public FastDateFormat(DateFormat df) 
{
       
        this.df = df;
}

 public StringBuffer format(Date date,
    StringBuffer toAppendTo,
    FieldPosition fieldPosition) 
{
        long dt = date.getTime();
        long ds = dt / 1000;
        if (ds != lastSec) {
            sb.setLength(0);
            df.format(date, sb, fp);
            lastSec = ds;
        } else {
            // munge current msec into existing string
            int ms = (int)(dt % 1000);
            int pos = fp.getEndIndex();
            int begin = fp.getBeginIndex();
            if (pos  > 0) {
                if (pos  > begin)
                    sb.setCharAt(--pos, Character.forDigit(ms % 10, 10));
                ms /= 10;
                if (pos  > begin)
                    sb.setCharAt(--pos, Character.forDigit(ms % 10, 10));
                ms /= 10;
                if (pos  > begin)
                    sb.setCharAt(--pos, Character.forDigit(ms % 10, 10));
            }
        }
        toAppendTo.append(sb.toString());
        return toAppendTo;
}

Note: breaks functionality of fieldPosition param. Also: there's a bug in SimpleDateFormat with "S" and "SS", use "SSS" instead if you want a msec field.

 public static  void main(String[] args) 
{
        String format = "yyyy-MM-dd HH:mm:ss.SSS";
        if (args.length  > 0)
            format = args[0];
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        FastDateFormat fdf = new FastDateFormat(sdf);
        Date d = new Date();
        d.setTime(1); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(20); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(500); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(543); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(999); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(1050); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(2543); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(12345); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        d.setTime(12340); System.out.println(fdf.format(d) + "\t" + sdf.format(d));
        final int reps = 100000;
        {
            long start = System.currentTimeMillis();
            for (int i = 0; i <  reps; i++) {
                d.setTime(System.currentTimeMillis());
                fdf.format(d);
            }
            long elap = System.currentTimeMillis() - start;
            System.out.println("fast: " + elap + " elapsed");
            System.out.println(fdf.format(d));
        }
        {
            long start = System.currentTimeMillis();
            for (int i = 0; i <  reps; i++) {
                d.setTime(System.currentTimeMillis());
                sdf.format(d);
            }
            long elap = System.currentTimeMillis() - start;
            System.out.println("slow: " + elap + " elapsed");
            System.out.println(sdf.format(d));
        }
}

 public Date parse(String text,
    ParsePosition pos) 
{
        return df.parse(text, pos);
}