How to deserialize generic classes with Jackson?

Question

I have some classes like so:

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonSubTypes(
              {
                  @JsonSubTypes.Type(value = LionCage.class, name = "LION"),
                  @JsonSubTypes.Type(value = TigerCage.class, name = "TIGER"),
              }
             )
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
public abstract class Cage<ANIMAL> {

  private AnimalType type;
  private ANIMAL animal;

}

public enum AnimalType {

    LION, TIGER

}

public LionCage extends Cage<Lion>{

    public LionCage(){
        super(AnimalType.LION);
    }
}

public Lion {

 private int maneLength;

}

public class Zoo {

   private List<Cage<?>> cages = new LinkedList<Cage<?>>();

}

Now, if I try to do the following:

String json = "{cages: [{\"type\": \"LION\", \"animal\": {\"maneLength\" : 10}}]}";

ObjectMapper om = new ObjectMapper();

Cage cage = om.readValue(json, Zoo.class);

The first value in cages should be of type LionCage and it is.

The problem is that cages.get(0).getAnimal() returns a LinkedHashMap. How can I make it return the correct Animal subclass?

I am using Jackson 1.

Thanks!

Answer 1

For your List problem, this is the one time where you want to use a raw type. Declare the Cage type argument as raw, instead of parameterized with a wildcard.

public class Zoo {

    private List<Cage> cages = new LinkedList<Cage>();
    // ... getters and setters similarly
}

With the wildcard, you are telling Jackson that it doesn't matter, so it uses its default, LinkedHashMap. If you remove the wildcard and use a raw type, it will need to look for hints elsewhere. In this case, it will look at the actual Cage implementation it uses.

---

Your JSON should look like this

String json = "{\"type\": \"LION\", \"animal\": {\"maneLength\" : 10}}";

Jackson will be able to determine that the animal is a Lion.

Through the type it will know that it is a LionCage and through the class definition of

public LionCage extends Cage<Lion>{

it will know that any reference to the type variable Animal should actually be a Lion.

_{I'm on Jackson 2.}

But it works with Jackson 1 as well. Code below

import org.codehaus.jackson.annotate.JsonIgnoreProperties;
import org.codehaus.jackson.annotate.JsonSubTypes;
import org.codehaus.jackson.annotate.JsonTypeInfo;
import org.codehaus.jackson.map.ObjectMapper;

public class Example {

    public static void main(String[] args) throws Exception {
        String json = "{\"type\": \"LION\", \"animal\": {\"maneLength\" : 10}}";

        ObjectMapper om = new ObjectMapper();

        Cage cage = om.readValue(json, Cage.class);
        System.out.println(cage.getClass());
        System.out.println(cage.getAnimal());

        System.out.println(((Lion) cage.getAnimal()).getManeLength());
    }
}

class Lion {

    private int maneLength;

    public int getManeLength() {
        return maneLength;
    }

    public void setManeLength(int maneLength) {
        this.maneLength = maneLength;
    }
}

class LionCage extends Cage<Lion> {

    public LionCage() {
        super(AnimalType.LION);
    }
}

class Tiger {
}

class TigerCage extends Cage<Tiger> {

    public TigerCage() {
        super(AnimalType.TIGER);
    }
}

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonSubTypes({ @JsonSubTypes.Type(value = LionCage.class, name = "LION"),
        @JsonSubTypes.Type(value = TigerCage.class, name = "TIGER"), })
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
abstract class Cage<Animal> {

    public Cage(AnimalType type) {
        this.setType(type);
    }

    public AnimalType getType() {
        return type;
    }

    public void setType(AnimalType type) {
        this.type = type;
    }

    public Animal getAnimal() {
        return animal;
    }

    public void setAnimal(Animal animal) {
        this.animal = animal;
    }

    private AnimalType type;
    private Animal animal;
}

enum AnimalType {
    LION, TIGER;
}

prints

class com.spring.LionCage
com.spring.Lion@15d16efc
10

Answer 2

I think you should simplify your solution by removing unnecessary types and by using JsonTypeInfo.As.EXTERNAL_PROPERTY instead of JsonTypeInfo.As.PROPERTY. With JsonTypeInfo.As.EXTERNAL_PROPERTY you do not have to create Cage subclasses. And you do not have to create AnimalType at all. You POJO classes could look like this:

@JsonIgnoreProperties(ignoreUnknown = true)
class Cage<ANIMAL> {

    @JsonSubTypes({
        @JsonSubTypes.Type(value = Lion.class, name = "LION"),
        @JsonSubTypes.Type(value = Tiger.class, name = "TIGER")
    })
    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "type")
    private ANIMAL animal;

    public ANIMAL getAnimal() {
        return animal;
    }

    public void setAnimal(ANIMAL animal) {
        this.animal = animal;
    }

    @Override
    public String toString() {
        return "Cage [animal=" + animal + "]";
    }
}

class Lion {

    private int maneLength;

    public int getManeLength() {
        return maneLength;
    }

    public void setManeLength(int maneLength) {
        this.maneLength = maneLength;
    }

    @Override
    public String toString() {
        return "Lion [maneLength=" + maneLength + "]";
    }
}

class Tiger {

    private int length;

    public int getLength() {
        return length;
    }

    public void setLength(int length) {
        this.length = length;
    }

    @Override
    public String toString() {
        return "Tiger [maneLength=" + length + "]";
    }
}

Simple usage:

String jsonLion = "{\"type\": \"LION\", \"animal\": {\"maneLength\" : 10}}";
String jsonTiger = "{\"type\": \"TIGER\", \"animal\": {\"length\" : 11}}";

ObjectMapper om = new ObjectMapper();

System.out.println(om.readValue(jsonLion, Cage.class));
System.out.println(om.readValue(jsonTiger, Cage.class));

Above code prints:

Cage [animal=Lion [maneLength=10]]
Cage [animal=Tiger [maneLength=11]]

I did not test my solution with Jackson 1.X but I hope it will work for you.