<i18n dev> Java should handle user defined TimeZone subclass instances supporting historic rule changes properly
y.umaoka at gmail.com
Fri Oct 17 20:43:29 UTC 2014
Dear Java i18n team,
I was looking at recent Russian time zone changes and noticed a design
issue in JDK. I think I knew this issue, but I forgot about this until
recently. The problem is - Java does not support user defined custom
TimeZone implementation properly.
java.util.TimeZone is quite old, and the original design assumed 'raw
offset' and 'daylight saving amount' will never changed. To implement a
subclass of TimeZone, you have to override an awkward abstract method -
abstract int getOffset(int era, int year, int month, int day, int
dayOfWeek, int milliseconds). This method signature itself does not
limit a custom TimeZone implementation to support historic raw/dst
offset changes. However, if you do so, you will see some problems in
I filed a bug below:
Java should handle user defined TimeZone subclass instances supporting
historic rule changes properly.
You can create a custom time zone implementation class by extending
java.util.TimeZone. By the contract, the subclass must implement a few
abstract int getOffset(int,int,int,int,int,int);
abstract int getRawOffset();
abstract boolean inDaylightTime(Date);
abstract void setRawOffset(int);
abstract boolean useDaylightTime();
Although, the interface above do not assume UTC offset or daylight
saving amount is not changing time to time, a user can provide an
implementation supporting historic UTC offset changes by int
getOffset(int,int,int,int,int,int). However, there are two Java
1. The default implementation of TimeZone#getOffset(long date) does not
use the abstract method - int getOffset(int,int,int,int,int,int).
Therefore, when a different 'raw' offset was used in the past, or will
be used in future, getOffset(long) always returns a result calculated
from getRawOffset() and getDSTSavings(). Although a subclass
implementation can provide TimeZone#getOffset(long date), the default
implementation should use getOffset(int,int,int,int,int,int).
2. java.util.GregorianCalendar checks if a TimeZone instance is
sun.util.calendar.TimZone or not, and if not, the offset field is
calculated based on getRawOffset() / getDSTSavings(). Therefore, even a
custom TimeZone implementation support different 'raw' offsets in the
past or future, current 'raw' offset (getRawOffset()) value is used.
A user may want to create a custom TimeZone instance from iCalendar
VTIMEZONE component. iCalendr VTIMEZONE can support historic time zone
1) Add API TimeZone#getRawOffset(long) with the default implementation -
2) Add API TimeZone#getDSTSavings(long) with the default implementation
- return getDSTSavings().
2) Update the default implementation of TimeZone#getOffset(long) to use
#getRawOffset(long) #getDSTSavings(long) above along is existing
3) Gregorian calendar to use #getRawOffset(long) and
#getDSTSavings(long), if not ZoneInfo.
There is an IETF working draft for timezone service protocol. In future,
someone may want to create a custom time zone, interacting with a
timezone service server to get the latest time zone rule data, instead
of applying tzdata update utility. In this case, such limitation becomes
a blocker. There are several ways to resolve the issue - I just
suggested one way in the bug report. Are there anyone willing to look
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