Thread safe SimpleDateFormat

Run this test class and you will see (as is not predictable run it few times):

/**
 * Please feel free to experiment - not only wrong data but sometimes number format exceptions...
 */
public class SimpleDateTest {

 static SimpleDateFormat df = new SimpleDateFormat("dd-MMM-yyyy");
 static String testdata[] = { "01-Jan-1999", "14-Feb-2001", "31-Dec-2007" };

 /**
  * Test method for SDF.
  */
 @Test
 public void testParse() {
  Runnable r[] = new Runnable[testdata.length];
  for (int i = 0; i < r.length; i++) {
   final int i2 = i;
   r[i] = new Runnable() {
    public void run() {
     try {
      for (int j = 0; j < 1000; j++) {
       String str = testdata[i2];
       String str2 = null;
//         synchronized(df) 
       {
        Date d = df.parse(str);
        str2 = df.format(d);
       }

       Assert.assertEquals("date conversion failed after "
         + j + " iterations.", str, str2);
      }
     } catch (ParseException e) {
      throw new RuntimeException("parse failed");
     }
    }
   };
   new Thread(r[i]).start();
  }
 }
}

Possible outputs are:

Exception in thread "Thread-0" junit.framework.ComparisonFailure: date conversion failed after 0 iterations. expected:<[01-Jan-1999]> but was:<[14-Feb-2001]>
 at junit.framework.Assert.assertEquals(Assert.java:85)

Exception in thread "Thread-0" Exception in thread "Thread-1" java.lang.NumberFormatException: For input string: "19992001.E199920014E"

Exception in thread "Thread-0" java.lang.NumberFormatException: multiple points
 at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)

Exception in thread "Thread-0" java.lang.NumberFormatException: For input string: ""
 at java.lang.NumberFormatException.forInputString(Unknown Source)

And solution is synchronize usage of SimpleDateFormat, use ThreadLocal like here: ThreadSafeSimpleDateFormat:

tsd

or my last findings use improved version of ThreadLocal  that creates HashMaps of SDF inside: SafeSimpleDateFormat

public class SafeSimpleDateFormat
{
    private final String _format;
    private static final ThreadLocal<Map<String, SimpleDateFormat>> _dateFormats = new ThreadLocal<Map<String, SimpleDateFormat>>()
    {
        public Map<String, SimpleDateFormat> initialValue()
        {
            return new HashMap<String, SimpleDateFormat>();
        }
    };

    private SimpleDateFormat getDateFormat(String format)
    {
        Map<String, SimpleDateFormat> formatters = _dateFormats.get();
        SimpleDateFormat formatter = formatters.get(format);
        if (formatter == null)
        {
            formatter = new SimpleDateFormat(format);
            formatters.put(format, formatter);
        }
        return formatter;
    }

    public SafeSimpleDateFormat(String format)
    {
        _format = format;
    }

    public String format(Date date)
    {
        return getDateFormat(_format).format(date);
    }

    public String format(Object date)
    {
        return getDateFormat(_format).format(date);
    }

    public Date parse(String day) throws ParseException
    {
        return getDateFormat(_format).parse(day);
    }

    public void setTimeZone(TimeZone tz)
    {
        getDateFormat(_format).setTimeZone(tz);
    }

    public void setCalendar(Calendar cal)
    {
        getDateFormat(_format).setCalendar(cal);
    }

    public void setNumberFormat(NumberFormat format)
    {
        getDateFormat(_format).setNumberFormat(format);
    }

    public void setLenient(boolean lenient)
    {
        getDateFormat(_format).setLenient(lenient);
    }

    public void setDateFormatSymbols(DateFormatSymbols symbols)
    {
        getDateFormat(_format).setDateFormatSymbols(symbols);
    }

    public void set2DigitYearStart(Date date)
    {
        getDateFormat(_format).set2DigitYearStart(date);
    }
}