java.time ISO date format with fixed millis digits (in Java 8 and later)

Question

By default, the toString method of Instant uses the DateTimeFormatter.ISO_INSTANT formatter. That formatter won’t print the digits for fraction-of-second if they happen to be 0.

Answer 1

Just create a DateTimeFormatter that keeps three fractional digits.

DateTimeFormatter formatter = new DateTimeFormatterBuilder().appendInstant(3).toFormatter();

Then use it. For example:

System.out.println(formatter.format(Instant.now()));
System.out.println(formatter.format(Instant.now().truncatedTo(ChronoUnit.SECONDS)));

…prints (at the time I run it):

2015-10-08T21:26:16.571Z
2015-10-08T21:26:16.000Z

Excerpt of the class doc:

… The fractionalDigits parameter allows the output of the fractional second to be controlled. Specifying zero will cause no fractional digits to be output. From 1 to 9 will output an increasing number of digits, using zero right-padding if necessary. The special value -1 is used to output as many digits as necessary to avoid any trailing zeroes. …