Getting Started with Custom Deserialization in Jackson

1. Overview

This quick tutorial will illustrate how to use Jackson 2 to deserialize JSON using a custom Deserializer.

To dig deeper into other cool things we can do with Jackson 2, head on over to the main Jackson tutorial.

2. Standard Deserialization

Let’s start by defining two entities and see how Jackson will deserialize a JSON representation to these entities without any customization:

public class User {
    public int id;
    public String name;
}
public class Item {
    public int id;
    public String itemName;
    public User owner;
}

Now let’s define the JSON representation we want to deserialize:

{
    "id": 1,
    "itemName": "theItem",
    "owner": {
        "id": 2,
        "name": "theUser"
    }
}

And finally, let’s unmarshal this JSON to Java Entities:

Item itemWithOwner = new ObjectMapper().readValue(json, Item.class);

3. Custom Deserializer on ObjectMapper

In the previous example, the JSON representation matched the Java entities perfectly.

Next, we will simplify the JSON:

{
    "id": 1,
    "itemName": "theItem",
    "createdBy": 2
}

When unmarshalling this to the exact same entities, by default, this will of course fail:

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: 
Unrecognized field "createdBy" (class org.baeldung.jackson.dtos.Item), 
not marked as ignorable (3 known properties: "id", "owner", "itemName"])
 at [Source: java.io.StringReader@53c7a917; line: 1, column: 43] 
 (through reference chain: org.baeldung.jackson.dtos.Item["createdBy"])

We’ll solve this by doing our own deserialization with a custom Deserializer:

public class ItemDeserializer extends StdDeserializer<Item> { 

    public ItemDeserializer() { 
        this(null); 
    } 

    public ItemDeserializer(Class<?> vc) { 
        super(vc); 
    }

    @Override
    public Item deserialize(JsonParser jp, DeserializationContext ctxt) 
      throws IOException, JsonProcessingException {
        JsonNode node = jp.getCodec().readTree(jp);
        int id = (Integer) ((IntNode) node.get("id")).numberValue();
        String itemName = node.get("itemName").asText();
        int userId = (Integer) ((IntNode) node.get("createdBy")).numberValue();

        return new Item(id, itemName, new User(userId, null));
    }
}

As we can see, the deserializer is working with the standard Jackson representation of JSON — the JsonNode. Once the input JSON is represented as a JsonNode, we can now extract the relevant information from it and construct our own Item entity.

Simply put, we need to register this custom deserializer and deserialize the JSON normally:

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Item.class, new ItemDeserializer());
mapper.registerModule(module);

Item readValue = mapper.readValue(json, Item.class);

4. Custom Deserializer on the Class

Alternatively, we can also register the deserializer directly on the class:

@JsonDeserialize(using = ItemDeserializer.class)
public class Item {
    ...
}

With the deserializer defined at the class level, there is no need to register it on the ObjectMapper — a default mapper will work fine:

Item itemWithOwner = new ObjectMapper().readValue(json, Item.class);

This type of per-class configuration is very useful in situations in which we may not have direct access to the raw ObjectMapper to configure.

5. Custom Deserializer for a Generic Type

Let’s now create a Wrapper class that contains a unique argument of the generic type T:

public class Wrapper<T> {

    T value;

    public T getValue() {
        return value;
    }

    public void setValue(T value) {
        this.value = value;
    }
}

The User attribute of our Item will now be of type Wrapper<User> instead:

public class Item {
    public int id;
    public String itemName;
    public Wrapper<User> owner;
}

Let’s implement a custom deserializer for this case.

First, we need to implement the ContextualDeserializer interface to get the entity type inside the Wrapper. We’ll do this by overriding the createContextual() method. When this method is called, the context is resolved, and the actual content of the Wrapper can be retrieved via the BeanProperty argument.

We also have to extend JsonDeserializer. Thus, we can set the concrete type of the Wrapper‘s value inside deserialize():

public class WrapperDeserializer extends JsonDeserializer<Wrapper<?>> implements ContextualDeserializer {

    private JavaType type;

    public WrapperDeserializer() {
        // Default constructor
    }

    private WrapperDeserializer(JavaType type) {
        this.type = type;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) {
        JavaType wrapperType = property.getType().containedType(0);
        return new WrapperDeserializer(wrapperType);
    }

    @Override
    public Wrapper<?> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        Wrapper<?> wrapper = new Wrapper<>();
        wrapper.setValue(deserializationContext.readValue(jsonParser, type));
        return wrapper;
    }
}

This JsonDeserializer can also handle multiple Wrapper properties with different types in the same class. Suppose we have a class that contains wrappers with several types:

public class ItemWithMultipleWrappers {
    private int id;
    private String itemName;
    private Wrapper<User> owner;
    private Wrapper<Integer> count;
}

In this case, the deserialization would still work because the createContextual() method creates and returns a new instance of WrapperDeserializer with the correct type for each property instead of setting the type field directly.

Finally, we need to register our custom deserializer to be able to deserialize the JSON:

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Wrapper.class, new WrapperDeserializer());
mapper.registerModule(module);

Item readValue = mapper.readValue(json, Item.class);

6. Conclusion

This article showed how to leverage Jackson 2 to read nonstandard JSON input as well as how to map that input to any Java entity graph with full control over the mapping.