Jackson - deserialize inner list of objects to list of one higher level

Question

With Spring Boot and Jackson, how can I deserialize a wrapped/inner list into a list directly in the outer level?

For example, I have:

{
    "transaction": {
    "items": {
        "item": [
            {
                "itemNumber": "193487654",
                "itemDescription": "Widget",
                "itemPrice": "599.00",
                "itemQuantity": "1",
                "itemBrandName": "ACME",
                "itemCategory": "Electronics",
                "itemTax": "12.95"
            },
            {
                "itemNumber": "193487654",
                "itemDescription": "Widget",
                "itemPrice": "599.00",
                "itemQuantity": "1",
                "itemBrandName": "ACME",
                "itemCategory": "Electronics",
                "itemTax": "12.95"
            }
        ]
    },
    ...
    }
}

In the JSON, item is a list under items; but I want to parse it as a list named items, directly under transaction, instead of defining a DTO Items which contains a list named item.

Is this possible? How to define this DTO Item?

public class TrasactionDTO {
    private List<Item> items;
    ...
}

public class Item {

}

This question is similar but does not solve the problem. Deserialize wrapped list using Jackson

Answer 1

We need to implement custom deserialiser. Because we want to skip one inner field our implementation should:

1. { - skip start object
2. "any_field_name" - skip any field name. We assume that we have only one inner field.
3. [{}, ..., {}] - use default deserialiser for List.
4. } - skip end object

Using above concept implementation should be easy:

public class InnerListDeserializer extends JsonDeserializer<List> implements ContextualDeserializer {

    private final JavaType propertyType;

    public InnerListDeserializer() {
        this(null);
    }

    public InnerListDeserializer(JavaType propertyType) {
        this.propertyType = propertyType;
    }

    @Override
    public List deserialize(JsonParser p, DeserializationContext context) throws IOException {
        p.nextToken(); // SKIP START_OBJECT
        p.nextToken(); // SKIP any FIELD_NAME

        List list = context.readValue(p, propertyType);

        p.nextToken(); // SKIP END_OBJECT

        return list;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext context, BeanProperty property) {
        return new InnerListDeserializer(property.getType());
    }
}

Let's assume we have JSON payload like this:

{
  "transaction": {
    "items": {
      "item": [
        {
          "itemNumber": "193487654",
          "itemDescription": "Widget",
          "itemPrice": "599.00",
          "itemQuantity": "1",
          "itemBrandName": "ACME",
          "itemCategory": "Electronics",
          "itemTax": "12.95"
        },
        {
          "itemNumber": "193487654",
          "itemDescription": "Widget",
          "itemPrice": "599.00",
          "itemQuantity": "1",
          "itemBrandName": "ACME",
          "itemCategory": "Electronics",
          "itemTax": "12.95"
        }
      ]
    },
    "name": "Pickle Rick"
  }
}

Above JSON we can map to below POJO classes:

@JsonRootName("transaction")
public class Transaction {

    private String name;
    private List<Item> items;

    @JsonDeserialize(using = InnerListDeserializer.class)
    public List<Item> getItems() {
        return items;
    }

    // getters, setters, toString
}

public class Item {

    private String itemNumber;

    // getters, setters, toString
}

To show it works for many different models let's introduce one more JSON payload:

{
  "product": {
    "products": {
      "innerArray": [
        {
          "id": "1234"
        }
      ]
    }
  }
}

and two more POJO classes:

@JsonRootName("product")
class Product {

    private List<ProductItem> products;

    @JsonDeserialize(using = InnerListDeserializer.class)
    public List<ProductItem> getProducts() {
        return products;
    }

    // getters, setters, toString
}

class ProductItem {

    private String id;

    // getters, setters, toString
}

Now we can test our solution:

import com.fasterxml.jackson.annotation.JsonRootName;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;

import java.io.File;
import java.io.IOException;
import java.util.List;

public class JSoupTest {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

        File jsonFile = new File("Path to 1-st JSON").getAbsoluteFile();
        File jsonFile1 = new File("Path to 2-nd JSON").getAbsoluteFile();

        System.out.println(mapper.readValue(jsonFile, Transaction.class));
        System.out.println(mapper.readValue(jsonFile1, Product.class));
    }
}

Above example prints:

Transaction{items=[Item{itemNumber=193487654}, Item{itemNumber=193487654}], name='Pickle Rick'}
Product{products=[ProductItem{id='1234'}]}

For more info read:

1. Custom Jackson Deserializer Getting Access to Current Field Class
2. Getting Started with Custom Deserialization in Jackson
3. Jackson Exceptions – Problems and Solutions
4. Jackson UNWRAP_ROOT_VALUE
5. Configuring ObjectMapper in Spring

Answer 2

It seems that @JsonUnwrapped is what I need.

https://www.baeldung.com/jackson-annotations

@JsonUnwrapped defines values that should be unwrapped/flattened when serialized/deserialized.

Let's see exactly how that works; we'll use the annotation to unwrap the property name:

public class UnwrappedUser {
    public int id;
 
    @JsonUnwrapped
    public Name name;
 
    public static class Name {
        public String firstName;
        public String lastName;
    }
 }

Let's now serialize an instance of this class:

@Test
public void whenSerializingUsingJsonUnwrapped_thenCorrect()
  throws JsonProcessingException, ParseException {
    UnwrappedUser.Name name = new UnwrappedUser.Name("John", "Doe");
    UnwrappedUser user = new UnwrappedUser(1, name);
 
    String result = new ObjectMapper().writeValueAsString(user);
     
    assertThat(result, containsString("John"));
    assertThat(result, not(containsString("name")));
}

Here's how the output looks like – the fields of the static nested class unwrapped along with the other field:

{
    "id":1,
    "firstName":"John",
    "lastName":"Doe"
}

So, it should be something like:

public class TrasactionDTO {
    
    private List<Item> items;
    ...
}

public static class Item {
    @JsonUnwrapped
    private InnerItem innerItem;
    ...

}

public static class InnerItem {
    private String itemNumber;
    ...
}