java中的Date和时区

Date本身没有时区概念

查看源码可以知道, Date对象中存储的是一个long型变量
这个变量的值为自1997-01-01 00:00:00(GMT)至Date对象记录时刻所经过的毫秒数
可以通过getTime()方法,获取这个变量值,且这个变量值和时区没有关系
全球任意地点同时执行new Date().getTime()获取到的值相同

Date源码

 private transient long fastTime;/*** Allocates a <code>Date</code> object and initializes it so that* it represents the time at which it was allocated, measured to the* nearest millisecond.** @see     java.lang.System#currentTimeMillis()*/public Date() {this(System.currentTimeMillis());}/*** Allocates a <code>Date</code> object and initializes it to* represent the specified number of milliseconds since the* standard base time known as "the epoch", namely January 1,* 1970, 00:00:00 GMT.** @param   date   the milliseconds since January 1, 1970, 00:00:00 GMT.* @see     java.lang.System#currentTimeMillis()*/public Date(long date) {fastTime = date;}/*** Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT* represented by this <tt>Date</tt> object.** @return  the number of milliseconds since January 1, 1970, 00:00:00 GMT*          represented by this date.*/public long getTime() {return getTimeImpl();}private final long getTimeImpl() {if (cdate != null && !cdate.isNormalized()) {normalize();}return fastTime;}/*** Converts this <code>Date</code> object to a <code>String</code>* of the form:* <blockquote><pre>* dow mon dd hh:mm:ss zzz yyyy</pre></blockquote>* where:<ul>* <li><tt>dow</tt> is the day of the week (<tt>Sun, Mon, Tue, Wed,*     Thu, Fri, Sat</tt>).* <li><tt>mon</tt> is the month (<tt>Jan, Feb, Mar, Apr, May, Jun,*     Jul, Aug, Sep, Oct, Nov, Dec</tt>).* <li><tt>dd</tt> is the day of the month (<tt>01</tt> through*     <tt>31</tt>), as two decimal digits.* <li><tt>hh</tt> is the hour of the day (<tt>00</tt> through*     <tt>23</tt>), as two decimal digits.* <li><tt>mm</tt> is the minute within the hour (<tt>00</tt> through*     <tt>59</tt>), as two decimal digits.* <li><tt>ss</tt> is the second within the minute (<tt>00</tt> through*     <tt>61</tt>, as two decimal digits.* <li><tt>zzz</tt> is the time zone (and may reflect daylight saving*     time). Standard time zone abbreviations include those*     recognized by the method <tt>parse</tt>. If time zone*     information is not available, then <tt>zzz</tt> is empty -*     that is, it consists of no characters at all.* <li><tt>yyyy</tt> is the year, as four decimal digits.* </ul>** @return  a string representation of this date.* @see     java.util.Date#toLocaleString()* @see     java.util.Date#toGMTString()*/public String toString() {// "EEE MMM dd HH:mm:ss zzz yyyy";BaseCalendar.Date date = normalize();StringBuilder sb = new StringBuilder(28);int index = date.getDayOfWeek();if (index == BaseCalendar.SUNDAY) {index = 8;}convertToAbbr(sb, wtb[index]).append(' ');                        // EEEconvertToAbbr(sb, wtb[date.getMonth() - 1 + 2 + 7]).append(' ');  // MMMCalendarUtils.sprintf0d(sb, date.getDayOfMonth(), 2).append(' '); // ddCalendarUtils.sprintf0d(sb, date.getHours(), 2).append(':');   // HHCalendarUtils.sprintf0d(sb, date.getMinutes(), 2).append(':'); // mmCalendarUtils.sprintf0d(sb, date.getSeconds(), 2).append(' '); // ss// 注意: 这里涉及到时区TimeZone zi = date.getZone();if (zi != null) {sb.append(zi.getDisplayName(date.isDaylightTime(), TimeZone.SHORT, Locale.US)); // zzz} else {sb.append("GMT");}sb.append(' ').append(date.getYear());  // yyyyreturn sb.toString();}

getTime()获取毫秒数,获取到的毫秒数和格式化后时间的关系

这个变量的值为自1997-01-01 00:00:00(GMT)至Date对象记录时刻所经过的毫秒数
这个毫秒数和格式化后时间是模型和视图的关系, 时区(TimeZone)决定了同一模型展示成什么样的视图(格式化Date)

 Date date = new Date();System.out.println(date);System.out.println(date.getTime());// 输出Tue Dec 10 18:44:24 CST 20191575974664352

格式化Date对象成字符串, 涉及时区

不管是调用Date对象的toString方法, 还是使用SimpleDateFormat的format方法去格式化Date对象,或者使用parse解析字符串成Date对象都会涉及到时区,
也就是说Date对象没有时区概念, 但是格式化Date对象, 或者解析字符串成Date对象时, 是有时区概念的

toString

 Date date = new Date();// 默认是系统时区System.out.println(date);// 修改默认时区TimeZone.setDefault(TimeZone.getTimeZone("GMT"));System.out.println(date);// 输出Tue Dec 10 19:03:46 CST 2019Tue Dec 10 11:03:46 GMT 2019

SimpleDateFormat

 Date date = new Date();// 默认是系统时区SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");System.out.println(dateFormat.format(date));// 设置时区dateFormat.setTimeZone(TimeZone.getTimeZone("GMT+1:00"));System.out.println(dateFormat.format(date));// 输出2019-12-10 19:06:312019-12-10 12:06:31

解析字符串成Date对象, 涉及时区

将同一个时间字符串按照不同的时区来解析, 得到的Date对象值不一样
很好理解: 东八区8点当然和0时区8点不一样

 String dateStr = "2019-12-10 08:00:00";// 默认是系统时区SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Date date1 = dateFormat.parse(dateStr);System.out.println(date1.getTime());// 设置时区dateFormat.setTimeZone(TimeZone.getTimeZone("GMT+1:00"));Date date2 = dateFormat.parse(dateStr);System.out.println(date2.getTime());// 输出15759360000001575961200000

将本地的时间字符串转换成另一时区的时间字符串

 String dateStr = "2019-12-10 08:00:00";// 按照本地时区解析字符串成DateSimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Date date1 = dateFormat.parse(dateStr);// 使用目标时区格式化DatedateFormat.setTimeZone(TimeZone.getTimeZone("GMT+9:00"));System.out.println(dateFormat.format(date1));// 输出2019-12-10 09:00:00