DecimalFormat.format(double) in different threads

Question

I have to print many formatted decimal values in many threads in parallel. To format the decimal values I use a java.text.DecimalFormat configured by a pattern. I am aware of the warning from the java doc of DecimalFormat:

Decimal 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.

But I don’t know if this warning applies to my scenario: I configure the java.text.DecimalFormat once when the application starts (and store the Formatter in a final field). After that I ONLY use the format(double) method.

The reason why I want to do this is: I don’t want to lose performance by creating a new DecimalFormat instance every time I need to print a formatted number.

I looked at the DecimalFormat.format(double) code and it looks to be thread safe, but I am not sure.

Could you please confirm that the usage of DecimalFormat.format(double) is eventually thread safe, when not changing the configuration of the formatter, or explain why it is not?

Answer 1

Just use this thread-safe snippet for NumberFormat:

static ThreadLocal<NumberFormat> numberFormat = new ThreadLocal<NumberFormat>() {     @Override     public NumberFormat initialValue() {         return new DecimalFormat("00000");     } }; 

Or in Java 8, as Jesper said in comment:

private static ThreadLocal<NumberFormat> numberFormatter =                    ThreadLocal.withInitial(() -> new DecimalFormat("00000")); 

Answer 2

While the current implementation may be eventually thread-safe, there is no such guarantee for coming implementations, or for other JREs.

Have you verified that avoiding new DecimalFormat() is a measurable performance gain in your application?