Java进阶(五十八)由时间转换报NumberFormatException异常引发的思考一：如何使用时间格式化工具SimpleDateFormat

前言

      在项目开发过程中，忽然遇到一个问题，数据库中的存放的时间原本是2018.03.21 08:08:08；后来存入的数据变为了2018.03.21 08:08:08.08。结果从数据库中取数进行处理时，报错如下：

      在排查问题过程中，发现该时间是从工作流转存过来的。

SimpleDateFormat简介

      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 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
M Month in year 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 in week Text Tuesday; Tue
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
      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 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 java.lang.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 java.util.Calendar.LONG is used. Otherwise, a calendar specific short or abbreviated form is used.
      Month: If the number of pattern letters is 3 or more, the month is interpreted as text; otherwise, it is interpreted as a number.
      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.
      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
      通过阅读以上文章，初步怀疑是工作流在写数据时，数据格式转换工具出现了问题，该问题有待进一步跟踪。

知识拓展

      在使用JDK1.7的时候，将SimpleDateFormat的参数写为YYYYMMdd，1.6如果Y大写是会报错的。
      1.7中YYYY代表的是当天所在的周属于的年份，一周从周日开始，周六结束，只要本周跨年，那么这周就算入下一年。（如20171231）

文章来源: shq5785.blog.csdn.net，作者：No Silver Bullet，版权归原作者所有，如需转载，请联系作者。

原文链接：shq5785.blog.csdn.net/article/details/79591713