Java 8: How to create DateTimeFormatter with milli, micro or nano seconds?

Question

I need to create the formatter for parsing timestamps with optional milli, micro or nano fractions of second.

For example, for my needs I see the following opportunity:

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
                                   .append(DateTimeFormatter.BASIC_ISO_DATE)
                                   .appendLiteral('-')
                                   .append(DateTimeFormatter.ISO_LOCAL_TIME)
                                   .appendOffset("+HH:mm", "Z")
                                   .toFormatter();

Or it is also possible to use appendFraction(field, minWidth, maxWidth, decimalPoint).

However in these cases will it be possible to parse timestamps with any number of decimals (up to 9 or maxWidth). How to achieve that we can parse (optionally) only 3, 6 or 9 numbers after the comma?

It should be possible to parse the following time parts:

• HH:mm:ss.SSS
• HH:mm:ss.SSSSSS
• HH:mm:ss.SSSSSSSSS

But impossible to parse: HH:mm:ss.SSSS.

Solution

DateTimeFormatter only supports width ranges, so this wouldn't be possible with a single instance. You could make 3 separate formatters using .appendFraction(NANO_OF_SECOND, #, #, true) where # is 3, 6, or 9. Then try them in sequence, ignoring any DateTimeParseException until the last one:

private static TemporalAccessor parse(String text) {
    try {
        return formatter3.parse(text);
    } catch (DateTimeParseException e) {
        // ignore
    }
    try {
        return formatter6.parse(text);
    } catch (DateTimeParseException e) {
        // ignore
    }
    return formatter9.parse(text); // let this one throw
}

Another option would be checking the input with a regular expression first, something like text.matches("[^.]+(.+\\.(\\d{3}|\\d{6}|\\d{9})\\b.*)?").