java中的日期格式
我们经常在字符串和date之间相互转化，通过指定pattern来指定格式，例如(yyyy-MM-dd)。但是我们可能只关注到用的比较多的格式，有一些可能冷门一点儿的其实也会很有用。下面就把jdk中源码拿出来总结一下。
/**
* 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.
*/
其中可能比较有用，但是容易被我们忽略的就是E(星期)和A(上午/下午)，这两个还是挺有用的，有的时候看到有些人还会专门写个工具类来将日期转化成星期几或者上午下午，其实人家都已经帮我们实现好了，知道了这个后就不用重复造轮子了。
下面来写一个格式来测试一下。
public class Dates {
public static void main(String[] args) {
String format = new SimpleDateFormat("yyyy-MM-dd E ahh:mm").format(new Date());
System.out.println(format);
}
}运行结果
2020-07-22 星期三 下午05:45我们平常还会想如何将上面的07变成7，直接将格式MM改成M即可。
public class Dates {
public static void main(String[] args) {
String format = new SimpleDateFormat("yyyy-M-dd E ahh:mm").format(new Date());
System.out.println(format);
}
}运行结果
2020-7-22 星期三 下午05:49类似的可以将日，时都可以将前面的0去掉。