HH:mm:ss
*/
private void test2() {
try {
String day = "2008年08月18日 20:07:33";
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss", Locale.getDefault());
Date date = simpleDateFormat.parse(day);
System.out.println("----> 格式化后的日期为: "+date);
day = "2008-08-18 20:07:33";
simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
date = simpleDateFormat.parse(day);
System.out.println("----> 格式化后的日期为: "+date);
day = "20131227085009";
simpleDateFormat = new SimpleDateFormat("yyyyMMddHHmmss", Locale.getDefault());
date = simpleDateFormat.parse(day);
simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
String time = simpleDateFormat.format(date);
System.out.println("----> 时间文本为: "+time);
System.out.println("----------------------------------");
} catch (Exception e) {
System.out.println("----> Exception: "+e.toString());
}
}
Output:
System.out: ----> 格式化后的日期为: Mon Aug 18 20:07:33 GMT+08:00 2008
System.out: ----> 格式化后的日期为: Mon Aug 18 20:07:33 GMT+08:00 2008
System.out: ----> 时间文本为: 2013-12-27 08:50:09
3. The time stamp is converted to
long timeStamp=System.currentTimeMillis();
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
Date date = new Date(timeStamp);
String time = simpleDateFormat.format(date);
System.out.println("----> 将时间戳转换为字符串: "+time);
System.out.println("----------------------------------");
Output:
System.out: ----> 将时间戳转换为字符串: 2019-05-23 22:36:27
4. converted into time stamp
long timeStamp = System.currentTimeMillis();
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
Date date = new Date(timeStamp);
String time = simpleDateFormat.format(date);
System.out.println("----> 当前时间戳为: "+timeStamp+" ,其字符串为:"+time);
Date parsedDate = simpleDateFormat.parse(time);
long ts = parsedDate.getTime();
System.out.println("----> 将字符串转换为时间戳: "+ts);
System.out.println("----------------------------------");
Output:
----> 当前时间戳为: 1558622317494 ,其字符串为:2019-05-23 22:38:37
----> 将字符串转换为时间戳: 1558622317000
5. java relationship between the timestamp and the timestamp unix
/**
* java中生成的时间戳精确到毫秒,但unix中精确到秒
* 所以两者相差1000倍
*/
long javaTimeStamp = System.currentTimeMillis();
long unixTimeStamp = javaTimeStamp/1000;
System.out.println("----> java时间戳: " + javaTimeStamp+", unix时间戳：" + unixTimeStamp);
System.out.println("----------------------------------");
Output:
System.out: ----> java时间戳: 1558622474893 ,unix时间戳：1558622474
6. Calculate the time difference between the two
private String time1="2016-01-02 00:00:00";
private String time2="2013-09-21 00:00:00";
private void getTimeDifference(String time1,String time2) {
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
try {
Date date1 = simpleDateFormat.parse(time1);
Date date2 = simpleDateFormat.parse(time2);
long difference = date1.getTime() - date2.getTime();
long days = difference / (1000 * 60 * 60 * 24);
System.out.println("----> 两个时间相距:"+days+"天");
} catch (Exception e) {
System.out.println("----> Exception="+e.toString());
}
System.out.println("----------------------------------");
}
Output:
System.out: ----> 两个时间相距:833天
7. comparing two time
private void compareTime(String time1, String time2) {
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
Calendar calendar1 = java.util.Calendar.getInstance();
Calendar calendar2 = java.util.Calendar.getInstance();
try {
calendar1.setTime(dateFormat.parse(time1));
calendar2.setTime(dateFormat.parse(time2));
} catch (java.text.ParseException e) {
System.out.println("----> Exception=" + e.toString());
}
int result = calendar1.compareTo(calendar2);
if (result == 0){
System.out.println("----> time1等于time2");
}else if (result < 0) {
System.out.println("----> time1小于time2");
}else {
System.out.println("----> time1大于time2");
}
}
Output:
System.out: ----> time1大于time2
2. Thread safe to use
2.1 ThreadLocal
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
public class ConcurrentDateUtil {
private static ThreadLocal<DateFormat> threadLocal = new ThreadLocal<DateFormat>() {
@Override
protected DateFormat initialValue() {
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
}
};
public static Date parse(String dateStr) throws ParseException {
return threadLocal.get().parse(dateStr);
}
public static String format(Date date) {
return threadLocal.get().format(date);
}
}
Use ThreadLocal, but also the shared variable becomes exclusive, exclusive thread certainly exclusive method than in a concurrent environment can reduce a lot of the overhead of creating objects. If is relatively high on the performance requirements, it is generally recommended to use this method.
Solution of 2.2 Java 8
Java 8 provides a new date and time API, including a date and time formatting DateTimeFormatter, and it is SimpleDateFormatthe biggest difference is: DateTimeFormatteris thread-safe, and SimpleDateFormatis not thread safe.
Parse date
String dateStr= "2018年06月20日";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日");
LocalDate date= LocalDate.parse(dateStr, formatter);
Date into a string
LocalDateTime now = LocalDateTime.now();
DateTimeFormatter format = DateTimeFormatter.ofPattern("yyyy年MM月dd日 hh:mm a");
String nowStr = now.format(format);