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Java-Standard-Library

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1 java.time

time_types

  • Instance: Represents a specific point in time on the UTC timeline (Coordinated Universal Time). It is often used for timestamps.
  • LocalDateTime: Represents a date and time without a timezone. It is used for storing date and time information in a local context.
  • ZonedDateTime: Represents a date and time with a timezone. It is used for storing date-time information that is associated with a specific timezone.

1.2 Parse time with local time zone and transfer to UTC timestamp

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String dateTimeStr = "2024-08-01 13:34:56";
LocalDateTime localDateTime = LocalDateTime.parse(dateTimeStr, java.time.format.DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));

ZoneId currentZoneId = ZoneId.systemDefault();

ZonedDateTime zonedDateTime = localDateTime.atZone(currentZoneId);

Instant utcInstant = zonedDateTime.toInstant();
long utcTimestamp = utcInstant.getEpochSecond();

1.3 Parse UTC timestamp and transfer to time with local time zone

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long utcTimestamp = 1722490496L;

Instant utcInstant = Instant.ofEpochSecond(utcTimestamp);

ZoneId currentZoneId = ZoneId.systemDefault();

LocalDateTime localDateTime = LocalDateTime.ofInstant(utcInstant, currentZoneId);

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formattedDateTime = localDateTime.format(formatter);

1.4 Transform between UTC and Local DateTime

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public static LocalDateTime normalizeToUTC(LocalDateTime localZonedDateTime) {
    Instant instant = localZonedDateTime.atZone(ZoneId.systemDefault()).toInstant();
    return LocalDateTime.ofInstant(instant, ZONE_UTC);
}

public static LocalDateTime localizeFromUTC(LocalDateTime utcDateTime) {
    Instant instant = utcDateTime.atZone(ZONE_UTC).toInstant();
    return LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
}

1.5 Get Time offset

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ZoneOffset offset = ZonedDateTime.now(ZoneId.systemDefault()).getOffset();

1.6 Default timezone config load order

The priorities from high to low:

  • -Duser.timezone: Jvm option
    • java -Duser.timezone=America/New_York -jar yourApp.jar
  • TZ: Env
    • export TZ=Asia/Tokyo
  • /etc/timezone
  • /etc/localtime