Thread-safe java.text.SimpleDateFormat format and parse

[Ben Evans]

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.

Thanks,

Ben

On Sat, Jun 27, 2015 at 8:36 AM, Paul Draper <paulddraper at gmail.com> wrote:
> 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 way,
> they can be called from multiple threads.
>
> The rationale is akin to calling ArrayList#get or HashMap#get from multiple
> 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: https://code.google.com/p/safe-simple-date-format/
> 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 condition.
>
> Making format() and parse() calls thread-safe would require either using a
> local Calendar variable -- one instance per call -- or using a thread-local
> 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.