How to serialize an Instant without nanoseconds using Spring Boot with Jackson?

Question

Spring uses Jackson's InstantSerializer to write out my Instant fields like this:

now: "2021-04-07T10:51:53.043320Z"

I don't want the nanoseconds, though - just the milliseconds. I guessed that setting the application property

spring.jackson.serialization.write-date-timestamps-as-nanoseconds=false

would achieve this, but it makes no difference.

How can I tell Spring/Jackson to omit the nanoseconds when serializing Instants?

(I'm using Spring Boot 2.2.11.RELEASE)

Update

I eventually got it work, based on this answer. I had to use the deprecated JSR310Module instead of JavaTimeModule, and override the createContextual(...) method to force it to always use my serializer.

@Bean
public ObjectMapper objectMapper(Jackson2ObjectMapperBuilder builder) {
    ObjectMapper objectMapper = builder.createXmlMapper(false).build();
    JSR310Module jsr310Module = new JSR310Module();
    jsr310Module.addSerializer(Instant.class, new MyInstantSerializer());
    objectMapper.registerModule(jsr310Module);
    return objectMapper;
}

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

    @Override
    public JsonSerializer<?> createContextual(SerializerProvider prov, BeanProperty property) {
        return this;
    }
}

And this works too (based on Volodya's answer below):

@Bean
public Jackson2ObjectMapperBuilderCustomizer addCustomTimeSerialization() {
    return jacksonObjectMapperBuilder -> 
            jacksonObjectMapperBuilder.serializerByType(Instant.class, new JsonSerializer<Instant>() {

        private final DateTimeFormatter formatter = 
                new DateTimeFormatterBuilder().appendInstant(3).toFormatter();

        @Override
        public void serialize(
                Instant instant, JsonGenerator generator, SerializerProvider provider) throws IOException {
            generator.writeString(formatter.format(instant));
        }
    });
}

Answer 1

For that you could use @JsonFormat(pattern = "yyyy-MM-dd'T'HH:mm:ss[.SSS]")

Full example:

    import com.fasterxml.jackson.annotation.JsonFormat;
    import java.time.LocalDateTime;
    import java.time.ZonedDateTime;
    
    public class Message {
    
        @JsonFormat(pattern = "yyyy-MM-dd'T'HH:mm:ss[.SSS]")
        private final LocalDateTime dateTime;
    
        @JsonFormat(pattern = "yyyy-MM-dd'T'HH:mm:ss[.SSS]")
        private final ZonedDateTime zonedDateTime;
    
        public Message(ZonedDateTime zonedDateTime) {
            this(zonedDateTime.toLocalDateTime(), zonedDateTime);
        }
    
        public Message(LocalDateTime dateTime, ZonedDateTime zonedDateTime) {
            this.dateTime = dateTime;
            this.zonedDateTime = zonedDateTime;
        }
    
        public LocalDateTime getDateTime() {
            return dateTime;
        }
    
        public ZonedDateTime getZonedDateTime() {
            return zonedDateTime;
        }
    }

Test:

    import com.fasterxml.jackson.core.JsonProcessingException;
    import com.fasterxml.jackson.databind.ObjectMapper;
    import org.junit.jupiter.api.Test;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.boot.test.autoconfigure.json.JsonTest;
    import java.time.*;
    import java.time.temporal.ChronoUnit;
    import static org.junit.jupiter.api.Assertions.*;
    
    @JsonTest
    class MessageTest {
    
        @Autowired
        ObjectMapper mapper;
    
        @Test
        public void serializationTest() throws JsonProcessingException {
            final LocalDate date = LocalDate.of(2000, Month.JANUARY, 1);
            final LocalTime time = LocalTime.of(12, 20, 10).plus(123, ChronoUnit.MILLIS);
            final Message message = new Message(ZonedDateTime.of(date, time, ZoneId.systemDefault()));
    
            final String res = mapper.writeValueAsString(message);
    
            assertEquals("{\"dateTime\":\"2000-01-01T12:20:10.123\",\"zonedDateTime\":\"2000-01-01T12:20:10.123\"}", res);
        }
    
    }

Update:

If you want to configure it centrally you could:

1. Try to set the date format to your ObjectMapper as described here

mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS"));

2. Customize your mapper like described here

@SpringBootApplication
public class InstantSerializerApplication {

    public static void main(String[] args) {
        SpringApplication.run(InstantSerializerApplication.class, args);
    }

    @Bean
    public Jackson2ObjectMapperBuilderCustomizer addCustomTimeSerialization() {
        return jacksonObjectMapperBuilder -> jacksonObjectMapperBuilder.serializerByType(ZonedDateTime.class, new JsonSerializer<ZonedDateTime>() {
            @Override
            public void serialize(ZonedDateTime zonedDateTime, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
                final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS");
                jsonGenerator.writeString(formatter.format(zonedDateTime));
            }
        });
    }
}