JSON consumer of polymorphic objects

Question

I am parsing JSON and am having difficulty with one structure that can have one of three forms. In my case it could be zero-dimensional, one-dimensional or two-dimensional.

Answer 1

Thanks to Eric's comment pointing me to programmerbruce I managed to crack it. Here's the code I used (cut down to simplify).

public static class Info {
  @JsonProperty("Product")
  public String product;
  // Empty in the 0d version, One entry in the 1d version, two entries in the 2d version.
  @JsonProperty("Dimensions")
  public String[] dimensions;

}

public static class Info0d extends Info {
}

public static class Info1d extends Info {
  @JsonProperty("Labels")
  public String[] labels;
}

public static class Info2d extends Info {
  @JsonProperty("Labels")
  public String[][] labels;
}

public static class InfoDeserializer extends StdDeserializer<Info> {
  public InfoDeserializer() {
    super(Info.class);
  }

  @Override
  public Info deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
    Class<? extends Info> variantInfoClass = null;
    ObjectMapper mapper = (ObjectMapper) jp.getCodec();
    ObjectNode root = (ObjectNode) mapper.readTree(jp);
    // Inspect the `diemnsions` field to decide what to expect.
    JsonNode dimensions = root.get("Dimensions");
    if ( dimensions == null ) {
      variantInfoClass = Info0d.class;
    } else {
      switch ( dimensions.size() ) {
        case 1:
          variantInfoClass = Info1d.class;
          break;

        case 2:
          variantInfoClass = Info2d.class;
          break;
      }
    }
    if (variantInfoClass == null) {
      return null;
    }
    return mapper.readValue(root, variantInfoClass);
  }
}

And to install this in the ObjectMapper:

// Register the special deserializer.
InfoDeserializer deserializer = new InfoDeserializer();
SimpleModule module = new SimpleModule("PolymorphicInfoDeserializerModule", new Version(1, 0, 0, null));
module.addDeserializer(Info.class, deserializer);
mapper.registerModule(module);
factory = new JsonFactory(mapper);

Answer 2

Is this structure what you want for AClass?

class Dimension {
    String name;
    List<String> possibleValues;
}

class Product {
    String name;
    List<Dimension> dimensions;
}

All you need to do is change the length of the dimensions list to account for the three types.

Parsing becomes a trivial problem of checking if the Dimensions property is present in the JSON, and if so, iterating over it and appending to the dimensions list.

---

Another idea would be to restructure the JSON (if you can) such that all cases are of the same form:

"d0":{
    "Product":"A zero-dimensional Product",
    "Dimensions": {}
},

"d1":{
    "Product":"A one-dimensional Product",
    "Dimensions": {
        "Size": [ "XS", "S", "M", "L" ]
    }
},

"d2":{
    "Product":"A two-dimensional Product",
    "Dimensions": {
        "Size": [ "XS", "S", "M", "L" ],
        "Fit": [ "26", "28", "30", "32" ]
    }
}