Question

I’d like to use a Calendar for some static methods and use a static field:

private static Calendar calendar = Calendar.getInstance();

Now I read java.util.Calendar isn’t thread safe. How can I make this thread safe (it should be static)?

Answer 1

You can’t make something thread-safe if it isn’t. In the case of Calendar, even reading data from it isn’t thread-safe, as it can update internal data structures.

If at all possible, I’d suggest using Joda Time instead:

• Most of the types are immutable
• The immutable types are thread-safe
• It’s a generally much better API anyway

If you absolutely have to use a Calendar, you could create a locking object and put all the access through a lock. For example:

private static final Calendar calendar = Calendar.getInstance();
private static final Object calendarLock = new Object();

public static int getYear()
{
    synchronized(calendarLock)
    {
        return calendar.get(Calendar.YEAR);
    }
}

// Ditto for other methods

It’s pretty nasty though. You could have just one synchronized method which created a clone of the original calendar each time it was needed, of course… it’s possible that by calling computeFields or computeTime you could make subsequent read-operations thread-safe, of course, but personally I’d be loathe to try it.