How to unit-test Jackson JsonSerializer and JsonDeserializer

Question

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)

Answer 1

JsonSerializer

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

@Test 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);     jsonGenerator.flush();     assertThat(jsonWriter.toString(), is(equalTo("\"2000-01-01T00:00:00\""))); } 

JsonDeserializer

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

    private ObjectMapper mapper;     private CustomerNumberDeserialiser deserializer;      @Before     public void setup() {         mapper = new ObjectMapper();         deserializer = new CustomerNumberDeserialiser();     }      @Test     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();         parser.nextToken();         parser.nextToken();         parser.nextToken();         return deserializer.deserialize(parser, ctxt);     } 

UPDATE

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()); doReturn(true).when(ctxt).isEnabled(any(MapperFeature.class)); 

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