How to unit-test Jackson JsonSerializer and JsonDeserializer

I've written custom JsonSerializer and JsonDeserializer for my app. Now I want to write some unit-tests for them.

How should a clean test case look like?

Are there some clean examples out there?

(clean means no dependencies to other frameworks or libraries)



The example is serialising a LocalDateTime but this can replaced by the required type.

public void serialises_LocalDateTime() throws JsonProcessingException, IOException {
    Writer jsonWriter = new StringWriter();
    JsonGenerator jsonGenerator = new JsonFactory().createGenerator(jsonWriter);
    SerializerProvider serializerProvider = new ObjectMapper().getSerializerProvider();
    new LocalDateTimeJsonSerializer().serialize(LocalDateTime.of(2000, Month.JANUARY, 1, 0, 0), jsonGenerator, serializerProvider);
    assertThat(jsonWriter.toString(), is(equalTo("\"2000-01-01T00:00:00\"")));


The example is deserialising a Number but this can replaced by the required type.

    private ObjectMapper mapper;
    private CustomerNumberDeserialiser deserializer;

    public void setup() {
        mapper = new ObjectMapper();
        deserializer = new CustomerNumberDeserialiser();

    public void floating_point_string_deserialises_to_Double_value() {
        String json = String.format("{\"value\":%s}", "\"1.1\"");
        Number deserialisedNumber = deserialiseNumber(json);
        assertThat(deserialisedNumber, instanceOf(Double.class));
        assertThat(deserialisedNumber, is(equalTo(1.1d)));

    @SneakyThrows({JsonParseException.class, IOException.class})
    private Number deserialiseNumber(String json) {
        InputStream stream = new ByteArrayInputStream(json.getBytes(StandardCharsets.UTF_8));
        JsonParser parser = mapper.getFactory().createParser(stream);
        DeserializationContext ctxt = mapper.getDeserializationContext();
        return deserializer.deserialize(parser, ctxt);


When upgrading to jackson 2.9.3 I received a NullPointerException in DeserializationContext in isEnabled(MapperFeature feature) when deserialising strings to numbers because new ObjectMapper() initialises _config to null.

To get around this, I used this SO answer to spy on the final class DeserializationContext:

DeserializationContext ctxt = spy(mapper.getDeserializationContext());

I feel there must be a better way, so please comment if you have one.

A deserializer can be unit tested like this:

public class CustomFooDeserializerTest {

  private Foo foo;

  public void setUp() {
    ObjectMapper objectMapper = new ObjectMapper();
    SimpleModule module = new SimpleModule();
    module.addDeserializer(Foo.class, new CustomFooDeserializer());
    foo = objectMapper.readValue(new File("path/to/file"), Foo.class);

  public void shouldSetSomeProperty() {
    assertThat(foo.getSomeProperty(), is("valueOfSomeProperty"));


Other answers have covered serialization very well. I have one other suggestion for deserialization.

If your domain class has the @JsonDeserialize annotation, then your deserializer will be picked up by the default ObjectMapper, without the need to explicitly register it.

public class Example {
    private String name;
    private BananaStore bananaStore;

    public Example(String name, BananaStore bananaStore) { = name;
        this.bananaStore = bananaStore;

    public String getName() {
        return name;

    public BananaStore getBananaStore() {
        return bananaStore;

and your unit test can become much shorter:

public class ExampleDeserializerTest {

    public void deserialize() throws IOException {
        String json = "{\"name\":\"joe\",\"bananas\":16}";

        Example example = new ObjectMapper().readValue(json, Example.class);

        assertEquals("joe", example.getName());
        assertEquals(16, example.getBananaStore().getBananaCount());

I found a way to unit test the Deserializers, was quite some trouble to figure out. Have a look at my repo

The unit test class is here: / src / test / java / de / denktmit / rest / hal / jackson /

An integration test, for testing the Deserializer in context can be found here: / src / test / java / de / denktmit / rest / hal /

EDIT due to comment Base class to setup mapper for easy unit testing

public abstract class AbstractJackson2MarshallingTest {

    protected ObjectMapper mapper;

    public void setUp() {
        mapper = new ObjectMapper();

    protected String write(Object object) throws Exception {
        Writer writer = new StringWriter();
        mapper.writeValue(writer, object);
        return writer.toString();

    protected <T> T read(String source, Class<T> targetType) throws Exception {
        return mapper.readValue(source, targetType);

    protected String getPackagePath() {
        return "/" + this.getClass().getPackage().getName().replace('.', '/');

The test class containing the unit tests

public class RelationDeserializerTest extends AbstractJackson2MarshallingIntegrationTest {

    @Rule public ResourceFile resourceFile = new ResourceFile(getPackagePath() + "/single_valued_relation.json");
    @Rule public ResourceFile resourceFile2 = new ResourceFile(getPackagePath() + "/multi_valued_relation.json");

    private RelationDeserializer deserializer = new RelationDeserializer();

    public void testDeserializeSingleValuedRelation() throws IOException {
        resourceFile = new ResourceFile(getPackagePath() + "/single_valued_relation.json");
        JsonParser parser = mapper.getFactory().createParser(resourceFile.getContent());
        DeserializationContext ctxt = mapper.getDeserializationContext();
        SingleValuedRelation rel = (SingleValuedRelation) deserializer.deserialize(parser, ctxt);
        assertEquals(rel.getName(), "invoiceAddress");
        assertEquals("invoiceAddressURL", rel.getLink().getHref());
        assertEquals("linkName", rel.getLink().getName());
        assertEquals("de", rel.getLink().getHreflang());

    public void testDeserializeMultiValuedRelation() throws IOException {
        resourceFile = new ResourceFile(getPackagePath() + "/multi_valued_relation.json");
        JsonParser parser = mapper.getFactory().createParser(resourceFile.getContent());
        DeserializationContext ctxt = mapper.getDeserializationContext();
        MultiValuedRelation rel = (MultiValuedRelation) deserializer.deserialize(parser, ctxt);
        assertEquals(rel.getName(), "images");
        Iterator<Link> linkIterator = rel.getLinks().iterator();
        Link link =;
        assertEquals("imageUrl1", link.getHref());
        link =;
        assertEquals("imageUrl2", link.getHref());


The class under test

public class RelationDeserializer extends StdDeserializer<Relation> {

    public RelationDeserializer() {

    public Relation deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        if (p.getCurrentToken() == null && p.nextToken() == null) {
            String msg = getClass().getCanonicalName()
                    + ": Can not deserialize without token";
            throw new IOException(msg);
        if (p.getCurrentToken() != JsonToken.START_OBJECT
                && p.getCurrentToken() != JsonToken.START_ARRAY) {
            String msg = getClass().getCanonicalName()
                    + ": Expected data to start with an Relation object or an array of Relation objects";
            throw new IOException(msg);
        if (p.nextToken() != JsonToken.FIELD_NAME) {
            String msg = getClass().getCanonicalName()
                    + ": Expected relation to be started by a field name";
            throw new IOException(msg);
        String relationName = p.getText();
        JsonToken tok = p.nextToken();
        Relation rel;
        switch (tok) {
        case START_ARRAY:
            rel = createMultiValuedRelation(relationName, p);
        case START_OBJECT:
            rel = createSingleValuedRelation(relationName, p);
            String msg = getClass().getCanonicalName() + "Expected relation content is a single link or array of links";
            throw new IOException(msg);
        return rel;

    private Relation createMultiValuedRelation(String relationName, JsonParser p)
            throws JsonParseException, IOException {
        List<Link> links = new ArrayList<Link>();
        if (p.nextToken() == JsonToken.START_OBJECT) {
            Iterator<DefaultLink> linkIterator = p.readValuesAs(DefaultLink.class);
            while (linkIterator.hasNext()) {
        if (p.getCurrentToken() != JsonToken.END_ARRAY) {
            String msg = getClass().getCanonicalName() + "Expected relation content is a single link or (possibly empty) array of links";
            throw new IOException(msg);
        return RelationFactory.createRelation(relationName, links);

    private Relation createSingleValuedRelation(String relationName,
            JsonParser p) throws JsonParseException, IOException {
        return RelationFactory.createRelation(relationName, p.readValueAs(DefaultLink.class));


Hope that helps and best regards

I don't find any example but you can try to create a generator like :

StringWriter stringJson = new StringWriter();
JsonGenerator generator = new JsonFactory().createGenerator(stringJson);

and you can get a SerializerProvider instance with

new ObjectMapper().getSerializerProvider();

I started using the solution in @RobertBain's answer but it didn't work for me as the JSON parser kept losing the value somehow. I ended up including Mockito and stubbing the method on the JsonParser that my deserializer uses and wrote my tests using that. See a small example below:

public class ZoneDateTimeDeserializerTest {
    private ZoneDateTimeDeserializer deserializer;

    private JsonParser parser;

    public MockitoRule mockitoRule = MockitoJUnit.rule();

    public void setUp() {
        deserializer = new ZoneDateTimeDeserializer();

    public void givenAZonedDateTimeInIsoFormat_whenTryingToDeserializeIt_itShouldBeSuccessful() {
        final String date = "1996-01-29T22:40:54+02:00";

        ZonedDateTime dateTime = deserialiseZoneDateTime(date);

        assertThat(dateTime, instanceOf(ZonedDateTime.class));
        Assert.assertEquals(1996, dateTime.getYear());
        Assert.assertEquals(Month.JANUARY, dateTime.getMonth());
        Assert.assertEquals(29, dateTime.getDayOfMonth());
        Assert.assertEquals(20, dateTime.getHour());
        Assert.assertEquals(40, dateTime.getMinute());
        Assert.assertEquals(54, dateTime.getSecond());

    @SneakyThrows({JsonParseException.class, IOException.class})
    private ZonedDateTime deserialiseZoneDateTime(String json) {
        // Mock the parser so that it returns the value when getValueAsString is called on it
        return deserializer.deserialize(parser, null);

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