Java：将时区添加到DateTimeFormatter

Question

public class TimeZone
{
    public static void main(String[] args)
    {
        LocalDateTime now = LocalDateTime.now();
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("hh:mm:ss a z");
        System.out.println(now.format(formatter));
    }
}

Exception in thread "main" java.time.DateTimeException: Unable to extract value: class java.time.LocalDateTime
at java.time.format.DateTimePrintContext.getValue(Unknown Source)
at java.time.format.DateTimeFormatterBuilder$ZoneTextPrinterParser.format(Unknown Source)
at java.time.format.DateTimeFormatterBuilder$CompositePrinterParser.format(Unknown Source)
at java.time.format.DateTimeFormatter.formatTo(Unknown Source)
at java.time.format.DateTimeFormatter.format(Unknown Source)
at java.time.LocalDateTime.format(Unknown Source)
at tz.TimeZone.main(TimeZone.java:12)

下面是DateTimeFormatter API部分：

All letters 'A' to 'Z' and 'a' to 'z' are reserved as pattern letters. The following pattern letters are defined: 
  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   G       era                         text              AD; Anno Domini; A
   u       year                        year              2004; 04
   y       year-of-era                 year              2004; 04
   D       day-of-year                 number            189
   M/L     month-of-year               number/text       7; 07; Jul; July; J
   d       day-of-month                number            10

   Q/q     quarter-of-year             number/text       3; 03; Q3; 3rd quarter
   Y       week-based-year             year              1996; 96
   w       week-of-week-based-year     number            27
   W       week-of-month               number            4
   E       day-of-week                 text              Tue; Tuesday; T
   e/c     localized day-of-week       number/text       2; 02; Tue; Tuesday; T
   F       week-of-month               number            3

   a       am-pm-of-day                text              PM
   h       clock-hour-of-am-pm (1-12)  number            12
   K       hour-of-am-pm (0-11)        number            0
   k       clock-hour-of-am-pm (1-24)  number            0

   H       hour-of-day (0-23)          number            0
   m       minute-of-hour              number            30
   s       second-of-minute            number            55
   S       fraction-of-second          fraction          978
   A       milli-of-day                number            1234
   n       nano-of-second              number            987654321
   N       nano-of-day                 number            1234000000

   V       time-zone ID                zone-id           America/Los_Angeles; Z; -08:30
   z       time-zone name              zone-name         Pacific Standard Time; PST
   O       localized zone-offset       offset-O          GMT+8; GMT+08:00; UTC-08:00;
   X       zone-offset 'Z' for zero    offset-X          Z; -08; -0830; -08:30; -083015; -08:30:15;
   x       zone-offset                 offset-x          +0000; -08; -0830; -08:30; -083015; -08:30:15;
   Z       zone-offset                 offset-Z          +0000; -0800; -08:00;

有人能看出问题出在哪里吗？

Answer 1

LocalDateTime有两个字段，类型为LocalDate和LocalTime。
LocalDate有字段日、月和年。
LocalTime有字段小时、分钟、秒和纳米

没有任何地方是给出的时区。这是本质上的，因为LocalDateTime的javadoc表示：

没有时区的日期时间

System.out.println(time.atZone(ZoneOffset.UTC)
                       .format(DateTimeFormatter.ofPattern("hh:mm:ss a z")));

System.out.println(time.format(DateTimeFormatter.ofPattern("hh:mm:ss a z")
                                                .withZone(ZoneOffset.UTC)));

System.out.println(time.format(DateTimeFormatter.ofPattern("hh:mm:ss a 'Z'")));

例如。如果时区为-04:00，则使用zoneoffset.offhours(-4)，您将得到：

晚上11:59:22-04:00

或者，如果您在纽约，使用zoneid.of（“america/new_york”），您将得到：

System.out.println(time.atZone(ZoneId.of("America/New_York"))
                       .format(DateTimeFormatter.ofPattern("hh:mm:ss a z")
                                                .withZone(ZoneOffset.UTC)));