Illustrates two problems with Jackson :jackson-datatype-jsr310 v2.6.2 when dealing with java.time.Instant.

jackson-jsr-310-instant-issues.java

package com.bakins_bits;

import static net.javacrumbs.jsonunit.fluent.JsonFluentAssert.assertThatJson;
import static org.assertj.core.api.StrictAssertions.assertThat;
import static org.assertj.core.api.StrictAssertions.fail;

import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.time.Instant;
import java.time.ZoneOffset;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

import org.testng.annotations.Test;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;

public class TestDateFormat
{
    public class HasInstantWithFormat
    {
        private Instant a;

        @JsonFormat(pattern = "yyyy-MM-dd'T'HH:mm:ss.SSSXXX", timezone = "Z")
        public Instant getA() {
            return a;
        }

        public void setA(Instant a) {
            this.a = a;
        }
    }

    @Test(enabled = true)
    public void serialize_an_instant_via_JsonFormat_annotation() {
        // ARRANGE
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new JavaTimeModule());

        HasInstantWithFormat sut = new HasInstantWithFormat();
        sut.setA(Instant.parse("2015-07-23T01:02:03.123Z"));

        // ACT
        String actual = null;
        try {
            actual = mapper.writeValueAsString(sut);
        }
        catch (IOException e) {
            fail("failed to serialize", e);
            // Possibly related to missing a timezone on the datetimeformatter?
            // See http://stackoverflow.com/a/27483371/751579
        }

        // ASSERT
        String expected = "{\"a\":\"2015-07-23T01:02:03.123Z\"}";
        assertThatJson(actual).isEqualTo(expected);
    }

    public class HasInstantWithoutFormat
    {
        private Instant a;

        public Instant getA() {
            return a;
        }

        public void setA(Instant a) {
            this.a = a;
        }
    }

    @Test(enabled = true)
    public void serialize_an_instant_via_configured_mapper() {
        // ARRANGE
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new JavaTimeModule());
        SimpleDateFormat formatter = (SimpleDateFormat) DateFormat.getDateTimeInstance(DateFormat.SHORT,
            DateFormat.SHORT, Locale.US);
        formatter.applyPattern("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");
        formatter.setTimeZone(TimeZone.getTimeZone(ZoneOffset.UTC));
        mapper.setDateFormat(formatter).setTimeZone(TimeZone.getTimeZone(ZoneOffset.UTC));

        HasInstantWithoutFormat sut1 = new HasInstantWithoutFormat();
        HasInstantWithoutFormat sut2 = new HasInstantWithoutFormat();
        HasInstantWithoutFormat sut3 = new HasInstantWithoutFormat();
        sut1.setA(Instant.parse("2015-07-23T01:02:03.123Z"));
        sut2.setA(Instant.parse("2015-07-23T01:02:03.5Z"));
        sut3.setA(Instant.parse("2015-07-23T01:02:03.000Z"));

        // ACT
        String actual1 = null;
        String actual2 = null;
        String actual3 = null;
        try {
            actual1 = mapper.writeValueAsString(sut1);
            actual2 = mapper.writeValueAsString(sut2);
            actual3 = mapper.writeValueAsString(sut3);
        }
        catch (IOException e) {
            fail("failed to serialize one or another", e);
        }

        // ASSERT
        String expected1 = "{\"a\":\"2015-07-23T01:02:03.123Z\"}";
        String expected2 = "{\"a\":\"2015-07-23T01:02:03.500Z\"}";
        String expected3 = "{\"a\":\"2015-07-23T01:02:03.000Z\"}";
        assertThatJson(actual1).isEqualTo(expected1);
        assertThatJson(actual2).isEqualTo(expected2); // OK, formats 500 ms as .500

        // The formatter formats 0ms as .000 but Jackson doesn't
        assertThat(formatter.format(Date.from(sut3.getA()))).isEqualTo("2015-07-23T01:02:03.000Z");
        assertThatJson(actual3).isEqualTo(expected3);
    }
}