Serializing org.joda.time.DateTime to JSON in ISO 8601 format comes with milliseconds

// Groovy Version: 2.4.5 JVM: 1.8.0_77 Vendor: Oracle Corporation OS: Mac OS X
@Grab('com.fasterxml.jackson.core:jackson-core:2.8.5')
@Grab('com.fasterxml.jackson.datatype:jackson-datatype-joda:2.8.5')

@Grab('joda-time:joda-time:2.9.6')

import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.databind.util.ISO8601DateFormat
import com.fasterxml.jackson.datatype.joda.JodaModule
import org.joda.time.DateTime

def now = new DateTime()
def bean = new Bean(date1: now.toDate(), date2: now)

def objectMapper = new ObjectMapper()
        .registerModule(new JodaModule())
        .setDateFormat(new ISO8601DateFormat())

def json = objectMapper.writeValueAsString(bean)
println json



class Bean {
    Date date1
    DateTime date2
}

result

{
  "date1": "2016-12-23T06:33:01Z",
  "date2": "2016-12-23T06:33:01.045Z"
}

Date1, which is java.util.Date, does not have milliseconds. It seems that you need to format it properly with @ com.fasterxml.jackson.annotation.JsonFormat.

bonus

def objectMapper = new ObjectMapper()
        .registerModule(new JodaModule())
//        .setDateFormat(new ISO8601DateFormat())
        .configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false)

If you try it in the above state, date1 will also have milliseconds. The timezone specifier has also changed to + 0000 instead of Z. ..

{
  "date1": "2016-12-23T06:38:26.270+0000",
  "date2": "2016-12-23T06:38:26.270Z"
}