Java LocalDateTime.parse with millisecond precision but optional microsecond precision

Question

Is there a way to create a LocalDateTime pattern that will parse a date/time that has at least millisecond precision but optional microsecond precision i.e. in the following

Answer 1

Hack no. 1

    String withMillis = "2019-02-14 11:04:52.784";
    String withMicros = "2019-02-14 11:04:52.784108";

    System.out.println(LocalDateTime.parse(withMillis.replace(' ', 'T')));
    System.out.println(LocalDateTime.parse(withMicros.replace(' ', 'T')));

2019-02-14T11:04:52.784
2019-02-14T11:04:52.784108

When we replace the space in the middle of your string with a T, the string conforms to ISO 8601, the standard format that LocalDateTime and the other classes of java.time parse (and also print) as their default, that is, without any explicit formatter. So this is an easy solution.

Hack no. 2

Something like what you tried can be made to work. Only you cannot split a sequence of SSSSSS with a square bracket in the middle.

static final DateTimeFormatter DATE_TIME_FORMATTER = 
        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.[SSSSSS][SSS]");

And then:

    System.out.println(LocalDateTime.parse(withMillis, DATE_TIME_FORMATTER));     
    System.out.println(LocalDateTime.parse(withMicros, DATE_TIME_FORMATTER));

I specify optionally 6 decimals and then optionally 3 decimals. We need that order. If we put [SSS][SSSSSS] and try to parse 6 decimals, the formatter will first parse 3 and then throw an exception because it cannot parse the remaining 3 with SSSSSS. It’s a bit of a hack since it will also accept a decimal point with no decimals at all, and will probably issue a very confusing error message (or possibly even give an incorrect result) if we give it 9 decimals.

The good solution: use a builder

Edit: I have improved the builder a little since the first version of this answer:

static final DateTimeFormatter DATE_TIME_FORMATTER = 
        new DateTimeFormatterBuilder().append(DateTimeFormatter.ISO_LOCAL_DATE)
                .appendLiteral(' ')
                .append(DateTimeFormatter.ISO_LOCAL_TIME)
                .toFormatter();

DateTimeFormatter.ISO_LOCAL_TIME parses from 0 to 9 decimals on the seconds, so we just reuse that formatter in our own formatter.

Original builder:

static final DateTimeFormatter DATE_TIME_FORMATTER = 
        new DateTimeFormatterBuilder().append(DateTimeFormatter.ISO_LOCAL_DATE)
                .appendLiteral(' ')
                .appendPattern("HH:mm:ss")
                .appendFraction(ChronoField.NANO_OF_SECOND, 1, 9, true)
                .toFormatter();

Here I have specified a minimum of 1 and a maximum of 9 decimals after the decimal point. You can specify 3 and 6 if you prefer. It will of course accept 4 or 5 too.

Link: Wikipedia article: ISO 8601