Java Instant to LocalDateTime trailing zero

Question

I converted an Instant to LocalDateTime in Java with Spring Boot as seen below

LocalDateTime.ofInstant(timeInUtc, zoneId);

In my test I got a Regex to check whether my resource returns a Json with a LocalDateTime. The Regex expects a JSON value in the format:

 2018-11-15T08:38:49.382

But it looks like the trailing zero is removed, meaning instead of

2018-11-15T08:38:49.380

which would comply to the regex, I get

 2018-11-15T08:38:49.38

How can I make sure that the trailing zero is not removed?

Answer 1

Formatting the date would help to retain the trailing zero

DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS")

Output looks like below:

2018-11-15T08:03:45.580

The code below:

public class Post2 {

    public static void main(String[] args) {

        String date = LocalDateTime.ofInstant(Instant.now(), ZoneId.of("UTC"))
           .format(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS"));
        System.out.println(date);

    }
}

EDIT Adding regex matching to match date time with and without milli seconds.

        String regex = "^\\d\\d\\d\\d-(0?[1-9]|1[0-2])-(0?[1-9]|[12][0-9]|3[01]) (00|[0-9]|1[0-9]|2[0-3]):([0-9]|[0-5][0-9]):([0-9]|[0-5][0-9])(\\.{0,1}[0-9]{1,3})$";

        String str1 = "2015-1-11 13:57:24";

        String str2 = "2015-1-11 13:57:24.0";
        String str3 = "2015-1-11 13:57:24.00";
        String str4 = "2015-1-11 13:57:24.000";
        String str5 = "2015-1-11 13:57:24.1";
        String str6 = "2015-1-11 13:57:24.12";
        String str7 = "2015-1-11 13:57:24.1222";
        String str8 = "2015-1-11 13:57:24.02";

        System.out.println( str1.matches(regex));
        System.out.println(str2.matches(regex));
        System.out.println(str3.matches(regex));
        System.out.println(str4.matches(regex));
        System.out.println(str5.matches(regex));
        System.out.println(str6.matches(regex));
        System.out.println(str7.matches(regex));
        System.out.println(str8.matches(regex));

output:

true
true
true
true
true
true
false
true