JDK源码阅读-------自学笔记(十三)(java.text.DateFormat和SimpleDateFormat类)

时间相关类总图

Date是学习的核心类,表示时间
DateFormat用于对日期的字符串转化
Calendar对于日期的转化,年月日对象的转化,日期表示和运算的时候使用
Calendar实现类SimpleDateFormat和Calendar的GergorianCalendar

时间在计算机中的表示

时间是一维的,只有前后之分
计算机标准纪元,1970年1月1日 00:00:00,每走一个毫秒为一个单位 (科普:由于早期的计算机存储单位比较小,32位能表示的最长时间是68年，从1970年开始的话，加上68.1，实际最终到2038年01月19日03时14分07秒，便会到达最大时间，过了这个时间点，所有32位操作系统时间便会变为 10000000 00000000 00000000 00000000，算下来也就是1901年12月13日20时45分52秒，这样便会出现时间回归的现象，很多软件便会运行异常了。时间回归的现象相信随着64为操作系统的产生逐渐得到解决，因为用64位操作系统可以表示到 292,277,026,596年12月4日15时30分08秒，相信我们的N代子孙，哪怕地球毁灭那天都不用愁不够用了，因为这个时间已经是千亿年以后了。)

获取当前时刻的毫秒值

1  long currentTimeMillis = System.currentTimeMillis();

实战

创建一个日期对象,包是java.util.Date
```
1  Date date = new Date();
```

源码解析

this(System.currentTimeMillis());相当于创建有参构造函数,并且赋值为当前毫秒值为对象的默认时间,相当于下边的构造方法

1  public Date(long date) {
2         fastTime = date;
3  }

测试

1         Date date = new Date(System.currentTimeMillis());
2 
3         Date date1 = new Date();
4 
5         System.out.println(date.getTime()==date1.getTime());

常用方法

boolean after(Date when) 测试此日期是否在指定日期之后核心思维:由于日期存储了当前时间的毫秒数,所以比较日期前后,可以使用getTime()的结果来进行比较得出

源码解析

源码

 1 /*
 2      * If cdate is null, then fastTime indicates the time in millis.
 3      * If cdate.isNormalized() is true, then fastTime and cdate are in
 4      * synch. Otherwise, fastTime is ignored, and cdate indicates the
 5      * time.
 6      */
 7     private transient BaseCalendar.Date cdate;
 8 
 9     /**
10      * Tests if this date is after the specified date.
11      *
12      * @param   when   a date.
13      * @return  true if and only if the instant represented
14      *          by this Date object is strictly later than the
15      *          instant represented by when;
16      *          false otherwise.
17      * @exception NullPointerException if when is null.
18      */
19     public boolean after(Date when) {
20         return getMillisOf(this) > getMillisOf(when);
21     }
22 
23 
24     /**
25      * Returns the millisecond value of this Date object
26      * without affecting its internal state.
27      */
28     static final long getMillisOf(Date date) {
29         if (date.cdate == null || date.cdate.isNormalized()) {
30             return date.fastTime;
31         }
32         BaseCalendar.Date d = (BaseCalendar.Date) date.cdate.clone();
33         return gcal.getTime(d);
34     }

解析

核心就是getMillisOf(Date date)这个方法之间的互相比较,getMillisOf(this) > getMillisOf(when)第一个this的对象的毫秒值是否大于when对象的毫秒追,是的话this对象的时间就晚于when的时间,即when对象的时间在this对象的时间之后.

测试

1         Date date = new Date(2000);
2 
3         Date date1 = new Date();
4 
5         System.out.println(date1.after(date));

booleanbefore(Date when) 测试此日期是否在指定日期之前。

源码解析

源码

 1     /*
 2      * If cdate is null, then fastTime indicates the time in millis.
 3      * If cdate.isNormalized() is true, then fastTime and cdate are in
 4      * synch. Otherwise, fastTime is ignored, and cdate indicates the
 5      * time.
 6      */
 7     private transient BaseCalendar.Date cdate;
 8 
 9      /**
10      * Tests if this date is before the specified date.
11      *
12      * @param   when   a date.
13      * @return  true if and only if the instant of time
14      *            represented by this Date object is strictly
15      *            earlier than the instant represented by when;
16      *          false otherwise.
17      * @exception NullPointerException if when is null.
18      */
19     public boolean before(Date when) {
20         return getMillisOf(this) < getMillisOf(when);
21     }
22 
23 
24     /**
25      * Returns the millisecond value of this Date object
26      * without affecting its internal state.
27      */
28     static final long getMillisOf(Date date) {
29         if (date.cdate == null || date.cdate.isNormalized()) {
30             return date.fastTime;
31         }
32         BaseCalendar.Date d = (BaseCalendar.Date) date.cdate.clone();
33         return gcal.getTime(d);
34     }

解析

核心就是getMillisOf(Date date)这个方法之间的互相比较,getMillisOf(this) < getMillisOf(when)第一个this的对象的毫秒值是否小于when对象的毫秒追,是的话this对象的时间就早于when的时间,即when对象的时间在this对象的时间之前.

测试

1   
2         Date date = new Date(2000);
3 
4         Date date1 = new Date();
5 
6         System.out.println(date.before(date1));

boolean equals(Object obj) 比较两个日期的相等性。

源码解析

源码

 1      /**
 2      * Compares two dates for equality.
 3      * The result is true if and only if the argument is
 4      * not null and is a Date object that
 5      * represents the same point in time, to the millisecond, as this object.
 6      * 
 7      * Thus, two Date objects are equal if and only if the
 8      * getTime method returns the same long
 9      * value for both.
10      *
11      * @param   obj   the object to compare with.
12      * @return  true if the objects are the same;
13      *          false otherwise.
14      * @see     java.util.Date#getTime()
15      */
16     public boolean equals(Object obj) {
17         return obj instanceof Date && getTime() == ((Date) obj).getTime();
18     }

解析

通过核心方法equals(Object obj),核心判断 obj instanceof Date && getTime() == ((Date) obj).getTime();同时满足两个对象的类型相同,获取的getTime()毫秒值相同,就判断两个日期相等

测试

1         Date date = new Date(2000);
2 
3         Date date1 = new Date();
4 
5         System.out.println(date.equals(date1));

long getTime() 返回自 1970 年 1 月 1 日 00:00:00 GMT 以来此 Date 对象表示的毫秒数。

源码解析

源码

 1     // The default value of gregorianCutover.
 2     static final long DEFAULT_GREGORIAN_CUTOVER = -12219292800000L;
 3 
 4     private static final BaseCalendar gcal = CalendarSystem.getGregorianCalendar();
 5 
 6     private static BaseCalendar jcal;
 7 
 8     private transient long fastTime;
 9 
10     /*
11      * If cdate is null, then fastTime indicates the time in millis.
12      * If cdate.isNormalized() is true, then fastTime and cdate are in
13      * synch. Otherwise, fastTime is ignored, and cdate indicates the
14      * time.
15      */
16     private transient BaseCalendar.Date cdate;
17 
18     /**
19      * Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT
20      * represented by this Date object.
21      *
22      * @return  the number of milliseconds since January 1, 1970, 00:00:00 GMT
23      *          represented by this date.
24      */
25     public long getTime() {
26         return getTimeImpl();
27     }
28 
29     private final long getTimeImpl() {
30         if (cdate != null && !cdate.isNormalized()) {
31             normalize();
32         }
33         return fastTime;
34     }
35 
36 
37     private final BaseCalendar.Date normalize() {
38         if (cdate == null) {
39             BaseCalendar cal = getCalendarSystem(fastTime);
40             cdate = (BaseCalendar.Date) cal.getCalendarDate(fastTime,
41                     TimeZone.getDefaultRef());
42             return cdate;
43         }
44 
45         // Normalize cdate with the TimeZone in cdate first. This is
46         // required for the compatible behavior.
47         if (!cdate.isNormalized()) {
48             cdate = normalize(cdate);
49         }
50 
51         // If the default TimeZone has changed, then recalculate the
52         // fields with the new TimeZone.
53         TimeZone tz = TimeZone.getDefaultRef();
54         if (tz != cdate.getZone()) {
55             cdate.setZone(tz);
56             CalendarSystem cal = getCalendarSystem(cdate);
57             cal.getCalendarDate(fastTime, cdate);
58         }
59         return cdate;
60     }
61 
62     private static final BaseCalendar getCalendarSystem(long utc) {
63         // Quickly check if the time stamp given by `utc' is the Epoch
64         // or later. If it's before 1970, we convert the cutover to
65         // local time to compare.
66         if (utc >= 0
67                 || utc >= GregorianCalendar.DEFAULT_GREGORIAN_CUTOVER
68                 - TimeZone.getDefaultRef().getOffset(utc)) {
69             return gcal;
70         }
71         return getJulianCalendar();
72     }
73 
74 
75     synchronized private static final BaseCalendar getJulianCalendar() {
76         if (jcal == null) {
77             jcal = (BaseCalendar) CalendarSystem.forName("julian");
78         }
79         return jcal;
80     }

解析
判断BaseCalendar是否为空,和它的格式是否符合设置的格式,为空和格式符合,就返回输入的毫秒值,不为空,不符合格式规范,进入normalize()方法,对BaseCalendar中的参数进行设置 GregorianCalendar.DEFAULTGREGORIANCUTOVER - TimeZone.getDefaultRef().getOffset(utc) 格里高利历默认时间减去系统时区默认的毫秒值时间
getJulianCalendar()获取儒略历(Julian Calendar)"规则的时间
normalize() 对象BaseCalendar为空的时候,获取格里高利历默认时间减去系统时区默认的毫秒值时间或者儒略历(Julian Calendar)"规则的时间放入对象再通过 cal.getCalendarDate的毫秒值和时区时间,获取对象日期,再通过对象的日期转化为毫秒值不是标准格式(!cdate.isNormalized()),通过 cdate = normalize(cdate);,获取日期对象 tz != cdate.getZone() 当前对象是否是默认时区,判断时区是否被更改时区更改了,改回当前的时区,然后通过对象获取日期
目的,就是获取BaseCalendar对象,获取其相关日期的参数,然后转化为儒略历的时间计算方式

测试

1         Date date = new Date(2000);
2 
3          System.out.println(date.getTime());

String toString() 把此 Date 对象转换为以下形式的 String：

dow mon dd hh:mm:ss zzz yyyy 其中： dow 是一周中的某一天 (Sun、 Mon、Tue、Wed、 Thu、 Fri、 Sat)

源码解析

源码

 1 /**
 2      * Converts this Date object to a String
 3      * of the form:
 4      *  5      * dow mon dd hh:mm:ss zzz yyyy
 6      * where:
 7      * dow is the day of the week (Sun, Mon, Tue, Wed,
 8      *     Thu, Fri, Sat).
 9      * 
mon is the month (Jan, Feb, Mar, Apr, May, Jun,
10      *     Jul, Aug, Sep, Oct, Nov, Dec).
11      * 
dd is the day of the month (01 through
12      *     31), as two decimal digits.
13      * 
hh is the hour of the day (00 through
14      *     23), as two decimal digits.
15      * 
mm is the minute within the hour (00 through
16      *     59), as two decimal digits.
17      * 
ss is the second within the minute (00 through
18      *     61, as two decimal digits.
19      * 
zzz is the time zone (and may reflect daylight saving
20      *     time). Standard time zone abbreviations include those
21      *     recognized by the method parse. If time zone
22      *     information is not available, then zzz is empty -
23      *     that is, it consists of no characters at all.
24      * 
yyyy is the year, as four decimal digits.
25      * 
26      *
27      * @return  a string representation of this date.
28      * @see     java.util.Date#toLocaleString()
29      * @see     java.util.Date#toGMTString()
30      */
31     public String toString() {
32         // "EEE MMM dd HH:mm:ss zzz yyyy";
33         BaseCalendar.Date date = normalize();
34         StringBuilder sb = new StringBuilder(28);
35         int index = date.getDayOfWeek();
36         if (index == BaseCalendar.SUNDAY) {
37             index = 8;
38         }
39         convertToAbbr(sb, wtb[index]).append(' ');                        // EEE
40         convertToAbbr(sb, wtb[date.getMonth() - 1 + 2 + 7]).append(' ');  // MMM
41         CalendarUtils.sprintf0d(sb, date.getDayOfMonth(), 2).append(' '); // dd
42 
43         CalendarUtils.sprintf0d(sb, date.getHours(), 2).append(':');   // HH
44         CalendarUtils.sprintf0d(sb, date.getMinutes(), 2).append(':'); // mm
45         CalendarUtils.sprintf0d(sb, date.getSeconds(), 2).append(' '); // ss
46         TimeZone zi = date.getZone();
47         if (zi != null) {
48             sb.append(zi.getDisplayName(date.isDaylightTime(), TimeZone.SHORT, Locale.US)); // zzz
49         } else {
50             sb.append("GMT");
51         }
52         sb.append(' ').append(date.getYear());  // yyyy
53         return sb.toString();
54     }

解析

拼接字符串按照格式:EEE MMM dd HH:mm:ss zzz yyyy,获取周/月/日/时分秒/时区/年

测试

1         Date date = new Date();
2 
3         System.out.println(date.toString());

注:
大部分方法已经被取代了,不推荐使用了 JDK8,已经推出了新的LocalDate系列取代的新方案,

详情查阅

源码阅读

JDK源码阅读-------自学笔记(十三)(java.text.DateFormat和SimpleDateFormat类)

时间相关类总图

时间在计算机中的表示

实战

测试

常用方法

源码解析

测试

源码解析

测试

源码解析

测试

源码解析

测试

源码解析

测试

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