Parse JSON to Java records with fasterxml.jackson

Question

Java records can not - by design - inherit from another object (see Why Java records do not support inheritance? ). So I wonder what would be the best way to achieve the following.

Given my JSON data contains objects that have some common data + unique data. For example, type, width and height are in all shapes, but depending on the type, they can have additional fields:

{
  "name": "testDrawing",
  "shapes": [
    {
      "type": "shapeA",
      "width": 100,
      "height": 200,
      "label": "test"
    },
    {
      "type": "shapeB",
      "width": 100,
      "height": 200,
      "length": 300
    },
    {
      "type": "shapeC",
      "width": 100,
      "height": 200,
      "url": "www.test.be",
      "color": "#FF2233"
    }
  ]
}

In "traditional" Java you would do this with

BaseShape with width and height
ShapeA extends BaseShape with label
ShapeB extends BaseShape with length
ShapeC extends BaseShape with URL and color

But I'm a bit stubborn and really would like to use records.

My solution now looks like this:

• No BaseShape
• The common fields are repeated in all records

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record Drawing(
        @JsonProperty("name")
        String name,

        @JsonProperty("shapes")
        @JsonDeserialize(using = TestDeserializer.class)
        List<Object> shapes // I don't like the Objects here... 
) {
}

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeA (
        @JsonProperty("type") String type,
        @JsonProperty("width") Integer width,
        @JsonProperty("height") Integer height,
        @JsonProperty("label") String label
) {
}

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeB(
        @JsonProperty("type") String type,
        @JsonProperty("width") Integer width,
        @JsonProperty("height") Integer height,
        @JsonProperty("length") Integer length
) {
}

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeC(
        @JsonProperty("type") String type,
        @JsonProperty("width") Integer width,
        @JsonProperty("height") Integer height,
        @JsonProperty("url") String url,
        @JsonProperty("color") String color
) {
}

I don't like repeated code and it's a bad practice... But in the end I can get this loaded with this helper class:

public class TestDeserializer extends JsonDeserializer {

    ObjectMapper mapper = new ObjectMapper();

    @Override
    public List<Object> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        List<Object> rt = new ArrayList<>();

        JsonNode node = jsonParser.getCodec().readTree(jsonParser);

        if (node instanceof ArrayNode array) {
            for (Iterator<JsonNode> it = array.elements(); it.hasNext(); ) {
                JsonNode childNode = it.next();
                rt.add(getShape(childNode));
            }
        } else {
            rt.add(getShape(node));
        }

        return rt;
    }

    private Object getShape(JsonNode node) {
        var type = node.get("type").asText();
        switch (type) {
            case "shapeA":
                return mapper.convertValue(node, ShapeA.class);
            case "shapeB":
                return mapper.convertValue(node, ShapeB.class);
            case "shapeC":
                return mapper.convertValue(node, ShapeC.class);
            default:
                throw new IllegalArgumentException("Shape could not be parsed");
        }
    }
}

And this test proves to be working OK:

@Test
    void fromJsonToJson() throws IOException, JSONException {
        File f = new File(this.getClass().getResource("/test.json").getFile());
        String jsonFromFile = Files.readString(f.toPath());

        ObjectMapper mapper = new ObjectMapper();
        Drawing drawing = mapper.readValue(jsonFromFile, Drawing.class);
        String jsonFromObject = mapper.writeValueAsString(drawing);

        System.out.println("Original:\n" + jsonFromFile.replace("\n", "").replace(" ", ""));
        System.out.println("Generated:\n" + jsonFromObject);

        assertAll(
                //() -> assertEquals(jsonFromFile, jsonFromObject),
                () -> assertEquals("testDrawing", drawing.name()),
                () -> assertTrue(drawing.shapes().get(0) instanceof ShapeA),
                () -> assertTrue(drawing.shapes().get(1) instanceof ShapeB),
                () -> assertTrue(drawing.shapes().get(2) instanceof ShapeC)
        );
    }

What would be the best way to achieve this with the Jackson library and Java Records?

Extra sidenote: I will also need to be able to write back to JSON in the same format as the original.

Answer 1

If you are using regular classes you can annotate the parent class to include the type name as a property.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, 
   include = JsonTypeInfo.As.PROPERTY, 
   property = "type")
public class Shape {
}

@JsonTypeName("shapeA")
public class ShapeA extends Shape {
}

---

If you need/want to use records the only option would be a custom deserializer for the Drawing type.

I think this doesnt makes any sence because you cannot store the shapes in one list (as long as you are not using List<Object> )