Why aren't milliseconds excluded from JSON when using dateFormat in Jackson's ObjectMapper with Java 8 dates?

Question

I am trying to figure out why Jackson (2.9.5) formats dates from Java 8 incorrectly.

data class Test(
        val zonedDateTim: ZonedDateTime = ZonedDateTime.now(),
        val offsetDateTim: OffsetDateTime = OffsetDateTime.now(),
        val date: Date = Date(),
        val localDateTime: LocalDateTime = LocalDateTime.now()
)

val mapper = ObjectMapper().apply {
    registerModule(KotlinModule())
    registerModule(JavaTimeModule())
    dateFormat = SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss")
    enable(SerializationFeature.INDENT_OUTPUT)
}

println(mapper.writeValueAsString(Test()))

From the date format I provided I would expect to get dates formatted without milliseconds but instead the result looks like this:

{
  "zonedDateTim" : "2018-07-27T13:18:26.452+02:00",
  "offsetDateTim" : "2018-07-27T13:18:26.452+02:00",
  "date" : "2018-07-27T13:18:26",
  "localDateTime" : "2018-07-27T13:18:26.452"
}

Why are milliseconds being included in the formatted dates?

Answer 1

The Answer by assylias is correct. Here are some further thoughts.

Truncate

If you really do not want the fractional second at all, truncate to whole seconds.

Instant instant = Instant.now().truncatedTo( ChronoUnit.SECONDS ) ;
OffsetDateTime odt = OffsetDateTime.now().truncatedTo( ChronoUnit.SECONDS ) ;
ZonedDateTime zdt = ZonedDateTime.now().truncatedTo( ChronoUnit.SECONDS ) ;

The formatters used by the various toString methods by default omit any text representing the fractional second if the value is zero.

So the value of:

2018-07-27T13:18:26.452+02:00

…becomes:

2018-07-27T13:18:26.000+02:00

…and its String representation will be generated as seconds without a fraction:

2018-07-27T13:18:26+02:00

Try it.

OffsetDateTime odt = OffsetDateTime.parse( "2018-07-27T13:18:26.452+02:00" ) ;
OffsetDateTime odtTrunc = odt.truncatedTo( ChronoUnit.SECONDS ) ;

System.out.println( "odt.toString(): " + odt ) ;
System.out.println( "odtTrunc.toString(): " + odtTrunc ) ;

Try that code live at IdeOne.com.

odt.toString(): 2018-07-27T13:18:26.452+02:00

odtTrunc.toString(): 2018-07-27T13:18:26+02:00

Avoid legacy classes

The code in your Question confuses me. Do not mix the troublesome old legacy date-time classes such as Date & SimpleDateFormat with the modern java.time classes. The legacy classes are entirely supplanted by the modern ones.

Timeline

Be clear that LocalDateTime serves a very different purpose than Instant, OffsetDateTime, and ZonedDateTime. A LocalDateTime object purposely lacks any concept of time zone or offset-from-UTC. So it cannot represent a moment, is not a point on the timeline.

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.