Java: Synchronization issue with NumberFormat?

Question

I use java.text.NumberFormat simply to convert numbers into more readable Strings, with commas separating thousands, etc. Basically I define it as:

public static NumberFormat nf = NumberFormat.getInstance(Locale.US); 

...and then I just call nf.format(some_number) in any thread where I want to make a readable version of a number. But looking at the JavaDoc, it says: "Number formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally."

If I am only using the format(number) method of the NumberFormat object, could there ever be a synchronization issue? I tried using NumberFormat.getInstance(Locale.US).format(number) instead, but there is overhead associated with doing that every time that I feel is probably not really needed. Does this really need external synchronization? Or is there a simpler, efficient way of accomplishing the same thing without NumberFormat?

Thanks!

Answer 1

Even if format is the only method you ever call, it's still not thread-safe. In fact, we've had bugs at work due to this. We usually either create NumberFormat objects on the fly, or use a ThreadLocal as Gerco suggested. If you wanted to get fancy, you could subclass NumberFormat and in the format method, either synchronize before calling format on a delegate NumberFormat or use a ThreadLocal to retrieve a delegate.

However, I believe the most straightforward way, especially if you're going to format/parse several numbers in a row, is to use a ThreadLocal manually.

Answer 2

Use a ThreadLocal<NumberFormat>. That way each thread will have it's own private NumberFormat instance and there will be no need to synchronise and only minimal overhead.