Dynamic polymorphic type handling with Jackson

Question

I have a class hierarchy similar to this one:

public static class BaseConfiguration {
}

public abstract class Base {
  private BaseConfiguration configurati         


        

Answer 1

Good answer provided by Programmer Bruce!

I have a case of polymorphism in which I want to keep the domain objects as POJOs and not use dependencies on Jackson annotations.

Therefore I preffer to use a custom deserializer and a Factory for decising the type or intantiating the concrete classes.

Here is my code ... (be aware that I have an Annotation Hierarchy which are in fact "User Tags" and not Java Annotations )

Here is the deserialization Method

public class AnnotationDeserializer extends StdDeserializer<Annotation> {

AnnotationDeserializer() {
    super(Annotation.class);
}

@Override
public Annotation deserialize(JsonParser jp, DeserializationContext ctxt)
        throws IOException, JsonProcessingException {

    ObjectMapper mapper = (ObjectMapper) jp.getCodec();
    ObjectNode root = (ObjectNode) mapper.readTree(jp);
    Class<? extends Annotation> realClass = null;

    Iterator<Entry<String, JsonNode>> elementsIterator = root.getFields();
    while (elementsIterator.hasNext()) {
        Entry<String, JsonNode> element = elementsIterator.next();
        if ("type".equals(element.getKey())) {
            realClass = AnnotationObjectFactory.getInstance()
                    .getAnnotationClass(element.getKey());
            break;
        }
    }

    if (realClass == null)
        return null;
    return mapper.readValue(root, realClass);
}
}

Answer 2

Apparently the answer was to implement something similar to the sixth solution posted at http://programmerbruce.blogspot.com/2011/05/deserialize-json-with-jackson-into.html, which uses unique JSON element names to identify the target type to deserialize to.

Answer 3

I had to do something similar, and ended up creating a generic polymorphic list serializer and deserialzer. Here is the deserialize that I think will work for you:

    public class PolymorphicListDeserializer extends JsonDeserializer<List<?>> implements ContextualDeserializer {
  private HashMap<String, Class> _typeMap = null;

  private Class _elementType;

  private static <T> List<T> getNewList(Class<T> clazz) {
    return new ArrayList<T>();
  }

  @Override
  public List<?> deserialize(final JsonParser jp, DeserializationContext ctxt) throws IOException {

    final List list = getNewList(_elementType);

    JsonToken nextToken = jp.getCurrentToken();

    if (nextToken == JsonToken.START_OBJECT) {
        if ( _typeMap.containsKey( currentFieldName )) {
          list.add( _elementType.cast( ctxt.readValue( jp, _typeMap.get( currentFieldName ) ) ) );
        }
        nextToken = jp.nextToken();
      } else if (currentFieldName != null && isEndToken(nextToken) && wrapperCount == 0) {
        break;
      } else {
        nextToken = jp.nextToken();
      }
    }
    return list;
  }

  public JsonDeserializer<List<?>> createContextual( DeserializationContext ctxt, BeanProperty property ) throws JsonMappingException {
    //In Jackson 2.6.3, this method is called once per instance and the exception is never thrown
    if ( _typeMap == null )
      _typeMap = new HashMap<String, Class>();
    else
      throw new RuntimeException("Unsupported version of Jackson. Code assumes context is created once and only once.");

    _elementType = property.getType().getContentType().getRawClass();

    //For now, requiring XmlElements annotation to work.  May add support for JsonElements (if needed) later.
    for (XmlElement e : property.getAnnotation(XmlElements.class).value()) {
      _typeMap.put(e.name(), e.type());
    }

    return this;
  }

  private static boolean isStartToken(JsonToken t) {
    boolean result = false;
    if (t == JsonToken.START_OBJECT) {
      result = true;
    } else if (t == JsonToken.START_ARRAY) {
      result = true;
    }
    return result;
  }

Answer 4

Above answers depicts a solution however lack what actually used annotations mean. If you are curious about what actually these annotation do, idea behind them & why they are required please go through the below link. Its explained very nicely in it. https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization