JSON Consumer of Polymorphic Objects

JSON consumer of polymorphic objects

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);

Deserialize a polymorphic map with Jackson, where Type information is given within the string key

You are looking for an ObjectMapper. Have a look at my answer to JSON POJO consumer of polymorphic objects for an example.

Jackson polymorphic deserialization with nested type info property

You are facing 2 issues here:

1. As you have seen Jackson cannot easily and just by annotations use a property in a nested JSON object to deduce the type to deserialize to.
2. @JsonTypeInfo and @JsonSubTypes are meant for inheritance e.g class FooEventPayload extends EventPayload. In your case EventPayload<T> is a generic class and Jackson needs to be told what T is with a TypeReference Look here for instance

Assuming you want a generic class I'd suggest serialize into a tree first, peek into the tree to get the property that specifies the type and then convert the tree to an object of this type. You can skip @JsonTypeInfo and @JsonSubTypes. E.g.

// user object mapper to parse JSON into a tree (node is the root) 
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(jsonString);

// use get as many times as needed by depth
// to get the value that defines the type to deserialise to
String type = node.get("metadata").get("eventName").textValue();

// convert JsonNode variable to the required type
if (type.equals("fooEvent")) {
    EventPayload<FooEvent> event = 
        mapper.convertValue(node, new TypeReference<EventPayload<FooEvent>>(){});
} else if (type.equals("barEvent")) {
    EventPayload<BarEvent> event =
        mapper.convertValue(node, new TypeReference<EventPayload<BarEvent>>(){});
}

Deserializing polymorphic types with Jackson based on the presence of a unique property

This feels like something @JsonTypeInfo and @JsonSubTypes should be used for but I've picked through the docs and none of the properties that can be supplied quite seem to match what you're describing.

You could write a custom deserializer that uses @JsonSubTypes' "name" and "value" properties in a non-standard way to accomplish what you want. The deserializer and @JsonSubTypes would be supplied on your base class and the deserializer would use the "name" values to check for the presence of a property and if it exists, then deserialize the JSON into the class supplied in the "value" property. Your classes would then look something like this:

@JsonDeserialize(using = PropertyPresentDeserializer.class)
@JsonSubTypes({
        @Type(name = "stringA", value = SubClassA.class),
        @Type(name = "stringB", value = SubClassB.class)
})
public abstract class Parent {
    private Long id;
    ...
}

public class SubClassA extends Parent {
    private String stringA;
    private Integer intA;
    ...
}

public class SubClassB extends Parent {
    private String stringB;
    private Integer intB;
    ...
}

Gson: How do I parse polymorphic values that can be either lists or strings?

If you want to handle both scenarios you can use a custom deserializer. Of course, you have to change the "cust_id" variable to be a list or an array.

Main:

String json1 = "{\"cust_id\": \"87655\"}";
String json2 = "{\"cust_id\": [\"12345\", \"45678\"]}";

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Customer.class, new CustomerDeserializer());
Gson gson = gsonBuilder.create();

Customer customer1 = gson.fromJson(json1, Customer.class);
System.out.println(customer1);

Customer customer2 = gson.fromJson(json2, Customer.class);
System.out.println(customer2);

Customer

public class Customer {

    @SerializedName("cust_id")
    private List<String> custId;

    public List<String> getCustId() {
        return custId;
    }

    public void setCustId(List<String> custId) {
        this.custId = custId;
    }
}

CustomerDeserializer

public class CustomerDeserializer implements JsonDeserializer<Customer> {

@Override
public Customer deserialize(JsonElement jsonElement, Type typeOf, JsonDeserializationContext context) throws JsonParseException {
    Customer result = null;
    Gson gson = new Gson();

    try {
        // try to deserialize by assuming JSON has a list
        result = gson.fromJson(jsonElement, Customer.class);
    } catch (JsonSyntaxException jse) {
        // error here means JSON has a single string instead of a list

        try {
            // get the single ID
            String custId = jsonElement.getAsJsonObject().get("cust_id").getAsString();

            result = new Customer();
            result.setCustId(Arrays.asList(new String[] {custId}));
        } catch (Exception e) {
            // more error handling here
            e.printStackTrace();
        }
    }

    return result;
}

}

Output

Customer [custId=[87655]]
Customer [custId=[12345, 45678]]

Jackson deserialization of type with different objects

No no no. You do NOT have to write a custom deserializer. Just use "untyped" mapping first:

public class Response {
  public long id;
  public Object rated;
}
// OR
public class Response {
  public long id;
  public JsonNode rated;
}
Response r = mapper.readValue(source, Response.class);

which gives value of Boolean or java.util.Map for "rated" (with first approach); or a JsonNode in second case.

From that, you can either access data as is, or, perhaps more interestingly, convert to actual value:

if (r.rated instanceof Boolean) {
    // handle that
} else {
    ActualRated actual = mapper.convertValue(r.rated, ActualRated.class);
}
// or, if you used JsonNode, use "mapper.treeToValue(ActualRated.class)

There are other kinds of approaches too -- using creator "ActualRated(boolean)", to let instance constructed either from POJO, or from scalar. But I think above should work.

Parsing an array of non-homogeneous JSON Objects with Jackson

I would use

com.fasterxml.jackson.databind.JsonNode.

JsonNode parsed = objectMapper
                   .readValue("[{\"name\": \"a\"},{\"type\":\"b\"}]", JsonNode.class);

This class has tons of utility methods to work with.

Or specific for arrays you can use:

com.fasterxml.jackson.databind.node.ArrayNode

ArrayNode value = objectMapper
                   .readValue("[{\"name\": \"a\"},{\"type\":\"b\"}]", ArrayNode.class);

EDIT

Sorry, I have misread your question, you can use @JsonTypeInfo for polymorphic serialization/deserialization:

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

    //language=JSON
    String data = "[{\"type\":\"type1\", \"type1Specific\":\"this is type1\"},{\"type\":\"type2\", \"type2Specific\":\"this is type2\"}]";
    ObjectMapper objectMapper = new ObjectMapper();

    List<BaseType> parsed = objectMapper.readValue(data, new TypeReference<List<BaseType>>() {});
    System.out.println(parsed);
}

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type", include = JsonTypeInfo.As.PROPERTY)
@JsonSubTypes(value = {
        @JsonSubTypes.Type(value = Type1.class, name = "type1"),
        @JsonSubTypes.Type(value = Type2.class, name = "type2")
})
static public abstract class BaseType {
    public String type;
}
static public class Type1 extends BaseType {
    public String type1Specific;
    @Override
    public String toString() {
        return "Type1{" +
                "type1Specific='" + type1Specific + '\'' +
                '}';
    }
}
static public class Type2 extends BaseType {
    public String type2Specific;

    @Override
    public String toString() {
        return "Type2{" +
                "type2Specific='" + type2Specific + '\'' +
                '}';
    }
}

Here are the docs:

https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization

Hope this helps.

And the result would be:

[Type1{type1Specific='this is type1'}, Type2{type2Specific='this is type2'}]

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