Jackson ObjectMapper

Dependencies

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.10.2</version>
</dependency>

Reading and Writing Using ObjectMapper

  • The simple readValue API of the ObjectMapper is a good entry point

  • we can use the writeValue API to serialize any Java object as JSON output

Java Object to JSON

ObjectMapper objectMapper = new ObjectMapper();
Car car = new Car("yellow", "renault");
objectMapper.writeValue(new File("target/car.json"), car);

String carAsString = objectMapper.writeValueAsString(car);

JSON to Java Object

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
Car car = objectMapper.readValue(json, Car.class);

Car car = objectMapper.readValue(new File("src/test/resources/json_car.json"), Car.class);

Car car = objectMapper.readValue(new URL("file:src/test/resources/json_car.json"), Car.class);

JSON to Jackson JsonNode

String json = "{ \"color\" : \"Black\", \"type\" : \"FIAT\" }";
JsonNode jsonNode = objectMapper.readTree(json);
String color = jsonNode.get("color").asText();
// Output: color -> Black

Creating a Java List from a JSON Array String

String jsonCarArray = "[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
List<Car> listCar = objectMapper.readValue(jsonCarArray, new TypeReference<List<Car>>(){});

Creating Java Map from JSON String

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
Map<String, Object> map 
  = objectMapper.readValue(json, new TypeReference<Map<String,Object>>(){});

Advanced Features

Configuring Serialization or Deserialization Feature

objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
Car car = objectMapper.readValue(jsonString, Car.class);
JsonNode jsonNodeRoot = objectMapper.readTree(jsonString);
JsonNode jsonNodeYear = jsonNodeRoot.get("year");
String year = jsonNodeYear.asText();

Creating Custom Serializer or Deserializer

public class CustomCarSerializer extends StdSerializer<Car> {
    public CustomCarSerializer() {
        this(null);
    }
 
    public CustomCarSerializer(Class<Car> t) {
        super(t);
    }
 
    @Override
    public void serialize(
      Car car, JsonGenerator jsonGenerator, SerializerProvider serializer) {
        jsonGenerator.writeStartObject();
        jsonGenerator.writeStringField("car_brand", car.getType());
        jsonGenerator.writeEndObject();
    }
}

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = 
  new SimpleModule("CustomCarSerializer", new Version(1, 0, 0, null, null, null));
module.addSerializer(Car.class, new CustomCarSerializer());
mapper.registerModule(module);
Car car = new Car("yellow", "renault");
String carJson = mapper.writeValueAsString(car);

public class CustomCarDeserializer extends StdDeserializer<Car> {
    public CustomCarDeserializer() {
        this(null);
    }
 
    public CustomCarDeserializer(Class<?> vc) {
        super(vc);
    }
 
    @Override
    public Car deserialize(JsonParser parser, DeserializationContext deserializer) {
        Car car = new Car();
        ObjectCodec codec = parser.getCodec();
        JsonNode node = codec.readTree(parser);
        // try catch block
        JsonNode colorNode = node.get("color");
        String color = colorNode.asText();
        car.setColor(color);
        return car;
    }
}

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
ObjectMapper mapper = new ObjectMapper();
SimpleModule module =
  new SimpleModule("CustomCarDeserializer", new Version(1, 0, 0, null, null, null));
module.addDeserializer(Car.class, new CustomCarDeserializer());
mapper.registerModule(module);
Car car = mapper.readValue(json, Car.class);

Handling Date Formats

ObjectMapper objectMapper = new ObjectMapper();
DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm a z");
objectMapper.setDateFormat(df);
String carAsString = objectMapper.writeValueAsString(request);
// output: {"car":{"color":"yellow","type":"renault"},"datePurchased":"2016-07-03 11:43 AM CEST"}

Handling Collections

String jsonCarArray = 
  "[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.USE_JAVA_ARRAY_FOR_JSON_ARRAY, true);
Car[] cars = objectMapper.readValue(jsonCarArray, Car[].class);

String jsonCarArray = 
  "[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
ObjectMapper objectMapper = new ObjectMapper();
List<Car> listCar = objectMapper.readValue(jsonCarArray, new TypeReference<List<Car>>(){});

Conclusion

Jackson is a solid and mature JSON serialization/deserialization library for Java.

Results

  • SerializationDeserializationFeatureTest

  • JavaReadWriteJsonTest

References