Jackson @JsonTypeInfo is making the field unable to read during the deserialization of JSON

Question

I am using Jackson to deserialize a JSON. The Jackson has to handle 2 things:

1. Identify if the provided JSON is a document and if so deserialize the elements in CustomerList one by one.
2. Identify if the provided JSON is a single Customer and if so then deserialize the customer directly.

I am able to achieve this and everything is working as expected but when I provide the CustomerList document then it's unable to read the @Context key-value pair.

Following is the JSON i am trying to deserialize:

{
  "@context": [
    "https://stackoverflow.com",
    {
      "example": "https://example.com"
    }
  ],
  "isA": "CustomerDocument",
  "customerList": [
    {
      "isA": "Customer",
      "name": "Batman",
      "age": "2008"
    }
  ]
}

Following is my Customer POJO class:

@Data
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, visible = true, property = "isA")
@JsonInclude(JsonInclude.Include.NON_NULL)
public class Customer implements BaseResponse {
    private String isA;
    private String name;
    private String age;
}


@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, visible = true, property = "isA")
@JsonSubTypes({
        @JsonSubTypes.Type(value = Customer.class, name = "Customer")})
interface BaseResponse {
}

Following is the Main:

public class JacksonMain {
    public static void main(String[] args) throws IOException {
        final InputStream jsonStream = JacksonMain.class.getClassLoader().getResourceAsStream("Customer.json");
        final JsonParser jsonParser = new JsonFactory().createParser(jsonStream);
        final ObjectMapper objectMapper = new ObjectMapper();
        jsonParser.setCodec(objectMapper);

        //Goto the start of the document
        jsonParser.nextToken();

        try {
            BaseResponse baseResponse = objectMapper.readValue(jsonParser, BaseResponse.class);
            System.out.println("SINGLE EVENT INPUT" + baseResponse.toString());
        } catch (Exception e) {
            System.out.println("LIST OF CUSTOMER INPUT");
            //Go until the customerList has been reached
            while (!jsonParser.getText().equals("customerList")) {
                System.out.println("Current Token Name : " + jsonParser.getCurrentName());
                if (jsonParser.getCurrentName() != null && jsonParser.getCurrentName().equalsIgnoreCase("@context")) {
                    System.out.println("WITHIN CONTEXT");
                }
                jsonParser.nextToken();
            }
            jsonParser.nextToken();

            //Loop through each object within the customerList and deserilize them
            while (jsonParser.nextToken() != JsonToken.END_ARRAY) {
                final JsonNode customerNode = jsonParser.readValueAsTree();
                final String eventType = customerNode.get("isA").asText();
                Object event = objectMapper.treeToValue(customerNode, BaseResponse.class);
                System.out.println(event.toString());
            }
        }
    }
}

When I run the application I get the following response:

LIST OF CUSTOMER INPUT
Current Token Name : isA
Customer(isA=Customer, name=Batman, age=2008)

As we can see it's printing only Current Token Name: isA I would expect it to print isA and @Context because it's present before the isA.

Now if I remove the following code I have on the interface BaseResponse:

@JsonTypeInfo( use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "isA")

Then I am able to read the @context but that would result in following error:

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `stackover.BaseResponse` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

I tried various things such as adding the following line:

@JsonTypeInfo( use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "@context")

But nothing seems to work as expected for me. Can someone please suggest some work-around on how to make it work?

I tried to create a copy of the Jackson JsonParser but even that does not seem to work for me.

Please Note:

1. The CustomerList can have a lot of Customers hence I do not want to store the whole CustomerList into some List as it can take a lot of memories. Hence, I am using JsonParser so I can read one JsonToken at a time.

2. Also, I do not want to create a CustomerList class rather than that I want to read one Customer at a time and deserialize it.

Answer 1

If I'm not mistaken you have only need Customer data. In my previous answer in here (Not the updated one) I thought you want de-serialize into two different classes. That's why I have added BaseResponse, Customer and CustomerDocument. Since that's not the case you don't want it anymore.

I would suggest you to create a Jackson JsonNode from the JSON response you receive. So you can decide how to travers the tree. If you do that you no longer need BaseResponse or any of these Jackson related annotations.

Your Customer class,

class Customer {
    private String isA;
    private String name;
    private String age;
    
    // Getters & Setters
}

You can use this approach to de-serialize.

Element @context cannot de-serialize directly. Because array not hold same type of elements within (one element a string other is an object).

String s = "{\"@context\":[\"https://stackoverflow.com\",{\"example\":\"https://example.com\"}],\"isA\":\"CustomerDocument\",\"customerList\":[{\"isA\":\"Customer\",\"name\":\"Batman\",\"age\":\"2008\"}]}";
//        String s = "{\"isA\":\"Customer\",\"name\":\"Superman\",\"age\":\"2013\"}";

ObjectMapper om = new ObjectMapper();
JsonNode tree = om.readTree(s);
String type = tree.get("isA").asText();

if (type.equals("Customer")) {
    Customer c = om.readValue(s, Customer.class);
    System.out.println(c);
} else if (type.equals("CustomerDocument")) {
    JsonNode customerListNode = tree.path("customerList");
    JsonNode contextNode = tree.path("@context");
    List<Customer> cl = om.convertValue(customerListNode, new TypeReference<List<Customer>>() {});
    cl.forEach(System.out::println);

    if (contextNode instanceof ArrayNode) {
        ArrayNode contextNodeArray = (ArrayNode) contextNode;
        for (JsonNode node : contextNodeArray) {
            if (node instanceof TextNode) {
                System.out.println(node.asText());
            } else if (node instanceof ObjectNode) {
                System.out.println(node.path("example").asText());
            }
        }
    }
}

Output,

Customer(isA=Customer, name=Batman, age=2008)
https://stackoverflow.com
https://example.com

Update

ObjectMapper om = new ObjectMapper();
JsonNode tree = om.readTree(s);
String type = tree.get("isA").asText();

if (type.equals("Customer")) {
    Customer c = om.readValue(s, Customer.class);
    System.out.println(c);
} else if (type.equals("CustomerDocument")) {
    JsonNode customerListNode = tree.path("customerList");
//    List<Customer> cl = om.convertValue(customerListNode, new TypeReference<List<Customer>>() {});
//    cl.forEach(System.out::println);
    ArrayNode customerArrayNode = (ArrayNode) customerListNode;
    for (JsonNode node : customerArrayNode) {
        Customer customer = om.convertValue(node, Customer.class);
        System.out.println(customer);
    }

    JsonNode contextNode = tree.path("@context");
    if (contextNode instanceof ArrayNode) {
        ArrayNode contextNodeArray = (ArrayNode) contextNode;
        for (JsonNode node : contextNodeArray) {
            if (node instanceof TextNode) {
                System.out.println(node.asText());
            } else if (node instanceof ObjectNode) {
                System.out.println(node.path("example").asText());
            }
        }
    }
}