在非同步静态方法中修改静态变量，是否存在线程安全的危险？

Question

I have a class with a static method modifying a static variable as follows, does this method needs to be synchonized for thread safe operations ?

public final class IdManager {

    private static int noOfIdsInReserveCurrently = 100;   
    private static final AtomicInteger allotedUserIdsCount; 

    public static int getNewId(){
         noOfIdsInReserveCurrently--;
         ....
         return allotedUserIdsCount.incrementAndGet();
    }
}

Should this method have been synchronized?

Answer 1

Well it's certainly not safe as it is. Two threads could both read the value, but decrement their local copy, then both write. Badness.

You could synchronize it (and all other aspects to the variable) - but it would be better to use AtomicInteger which is designed for exactly this sort of thing. That's fine if the only shared state you're modifying is that one value; if you're trying to modify more shared state atomically (eg some "next ID" counter as well as the number of outstanding IDs) then you would need to either thing really, really carefully about the various interleavings, or use a synchronized block instead.