Making DateFormat Threadsafe. What to use, synchronized or Thread local

Question

I want to make following code thread safe. What is the best way to achieve it?

private static final DateFormat DATE_FORMAT = DateFormat.getDateTimeInstance();

public static final String eventTypeToDateTimeString(long timestamp)
{
   return DATE_FORMAT.format(new Date(timestamp));
}

Answer 1

You can

1. Create a new DateFormat instance every time you need one.

2. Use a synchronized block, as pointed by @Giovanni Botta.

3. Use ThreadLocal:

   private static final ThreadLocal<DateFormat> THREADLOCAL_FORMAT =
       new ThreadLocal<DateFormat>() {
           @Override protected DateFormat initialValue() {
               return DateFormat.getDateTimeInstance();
           }
       };
   
   public static final String eventTypeToDateTimeString(long timestamp) {
       return THREADLOCAL_FORMAT.get().format(new Date(timestamp));
   }
   

Actually, using ThreadLocal might give you the best performance if you have a thread pool (meaning threads are reused), which most web containers do.

Reference

http://www.javacodegeeks.com/2010/07/java-best-practices-dateformat-in.html