Re: Calendar.getInstance() thread safe?

From:

"Mike Schilling" <mscottschilling@hotmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 7 Mar 2010 09:22:03 -0800

Message-ID:

<hn0nbv$hf7$1@news.eternal-september.org>

endymion wrote:

Hi,

In a multi-threaded application, I encountered problems with dates
being "reset" to 1970, or being negative when converted into ms since
1970.

I use Calendar to do operations on dates, and as I see no obvious bug
in the code, and as the problem occurs randomly and after a certain
time, I was wondering if that was a multi-threading issue.

So the question is: Is Calendar.getInstance() thread safe?

It would be odd as hell for a static factory method not to be thread-safe.
But since this isn't documented anywhere, let's have a look at the source
code (for 1.5.0_16-b02):

   public static Calendar getInstance()
    {
        Calendar cal = createCalendar(TimeZone.getDefaultRef(),
Locale.getDefault());
        cal.sharedZone = true;
        return cal;
    }

    private static Calendar createCalendar(TimeZone zone,
        Locale aLocale)
    {
         // If the specified locale is a Thai locale, returns a
BuddhistCalendar
         // instance.
         if ("th".equals(aLocale.getLanguage())
             && ("TH".equals(aLocale.getCountry()))) {
             return new sun.util.BuddhistCalendar(zone, aLocale);
         }

         // else create the default calendar
        return new GregorianCalendar(zone, aLocale);
    }

Not much doubt that it always returns a newly constructed one. If the
version you're using might return the same Calendar twice, it has a pretty
significant bug in it.