源碼閱讀:new Date之旅

風起Java Date

令天問了同事有咩有對Date源碼瞭解，同事說沒有了解過。作爲日期工具裏面的常用選手Date，大概也是不會滿意這番回答，那我們接下來就對這位健將做一番專訪，讓我們重新認識java.util 包提供了 Date 類吧。

雨來

public static void main(String[] args) {
    Date date = new Date();
    System.out.println(date);
}

先讓我們看下打印結果

Tue Jan 14 14:51:54 CST 2020

不應該輸出Date的類的信息嗎？爲毛這樣的呢，看來我們還得繼續深入Date這位老將，一層層的揭開它的迷霧。

霧來

 /**
     * 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，我們看到Date的構造方法還是調用了System.currentTimeMillis()系統時間，最後對fastTime 進行了賦值就完了，可爲什麼結果是字符串呢，既然Date本身沒有問題，那麼就是System.out.println()從中做了我們不知的事情，看來源碼之路路漫漫繼續吧。

	/**
     * 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，讓我們看下 String.valueOf(x)做了什麼吧

	/**
     * 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();
    }

雲散霧開

至此真相大白於天下，打印一個類對象時會自動調用toString方法，就出現了最後的結果。當然在我們揭祕Date的同時，也離不開fastTime和cdate兩位夥伴的幫助。

    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;

特別提名友情出演來自於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;
    ...
    }

醫理上常說“痛則不通，通則不痛”，學問亦是如此，不通則不明，正所謂一理通而百理明。

番外

在我們debug的路途中發現初始時間是1970年1月1日，爲什麼這樣呢，其實不僅僅是Java，幾乎所有的語言的時間都是從這一刻開始算起的。因爲java起源於UNIX系統，而UNIX認爲1970年1月1日0點是時間紀元。所有有了我們看到的結果。