java.text.SimpleDateFormat 源码注释，看了这个还不会用的自我面壁

SimpleDateFormat is a concrete class for formatting and
parsing dates in a locale-sensitive manner. It allows for formatting
(date → text), parsing (text → date), and normalization.

SimpleDateFormat allows you to start by choosing any user-defined patterns for date-time formatting. However, you are encouraged to create a date-time formatter with either getTimeInstance, getDateInstance, or getDateTimeInstance in DateFormat. Each of these class methods can return a date/time formatter initialized with a default format pattern. You may modify the format pattern using the applyPattern methods as desired. For more information on using these methods, see {@link DateFormat}.

Date and Time Patterns

Date and time formats are specified by date and time pattern strings. Within date and time pattern strings, unquoted letters from 'A' to 'Z' and from 'a' to 'z' are interpreted as pattern letters representing the components of a date or time string. Text can be quoted using single quotes (') to avoid interpretation. "''" represents a single quote. All other characters are not interpreted; they're simply copied into the output string during formatting or matched against the input string during parsing.

The following pattern letters are defined (all other characters from 'A' to 'Z' and from 'a' to 'z' are reserved):

Letter	Date or Time Component	Presentation	Examples
G	Era designator	Text	AD
y	Year	Year	1996; 96
Y	Week year	Year	2009; 09
M	Month in year (context sensitive)	Month	July; Jul; 07
L	Month in year (standalone form)	Month	July; Jul; 07
w	Week in year	Number	27
W	Week in month	Number	2
D	Day in year	Number	189
d	Day in month	Number	10
F	Day of week in month	Number	2
E	Day name in week	Text	Tuesday; Tue
u	Day number of week (1 = Monday, ..., 7 = Sunday)	Number	1
a	Am/pm marker	Text	PM
H	Hour in day (0-23)	Number	0
k	Hour in day (1-24)	Number	24
K	Hour in am/pm (0-11)	Number	0
h	Hour in am/pm (1-12)	Number	12
m	Minute in hour	Number	30
s	Second in minute	Number	55
S	Millisecond	Number	978
z	Time zone	General time zone	Pacific Standard Time; PST; GMT-08:00
Z	Time zone	RFC 822 time zone	-0800
X	Time zone	ISO 8601 time zone	-08; -0800; -08:00

Pattern letters are usually repeated, as their number determines the exact presentation:

• Text: For formatting, if the number of pattern letters is 4 or more, the full form is used; otherwise a short or abbreviated form is used if available. For parsing, both forms are accepted, independent of the number of pattern letters.
  
  
• Number: For formatting, the number of pattern letters is the minimum number of digits, and shorter numbers are zero-padded to this amount. For parsing, the number of pattern letters is ignored unless it's needed to separate two adjacent fields.
  
  
• Year: If the formatter's {@link #getCalendar() Calendar} is the Gregorian calendar, the following rules are applied.
  
  • For formatting, if the number of pattern letters is 2, the year is truncated to 2 digits; otherwise it is interpreted as a number.
  • For parsing, if the number of pattern letters is more than 2, the year is interpreted literally, regardless of the number of digits. So using the pattern "MM/dd/yyyy", "01/11/12" parses to Jan 11, 12 A.D.
  • For parsing with the abbreviated year pattern ("y" or "yy"), SimpleDateFormat must interpret the abbreviated year relative to some century. It does this by adjusting dates to be within 80 years before and 20 years after the time the SimpleDateFormat instance is created. For example, using a pattern of "MM/dd/yy" and a SimpleDateFormat instance created on Jan 1, 1997, the string "01/11/12" would be interpreted as Jan 11, 2012 while the string "05/04/64" would be interpreted as May 4, 1964. During parsing, only strings consisting of exactly two digits, as defined by {@link Character#isDigit(char)}, will be parsed into the default century. Any other numeric string, such as a one digit string, a three or more digit string, or a two digit string that isn't all digits (for example, "-1"), is interpreted literally. So "01/02/3" or "01/02/003" are parsed, using the same pattern, as Jan 2, 3 AD. Likewise, "01/02/-3" is parsed as Jan 2, 4 BC.
  Otherwise, calendar system specific forms are applied. For both formatting and parsing, if the number of pattern letters is 4 or more, a calendar specific {@linkplain Calendar#LONG long form} is used. Otherwise, a calendar specific {@linkplain Calendar#SHORT short or abbreviated form} is used.
  
  If week year {@code 'Y'} is specified and the {@linkplain #getCalendar() calendar} doesn't support any week years, the calendar year ({@code 'y'}) is used instead. The support of week years can be tested with a call to {@link DateFormat#getCalendar() getCalendar()}.{@link java.util.Calendar#isWeekDateSupported() isWeekDateSupported()}.
  
  
• Month: If the number of pattern letters is 3 or more, the month is interpreted as text; otherwise, it is interpreted as a number.
  
  • Letter M produces context-sensitive month names, such as the embedded form of names. If a {@code DateFormatSymbols} has been set explicitly with constructor {@link #SimpleDateFormat(String, DateFormatSymbols)} or method {@link #setDateFormatSymbols(DateFormatSymbols)}, the month names given by the {@code DateFormatSymbols} are used.
  • Letter L produces the standalone form of month names.
  
  
• General time zone: Time zones are interpreted as text if they have names. For time zones representing a GMT offset value, the following syntax is used:

     GMTOffsetTimeZone:
             GMT Sign Hours : Minutes
     Sign: one of
             + -
     Hours:
             Digit
             Digit Digit
     Minutes:
             Digit Digit
     Digit: one of
             0 1 2 3 4 5 6 7 8 9

  Hours must be between 0 and 23, and Minutes must be between 00 and 59. The format is locale independent and digits must be taken from the Basic Latin block of the Unicode standard.

  For parsing, RFC 822 time zones are also accepted.
  
  

• RFC 822 time zone: For formatting, the RFC 822 4-digit time zone format is used:

   RFC822TimeZone:
           Sign TwoDigitHours Minutes
   TwoDigitHours:
           Digit Digit

TwoDigitHours must be between 00 and 23. Other definitions
are as for general time zones.

For parsing, general time zones are also accepted.

ISO 8601 Time zone: The number of pattern letters designates the format for both formatting and parsing as follows:

   ISO8601TimeZone:
           OneLetterISO8601TimeZone
           TwoLetterISO8601TimeZone
           ThreeLetterISO8601TimeZone
   OneLetterISO8601TimeZone:
           Sign TwoDigitHours
           {@code Z}
   TwoLetterISO8601TimeZone:
           Sign TwoDigitHours Minutes
           {@code Z}
   ThreeLetterISO8601TimeZone:
           Sign TwoDigitHours {@code :} Minutes
           {@code Z}

Other definitions are as for general time zones or RFC 822 time zones.

For formatting, if the offset value from GMT is 0, {@code "Z"} is produced. If the number of pattern letters is 1, any fraction of an hour is ignored. For example, if the pattern is {@code "X"} and the time zone is {@code "GMT+05:30"}, {@code "+05"} is produced.

For parsing, {@code "Z"} is parsed as the UTC time zone designator. General time zones are not accepted.

If the number of pattern letters is 4 or more, {@link IllegalArgumentException} is thrown when constructing a {@code SimpleDateFormat} or {@linkplain #applyPattern(String) applying a pattern}. SimpleDateFormat also supports localized date and time pattern strings. In these strings, the pattern letters described above may be replaced with other, locale dependent, pattern letters. SimpleDateFormat does not deal with the localization of text other than the pattern letters; that's up to the client of the class.

Examples

The following examples show how date and time patterns are interpreted in
the U.S. locale. The given date and time are 2001-07-04 12:08:56 local time
in the U.S. Pacific Time time zone.

Date and Time Pattern	Result
"yyyy.MM.dd G 'at' HH:mm:ss z"	2001.07.04 AD at 12:08:56 PDT
"EEE, MMM d, ''yy"	Wed, Jul 4, '01
"h:mm a"	12:08 PM
"hh 'o''clock' a, zzzz"	12 o'clock PM, Pacific Daylight Time
"K:mm a, z"	0:08 PM, PDT
"yyyyy.MMMMM.dd GGG hh:mm aaa"	02001.July.04 AD 12:08 PM
"EEE, d MMM yyyy HH:mm:ss Z"	Wed, 4 Jul 2001 12:08:56 -0700
"yyMMddHHmmssZ"	010704120856-0700
"yyyy-MM-dd'T'HH:mm:ss.SSSZ"	2001-07-04T12:08:56.235-0700
"yyyy-MM-dd'T'HH:mm:ss.SSSXXX"	2001-07-04T12:08:56.235-07:00
"YYYY-'W'ww-u"	2001-W27-3

Synchronization

Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

@see Java Tutorial
@see java.util.Calendar
@see java.util.TimeZone
@see DateFormat
@see DateFormatSymbols
@author Mark Davis, Chen-Lieh Huang, Alan Liu