Deserialising Abstract Classes with Jackson

Jackson can automatically detect the relevant abstract class to initialise when parsing some JSON if you give it a little bit of help. There are multiple methods Jackson can use to detect and indicate the class it should use but the method I have chosen is to tell it to look for a type field.

Consider the following json snippets

{
   "id": "123",
   "type": "name",
   "name": "Alex"
}

{
   "id": "345",
   "type": "value",
   "value": 123.4
}

Which can be represented by the following classes

public class IdName extends IdField {
   public static final String FIELD_TYPE = "name";
   private String name;

   public IdName() {
      super(FIELD_TYPE);
   }

   ...
}

public class IdValue extends IdField {
   public static final String FIELD_TYPE = "value";
   private float value;

   public IdValue() {
      super(FIELD_TYPE);
   }

   ...
}

We can then add a few annotations to the base abstract class telling Jackson to check the value of the type field and have it pick the correct concrete class automatically.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type")
@JsonSubTypes({
      @JsonSubTypes.Type(value = IdName.class, name = IdName.FIELD_TYPE),
            @JsonSubTypes.Type(value = IdValue.class, name = IdValue.FIELD_TYPE)
})
public abstract class IdField {
   private String id;
   private String type;

   public IdField(String type) {
      this.id = UUID.randomUUID().toString();
      this.type = type;
   }

   ...
}

Then we can just parse the json as we normally would.

String jsonStr = "{\"id\":\"123\",\"name\":\"Alex\"}";
ObjectMapper mapper = new ObjectMapper();

IdField field = mapper.readValue(jsonStr, IdField.class);
System.out.println(field.getType());                        // => "name"

IdName nameField = (IdName) field;
System.out.println(nameField.getName());                    // => "Alex"