风起Java Date
令天问了同事有咩有对Date源码了解，同事说没有了解过。作为日期工具里面的常用选手Date，大概也是不会满意这番回答，那我们接下来就对这位健将做一番专访，让我们重新认识java.util 包提供了 Date 类吧。
雨来
public static void main(String[] args) {
Date date = new Date();
System.out.println(date);
}
先让我们看下打印结果
Tue Jan 14 14:51:54 CST 2020
不应该输出Date的类的信息吗？为毛这样的呢，看来我们还得继续深入Date这位老将，一层层的揭开它的迷雾。
雾来
/**
* 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;
}
至此，我们看到Date的构造方法还是调用了System.currentTimeMillis()系统时间，最后对fastTime 进行了赋值就完了，可为什么结果是字符串呢，既然Date本身没有问题，那么就是System.out.println()从中做了我们不知的事情，看来源码之路路漫漫继续吧。
/**
* Prints an Object and then terminate the line. This method calls
* at first String.valueOf(x) to get the printed object's string value,
* then behaves as
* though it invokes <code>{@link #print(String)}</code> and then
* <code>{@link #println()}</code>.
*
* @param x The <code>Object</code> to be printed.
*/
public void println(Object x) {
String s = String.valueOf(x);
synchronized (this) {
print(s);
newLine();
}
}
似乎真相快要浮出水面了，让我们看下 String.valueOf(x)做了什么吧
/**
* Returns the string representation of the {@code Object} argument.
*
* @param obj an {@code Object}.
* @return if the argument is {@code null}, then a string equal to
* {@code "null"}; otherwise, the value of
* {@code obj.toString()} is returned.
* @see java.lang.Object#toString()
*/
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}
/**
* 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
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();
}
云散雾开
至此真相大白于天下，打印一个类对象时会自动调用toString方法，就出现了最后的结果。当然在我们揭秘Date的同时，也离不开fastTime和cdate两位伙伴的帮助。
private transient long fastTime;
/*
* If cdate is null, then fastTime indicates the time in millis.
* If cdate.isNormalized() is true, then fastTime and cdate are in
* synch. Otherwise, fastTime is ignored, and cdate indicates the
* time.
*/
private transient BaseCalendar.Date cdate;
特别提名友情出演来自于sun.util.calendar的BaseCalendar。
public abstract class BaseCalendar extends AbstractCalendar {
public static final int JANUARY = 1;
public static final int FEBRUARY = 2;
public static final int MARCH = 3;
public static final int APRIL = 4;
public static final int MAY = 5;
public static final int JUNE = 6;
public static final int JULY = 7;
public static final int AUGUST = 8;
public static final int SEPTEMBER = 9;
public static final int OCTOBER = 10;
public static final int NOVEMBER = 11;
public static final int DECEMBER = 12;
public static final int SUNDAY = 1;
public static final int MONDAY = 2;
public static final int TUESDAY = 3;
public static final int WEDNESDAY = 4;
public static final int THURSDAY = 5;
public static final int FRIDAY = 6;
public static final int SATURDAY = 7;
private static final int BASE_YEAR = 1970;
...
}
医理上常说“痛则不通，通则不痛”，学问亦是如此，不通则不明，正所谓一理通而百理明。
番外
在我们debug的路途中发现初始时间是1970年1月1日，为什么这样呢，其实不仅仅是Java，几乎所有的语言的时间都是从这一刻开始算起的。因为java起源于UNIX系统，而UNIX认为1970年1月1日0点是时间纪元。所有有了我们看到的结果。
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