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

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