如何判断Java 8 DateTimeFormatter的时区信息并设置默认时区？

Great question! Relying on the toString() output of DateTimeFormatter is risky because that string is an internal implementation detail—Oracle doesn't guarantee its format will stay consistent across Java versions, so your code could break unexpectedly when you update the JDK.

Here's the clean, officially supported approach to solve your problem:

ZoneId defaultZone = ZoneOffset.UTC;
if (formatter.getZone() == null) {
    formatter = formatter.withZone(defaultZone);
    LOG.error("The timestamp formatter has no default timezone configured, so we're using {} as fallback.", 
              defaultZone.getDisplayName(TextStyle.SHORT, Locale.ENGLISH));
}

Why this works: #

• Official API compliance: getZone() is a documented, public method of DateTimeFormatter that returns the default timezone used for parsing when the input text doesn't contain timezone information. Checking if it's null tells you exactly whether a default timezone is already configured.
• Safe for all formatter patterns:
  • If your formatter's pattern includes timezone/offset fields (like Offset(...) or ZoneText(...)), setting a default timezone won't interfere with parsing—any timezone information in the input text will take precedence over the formatter's default zone.
  • If the pattern has no timezone fields, the default zone will be automatically applied to the parsed temporal (e.g., converting a LocalDateTime to ZonedDateTime without throwing an exception).

Your original approach of parsing the toString() output is fragile because the format of that string isn't part of the public contract. For example, Java might change how it formats formatter expressions in a future release, breaking your substring checks.

内容的提问来源于stack exchange，提问作者Niels Basjes