java中的Date和时区

超、凢脫俗 2023-06-20 15:52 11阅读 0赞

## 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(' ');                        // EEE
            convertToAbbr(sb, wtb[date.getMonth() - 1 + 2 + 7]).append(' ');  // MMM
            CalendarUtils.sprintf0d(sb, date.getDayOfMonth(), 2).append(' '); // dd
    
            CalendarUtils.sprintf0d(sb, date.getHours(), 2).append(':');   // HH
            CalendarUtils.sprintf0d(sb, date.getMinutes(), 2).append(':'); // mm
            CalendarUtils.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());  // yyyy
            return 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 2019
    	1575974664352

## 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 2019
    	Tue 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 19:06:31
    	2019-12-10 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
    	1575936000000
    	1575961200000

## 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Date
    	SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date date1 = dateFormat.parse(dateStr);
        // 使用目标时区格式化Date
        dateFormat.setTimeZone(TimeZone.getTimeZone("GMT+9:00"));
        System.out.println(dateFormat.format(date1));
    
    	// 输出
    	2019-12-10 09:00:00