Jackson Serialization for subclasses

Question

In the below example, I have a primary class - A and its subclass - B. Both can be used as a property in the general class X.

public class A
{
   @JsonProperty("primary_key")
   public final String primaryKey;

   @JsonCreator
   A(@JsonProperty("primary_key") String primaryKey)
   {
        this.primaryKey = primaryKey;
   }
}
public class B extends A
{
   @JsonProperty("secondary_key")
   public final String secondaryKey;

   @JsonCreator
   B(@JsonProperty("primary_key") String primaryKey, @JsonProperty("secondary_key") String secondaryKey)
   {
        super(primaryKey);
        this.secondaryKey = secondaryKey;
   }
}

public class X
{
    @JsonProperty("keys")
    public final A keys;

    @JsonCreator
    X(@JsonProperty("keys") A keys)
    {
         this.keys = keys;
    }
}

How can I use Jackson Polymorphic feature in order to correctly deserialize the below given json into their respective classes:

JSON A :

 { "keys" :{
              "primary_key" : "abc"
          }
 }

JSON B :

 { "keys" : {
              "primary_key" : "abc",
              "secondary_key" : "xyz"
         }
    }

Expected Result: Map keys object to Class A for JSON A and Class B for JSON B.

Please suggest alternative suggestions too.

Answer 1

It feels like a pretty common problem and there is no easy annotations way to solve it (Or maybe i just cant find one):

Jackson Polymorphic Deserialization - Can you require the existence of a field instead of a specific value?

Deserializing polymorphic types with Jackson

One thing you can do is to add custom deserializer to your object mapper. Here is nice demo of this approach: https://stackoverflow.com/a/19464580/1032167

Here is demo related to your example:

import com.fasterxml.jackson.annotation.*;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.ObjectCodec;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.module.SimpleModule;

import java.io.IOException;

public class Main4 {


private static final String jsonA = "{ \"keys\" : { \"primary_key\" : \"abc\" } }";
private static final String jsonB = 
        "{ \"keys\" : { \"primary_key\" : \"abc\", \"secondary_key\" : \"xyz\" } }";

public static void main(String[] args) throws IOException {
    ObjectMapper mapper = new ObjectMapper();

    SimpleModule idAsRefModule = new SimpleModule("ID-to-ref");
    idAsRefModule.addDeserializer(A.class, new AJsonDeserializer());
    mapper.registerModule(idAsRefModule);

    X tl = mapper.readValue(jsonA, X.class);
    System.out.println(tl);

    X t2 = mapper.readValue(jsonB, X.class);
    System.out.println(t2);
}

public static class AJsonDeserializer  extends JsonDeserializer<A>{

    @Override
    public A deserialize(JsonParser jp, DeserializationContext dc)
    throws IOException {

        ObjectCodec codec = jp.getCodec();
        JsonNode node = codec.readTree(jp);

        if (node.has("secondary_key")) {
            return codec.treeToValue(node, B.class);
        }
        return new A(node.findValue("primary_key").asText());
    }
}

public static class A
{
    @JsonProperty("primary_key")
    public final String primaryKey;

    @JsonCreator
    A(@JsonProperty("primary_key") String primaryKey)
    {
        this.primaryKey = primaryKey;
    }

    @Override
    public String toString() {
        return "A{" +
                "primaryKey='" + primaryKey + '\'' +
                '}';
    }
}

public static class B extends A
{
    @JsonProperty("secondary_key")
    public final String secondaryKey;

    @JsonCreator
    B(@JsonProperty("primary_key") String primaryKey, 
      @JsonProperty("secondary_key") String secondaryKey)
    {
        super(primaryKey);
        this.secondaryKey = secondaryKey;
    }

    @Override
    public String toString() {
        return "B{" +
                "primaryKey='" + primaryKey + '\'' +
                "secondaryKey='" + secondaryKey + '\'' +
                '}';
    }
}

public static class X
{
    @JsonProperty("keys")
    public final A keys;

    @JsonCreator
    X(@JsonProperty("keys") A keys)
    {
        this.keys = keys;
    }

    @Override
    public String toString() {
        return "X{" +
                "keys=" + keys +
                '}';
    }
}
}

But you will have to create one more super class if you want to use default A deserializer or look here how you can solve this: https://stackoverflow.com/a/18405958/1032167

Answer 2

If I understoon correctly, simply passing the values will work, without any config. I believe this is what you are looking for:

public class Test {

    private static final String JSON = "{\"keys\":{\"primary_key\":\"abc\",\"secondary_key\":\"xyz\"}}";

    public static void main(String[] args) throws IOException {

        ObjectMapper mapper = new ObjectMapper();
        X x = mapper.readValue(JSON, X.class);
        System.out.println(mapper.writeValueAsString(x));

    }
}

class A {
    private String primary_key;

    public String getPrimary_key() {
        return primary_key;
    }

    public void setPrimary_key(String primary_key) {
        this.primary_key = primary_key;
    }
}

class B extends A {

    private String secondary_key;

    public String getSecondary_key() {
        return secondary_key;
    }

    public void setSecondary_key(String secondary_key) {
        this.secondary_key = secondary_key;
    }
}

class X {

    private B keys;

    public B getKeys() {
        return keys;
    }

    public void setKeys(B keys) {
        this.keys = keys;
    }
}

Output will be:

{"keys":{"primary_key":"abc","secondary_key":"xyz"}}

In case this is not what you expect, please provide another explanation and I will edit the answer as needed.