Java Date objects can mutate, even when read

Ran into this problem a couple of months ago, when we saw some strange dates in production. So I dug into the Java library sources (thank you Sun for providing those!) and found that Date objects aren’t always “normalized”. Rather, sometimes a “denormalized” value is stored which is later (lazily) normalized. The normalized value isn’t properly synchronized with regards to the Java memory model however, which means that sometimes you can get weir (and incorrect!) results.

To illustrate the problem, I’ve created a small program. It does the following:

  1. It creates a Date object and sets it to certain values
  2. Schedules multiple Runnable’s which examine the value of the object on a threadpool

Everything looks fine and dandy, right? The object isn’t changed (apparently) after being handed of to the threadpool, yet sometimes wrong answers still appear (it takes around ~30 min on my laptop for such an event). So what are the lessons here?

  • Get your API right! If the user doesn’t seem to be doing writing, don’t do writing!
  • You can still do lazy initialization (if you really want to), but be sure to make it thread-correct (volatile, synchronized, etc) or at least document it (even though nobody reads the documentation)
  • Source code FTW! I couldn’t have debugged this without source code. Ok, maybe I could (decompiling class files is not that hard), but probably I wouldn’t have bothered.
  • Finally, the solution (hack) in this particular situation is to call getTime() after setting the values, which preemptively normalizes the internal representation. Of course the proper solution would be to pass around truly immutable objects (like timestamps or value objects from Joda Time).
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