Jackson Objectmapper is not initialized by Spring after configured as bean when I try to de serialize abstarct class

Question

I am using Jackson xml version 2.9.9 in spring boot application.

I have a controller that accepts Vehicle which is an abstract class that has 2 child classes. When I pass JSON data from postman I am not able deserialize Vehicle since object mapper which is auto wired is always null(I configured objectmapper as a bean since I am setting deserialiers as simple modules to mapper). Please look at the example code in https://github.com/vin0010/Jackson-Mapper-Issue

Controller

@RequestMapping(path = "/jackson", method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE)
    public void forSingleObject(@RequestBody MyModel myModel){
        System.out.println("Deserialization successful");
    }

MyModel.java

public class MyModel {
    private String name;
    private Vehicle value;
}

Vehicle.java

public abstract class Vehicle {
    private String vehicleName;
}

Car.java

public class Car extends Vehicle{
    private String carType;
    private String carDriverName;
}

Auto.java

public class Auto extends Vehicle {
    private String autoType;
    private String autoDriverName;
}

ObjectMapper bean configuration

@Configuration
public class MapperConfig {
    @Bean
    public ObjectMapper objectMapper(){
        ObjectMapper objectMapper = new ObjectMapper();//Jackson2ObjectMapperBuilder.json().build();
        List<Module> modules = new ArrayList<>();
        modules.add(new SimpleModule().addDeserializer(Vehicle.class, new VehicleDeserializer()));
        objectMapper.registerModules(modules);
        return objectMapper;
    }
}

VehicleDeserializer.java

public class VehicleDeserializer extends JsonDeserializer<Vehicle> {

    @Autowired
    private ObjectMapper objectMapper;
    @Override
    public Vehicle deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        JsonNode jsonNode = jsonParser.getCodec().readTree(jsonParser);
        Vehicle vehicle=null;
        if (!jsonNode.findPath("carDriverName").equals("")){
            vehicle = objectMapper.readValue(jsonParser, Car.class);
        }else if (!jsonNode.findPath("autoDriverName").equals("")){
            vehicle = objectMapper.readValue(jsonParser, Auto.class);
        }
        return vehicle;
    }
}

JSON input

{
    "name": "Bajaj",
    "value": {
        "autoDriverName": "Mr Auto Driver",
        "autoType": "commercial"
    }
}

Problem : Object mapper is not injected and its null.

I just found out that neither object mapper is initialized in VehicleDeserializer.java nor I found a way to deserialize string to Car/Auto without the help of ObjectMapper.

I tried every method in How do I obtain the Jackson ObjectMapper in use by Spring 4.1? but nothing seems to work.

I added this as a separate question because either I need a fix the injection Objectmapper or deserialize Car/Auto without using Objectmapper(still I can only have Fasterxml).

Creating a new ObjectMapper inside VehicleDeserializer.java is not helping since it always return null object for Car/Auto even if I add deserializers to it. I made this example since its just a https://stackoverflow.com/help/minimal-reproducible-example. I need jackson to complete the de serialization

Answer 1

There's no need to attempt to inject ObjectMapper in your custom deserializer. You could use:

ObjectMapper mapper = (ObjectMapper) jsonParser.getCodec();

If you don't need any method from ObjectMapper, casting is not necessary:

ObjectCodec codec = jsonParser.getCodec();

Answer 2

the reason for your issue, you are doing this:

modules.add(new SimpleModule().addDeserializer(Vehicle.class, new VehicleDeserializer()));

you are instantiating the object yourself so the mapper will be null.

Answer 3

You made some mistakes in your deserialiser:

• Because you registered your deserialiser to ObjectMapper object inside deserialiser you can get it by (ObjectMapper) parser.getCodec() where parser is JsonParser type.
• findPath method returns JsonNode not String so .equals("") always return false. You should use isMissingNode method instead.
• When you use readTree method on parser object you can not use it again because deserialisation was already done and you have a JsonNode. Since now, you should use it. You can use convertValue method to convert it to desired Class.

This is how it should be:

class VehicleDeserializer extends JsonDeserializer<Vehicle> {

    @Override
    public Vehicle deserialize(JsonParser parser, DeserializationContext deserializationContext) throws IOException {
        // cast to ObjectMapper
        ObjectMapper mapper = (ObjectMapper) parser.getCodec();

        // read JSON as tree to JsonNode object
        // since now `parser` should not be used anymore.
        JsonNode root = mapper.readTree(parser);
        Vehicle vehicle = null;
        if (!root.findPath("carDriverName").isMissingNode()) {
            vehicle = mapper.convertValue(root, Car.class);
        } else if (!root.findPath("autoDriverName").isMissingNode()) {
            vehicle = mapper.convertValue(root, Auto.class);
        }
        return vehicle;
    }
}