Force Milliseconds When Serializing Instant to ISO8601 using Jackson

There appears to be a Jackson issue open for this here*. That link contains two workarounds

Workaround 1

 ObjectMapper objectMapper = new ObjectMapper();    objectMapper.registerModule(new JavaTimeModule());    SimpleModule module = new SimpleModule();    module.addSerializer(ZonedDateTime.class, new JsonSerializer<ZonedDateTime>() {        @Override        public void serialize(ZonedDateTime zonedDateTime, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {            jsonGenerator.writeString(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSZZZ").format(zonedDateTime));        }    });    objectMapper.registerModule(module);    objectMapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);    objectMapper.enable(SerializationFeature.INDENT_OUTPUT);

Workaround 2

JavaTimeModule javaTimeModule = new JavaTimeModule();javaTimeModule.addSerializer(ZonedDateTime.class,  new ZonedDateTimeSerializer(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSX")));ObjectMapper mapper = new ObjectMapper().registerModule(javaTimeModule);

^{*Link is dead because they deprecated FasterXML/jackson-datatype-jsr310 and moved it to jackson-modules-java8. See https://github.com/FasterXML/jackson-modules-java8/issues/76}

I solve using this aproach:

ObjectMapper objectMapper = new ObjectMapper();JavaTimeModule module = new JavaTimeModule();module.addSerializer(Instant.class, new InstantSerializerWithMilliSecondPrecision());objectMapper.registerModule(module);

And for InstantSerializerWithMilliSecondPrecision i used this:

public class InstantSerializerWithMilliSecondPrecision extends InstantSerializer {    public InstantSerializerWithMilliSecondPrecision() {        super(InstantSerializer.INSTANCE, false, new DateTimeFormatterBuilder().appendInstant(3).toFormatter());    }}

Now the Instant serialization always includes milliseconds. Example: 2019-09-27T02:59:59.000Z

None of two workarounds mentioned by Sean Carroll works me. I end up with writing my own serializer for Instant.

final ObjectMapper mapper = new ObjectMapper();final JavaTimeModule javaTimeModule = new JavaTimeModule();javaTimeModule.addSerializer(Instant.class, new KeepMillisecondInstantSerializer());mapper.registerModule(javaTimeModule);public class KeepMillisecondInstantSerializer extends JsonSerializer<Instant> {    private final DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSX")            .withZone(ZoneId.of("UTC"));    @Override    public void serialize(final Instant instant, final JsonGenerator jsonGenerator, final SerializerProvider serializerProvider) throws IOException {        final String serializedInstant = dateTimeFormatter.format(instant);        jsonGenerator.writeString(serializedInstant);    }}

I guess Jackson use Instant.toString() method to serialize Instant objects by default. I also find some discussions about Instant.toString() method on StackOverflow.