Thread-safe java.text.SimpleDateFormat format and parse
Otávio Gonçalves de Santana
otaviopolianasantana at gmail.com
Wed Jul 1 15:52:11 UTC 2015
I agree with Ben.
On Jul 1, 2015 12:39 PM, "Ben Evans" <benjamin.john.evans at gmail.com> wrote:
> Given the other problems with the legacy date and time classes, why
> spend engineering time tidying this up?
> Everyone should be migrating to the new date & time support in
> java.time, so this would just be a distraction.
> On Sat, Jun 27, 2015 at 8:36 AM, Paul Draper <paulddraper at gmail.com>
> > While it's often understood that SimpleDateFormat isn't thread safe with
> > its setters, etc. it is frequently incorrectly assumed (despite the docs)
> > that since format() and parse() do not mutate the object in a visible
> > they can be called from multiple threads.
> > The rationale is akin to calling ArrayList#get or HashMap#get from
> > threads. The entire class is not thread-safe, but you can call that
> > non-mutating accessor from multiple threads without issue.
> > The trouble is that SimpleDateFormat has a private Calendar instance
> > variable, which is mutated during the format() and parse() methods.
> > This is a very common mistake. There is a project whose entire purpose
> is a
> > thread-safe formatter:
> > And Apache Commons and Joda Time provide similar classes.
> > Currently, users of SimpleDateFormat have to synchronize format() and
> > parse(), or use a separate SimpleDateFormat for every thread.
> > Or, too commonly, do neither and have a relatively unobvious race
> > Making format() and parse() calls thread-safe would require either using
> > local Calendar variable -- one instance per call -- or using a
> > Calendar -- one instance per thread. The former option seems the best.
> > The change would be fully backwards compatible. I have profiled a change
> > with a local Calendar variable, and measured no difference in the
> > performance (format and parse are by their nature rather involved methods
> > to begin with).
> > This change would improve the intuitive behavior of SimpleDateFormat and
> > eliminate one of the most common mistakes of JDK users.
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