Force Milliseconds When Serializing Instant to ISO8601 using Jackson

Question

I have some questions related to JSON serialization using Jackson in a project where I use Spring Boot 2.0.0.M6, Spring Framework 5.0.1.RELEASE and Jackson 2.9.2.

I have configured the following Jackson-related settings in application.properties:

spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS=false

Serialization works mostly as I need. Nevertheless, I have noticed that Jackson seems to cut-off milliseconds if they are 000.

Test 1: Serialize Instant with milliseconds set to 000:

• Initialize Instant field using Instant.parse("2017-09-14T04:28:48.000Z")
• Serialize it using Jackson
• Output will be "2017-09-14T04:28:48Z"

Test 2: Serialize Instant with milliseconds set to some non-000 value:

• Initialize Instant field using Instant.parse("2017-09-14T04:28:48.100Z")
• Serialize it using Jackson
• Output will be "2017-09-14T04:28:48.100Z"

Questions:

• Is that behavior by design?
• Is there anything I can do to force serialization of 000?

Answer 1

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}

Answer 2

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.

Answer 3

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