Immutable/polymorphic POJO <-> JSON serialization with Jackson

Question

I'm trying to serialize a immutable POJO to and from JSON, using Jackson 2.1.4, without having to write a custom serializer and with as few annotations as possible. I also like to avoid having to add unnecessary getters or default constructors just to satisfy the Jackson library.

I'm now stuck on the exception:

JsonMappingException: No suitable constructor found for type [simple type, class Circle]: can not instantiate from JSON object (need to add/enable type information?)

The code:

public abstract class Shape {}


public class Circle extends Shape {
  public final int radius; // Immutable - no getter needed

  public Circle(int radius) {
    this.radius = radius;
  }
}


public class Rectangle extends Shape {
  public final int w; // Immutable - no getter needed
  public final int h; // Immutable - no getter needed

  public Rectangle(int w, int h) {
    this.w = w;
    this.h = h;
  }
}

The test code:

ObjectMapper mapper = new ObjectMapper();
mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.PROPERTY); // Adds type info

Shape circle = new Circle(10);
Shape rectangle = new Rectangle(20, 30);

String jsonCircle = mapper.writeValueAsString(circle);
String jsonRectangle = mapper.writeValueAsString(rectangle);

System.out.println(jsonCircle); // {"@class":"Circle","radius":123}
System.out.println(jsonRectangle); // {"@class":"Rectangle","w":20,"h":30}

// Throws:
//  JsonMappingException: No suitable constructor found.
//  Can not instantiate from JSON object (need to add/enable type information?)
Shape newCircle = mapper.readValue(jsonCircle, Shape.class);
Shape newRectangle = mapper.readValue(jsonRectangle, Shape.class);

System.out.println("newCircle = " + newCircle);
System.out.println("newRectangle = " + newRectangle);

Any help is greatly appreciated, thanks!

Answer 1

You could (according to the API) annotate the constructor with @JsonCreator and the parameters with @JsonProperty.

public class Circle extends Shape {
    public final int radius; // Immutable - no getter needed

    @JsonCreator
    public Circle(@JsonProperty("radius") int radius) {
        this.radius = radius;
    }
}

public class Rectangle extends Shape {
    public final int w; // Immutable - no getter needed
    public final int h; // Immutable - no getter needed

    @JsonCreator        
    public Rectangle(@JsonProperty("w") int w, @JsonProperty("h") int h) {
        this.w = w;
        this.h = h;
    }
}

Edit: Maybe you have to annotate the Shape class with @JsonSubTypes so that the concrete subclass of Shape could be determined.

@JsonSubTypes({@JsonSubTypes.Type(Circle.class), @JsonSubTypes.Type(Rectangle.class)})
public abstract class Shape {}

Answer 2

Have a look at Genson library some of its key features are adressing your exact problem: polymorphism, not requiring annotations and most important immutable pojos. Everything works in your example with 0 annotations or heavy conf.

Genson genson = new Genson.Builder().setWithClassMetadata(true)
                            .setWithDebugInfoPropertyNameResolver(true)
                            .create();

String jsonCircle = genson.serialize(circle);
String jsonRectangle = genson.serialize(rectangle);

System.out.println(jsonCircle); // {"@class":"your.package.Circle","radius":123}
System.out.println(jsonRectangle); // {"@class":"your.package.Rectangle","w":20,"h":30}

// Throws nothing :)
Shape newCircle = genson.deserialize(jsonCircle, Shape.class);
Shape newRectangle = genson.deserialize(jsonRectangle, Shape.class);

Genson gives you also the ability to use aliases (used instead classes names).

new Genson.Builder().addAlias("shape", Shape.class)
                .addAlias("circle", Circle.class)
                .create();