“Java DateFormat is not threadsafe” what does this leads to?

Question

Everybody cautions regarding Java DateFormat not being thread safe and I understand the concept theoretically.

But I'm not able to visualize what actual issues we can face due to this. Say, I've a DateFormat field in a class and the same is used in different methods in the class (formatting dates) in a multi-threaded environment.

Will this cause:

• any exception like format exception
• discrepancy in data
• any other issue?

Also, please explain why.

Answer 1

I would expect data corruption - e.g. if you're parsing two dates at the same time, you could have one call polluted by data from another.

It's easy to imagine how this could happen: parsing often involves maintaining a certain amount of state as to what you've read so far. If two threads are both trampling on the same state, you'll get problems. For example, DateFormat exposes a calendar field of type Calendar, and looking at the code of SimpleDateFormat, some methods call calendar.set(...) and others call calendar.get(...). This is clearly not thread-safe.

I haven't looked into the exact details of why DateFormat isn't thread-safe, but for me it's enough to know that it is unsafe without synchronization - the exact manners of non-safety could even change between releases.

Personally I would use the parsers from Joda Time instead, as they are thread safe - and Joda Time is a much better date and time API to start with :)