Confusion about thread safety - SimpleDateFormat example

Question

I have a question about thread safety. From what I have been told, SimpleDateFormat is not thread safe. I was wondering what effects it would have if I use it the following way in my spring controller:

private final static SimpleDateFormat dateFormat = new SimpleDateFormat("EEE MMM dd yyyy", Locale.US);

Later in my of my controller functions I use it as follows:

  try {
        changedate = changedate.substring(0, 15);                                                
        calcDate = dateFormat.parse(changedate);
    } catch (ParseException e2) {
        logger.error("Date Parsing Problem", e2); 
    }

calcDate then gets added to my model object and a ModelAndView is returned.

So what kind of problems will I see using it this way? Would simply removing the static keyword fix any issues because then each thread will use its own instance of dateFormat? Any clarity on this subejct in regards to thread safety would be much appreciated.

Thanks

Answer 1

SimpleDateFormat.parse() uses an instance variable called calendar to build the date from the string. If two threads try to parse at the same time, the calendar variable will get clobbered and you'll get wrong results.

Making the variable not static won't necessarily help, since two threads could still be using the same controller. A better solution is to either create a new DateFormat object each time you parse a date, or use thread local storage. Better still, use JodaTime which has thread safe parsers.

Answer 2

So what kind of problems will I see using it this way?

Developers of SimpleDateFormat took a very strange decision - during work of parse() they store partially parsed date in a field of SimpleDateFormat. Obviously, it means that you can't call parse() from several threads simultaneously.

Would simply removing the static keyword fix any issues because then each thread will use its own instance of dateFormat?

Removing static won't help you, since Spring controllers are singleton scoped by default, so Spring uses a single instance of your controller to serve all requests.

Answer 3

Personally, I would avoid all these issues by just using JodaTime. The API is much richer, it doesn't have the threading issues, and it's a lot faster.

Answer 4

SimpleDateFormat has instance-wide state while parsing and is therefore not thread safe. If you use it from multiple threads it will crash (well like java crashes :-), no process crash and the like). Removing static keyword doesn't necessarily cure the problem because it depends on the instance and it still might be used from several threads.

You can create a local instance within the method above so that each parsing happens with its own formatter or play with threadlocal variables.