Jackson Deserializer based on several StackOverflow posts.

ArrayOrObject.java

import java.util.List;

@Data
public class ArrayOrObject<T> {
    private List<T> data;

    private Boolean isObject;
}

ArrayOrObjectDeserializer.java

/*
 * http://stackoverflow.com/questions/20230114/jersey-jackson-and-jax-rs-post-multiple-json-formats
 * http://stackoverflow.com/questions/36159677/how-to-create-a-custom-deserializer-in-jackson-for-a-generic-type
 * http://stackoverflow.com/questions/31300046/how-to-use-jacksons-contextualdeserializer-for-root-values
 */
public static class ArrayOrObjectDeserializer
    extends JsonDeserializer<ArrayOrObject>
    implements ContextualDeserializer {

    private Class<?> contentType;

    @Override
    public JsonDeserializer<?> createContextual(
        DeserializationContext ctxt,
        BeanProperty property
    ) throws JsonMappingException {
        final JavaType wrapperType;

        if (property == null) {
            wrapperType = ctxt.getContextualType();
        } else {
            wrapperType = property.getType();
        }

        final JavaType contentType = wrapperType.containedType(0);
        final ArrayOrObjectDeserializer deserializer = new ArrayOrObjectDeserializer();
        deserializer.contentType = contentType.getRawClass();
        return deserializer;
    }

    @Override
    public ArrayOrObject deserialize(
        final JsonParser jp,
        final DeserializationContext ctxt
    ) throws IOException, JsonProcessingException {
        final ArrayOrObject wrapper = new ArrayOrObject();

        // Retrieve the object mapper and read the tree.
        ObjectMapper mapper = (ObjectMapper) jp.getCodec();
        JsonNode root = mapper.readTree(jp);

        if (root.isArray()) {
            final List list = new LinkedList();

            // Deserialize each node of the array using the type expected.
            final Iterator<JsonNode> rootIterator = root.iterator();
            while (rootIterator.hasNext()) {
                list.add(mapper.treeToValue(rootIterator.next(), this.contentType));
            }

            wrapper.setData(list);
            wrapper.setIsObject(false);
        } else {
            wrapper.setData(Arrays.asList(mapper.treeToValue(root, this.contentType)));
            wrapper.setIsObject(true);
        }

        return wrapper;
    }
}