Jackson deserialize single item into list

Question

I'm trying to consume a service that gives me an entity with a field that it's an array.

{
  "id": "23233",
  "items": [
    {
      "name": "item 1"
    },
    {
      "name": "item 2"
    }
  ]
}

But when the array contains a single item, the item itself is returned, instead of an array of one element.

{
  "id": "43567",
  "items": {
      "name": "item only"
    }
}

In this case, Jackson fails to convert to my Java object.

public class ResponseItem {

   private String id;
   private List<Item> items;

   //Getters and setters...
}

Is there an straightforward solution for it?

Answer 1

You are not the first to ask for this problem. It seems pretty old.

After looking at this problem, you can use the DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY :

Look at the documentation : http://fasterxml.github.io/jackson-databind/javadoc/2.5/com/fasterxml/jackson/databind/DeserializationFeature.html#ACCEPT_SINGLE_VALUE_AS_ARRAY

You need to add this jackson feature to your object mapper.

I hope it will help you.

Answer 2

I think the anotation suits the code better.

@JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
private List<Item> items;

Answer 3

A custom JsonDeserializer can be used to solve this.

E.g.

class CustomDeserializer extends JsonDeserializer<List<Item>> {

    @Override
    public List<Item> deserialize(JsonParser jsonParser, DeserializationContext context)
            throws IOException, JsonProcessingException {

        JsonNode node = jsonParser.readValueAsTree();

        List<Item> items = new ArrayList<>();
        ObjectMapper mapper = new ObjectMapper();

        if (node.size() == 1) {
            Item item = mapper.readValue(node.toString(), Item.class);
            items.add(item);

        } else {
            for (int i = 0; i < node.size(); i++) {
                Item item = mapper.readValue(node.get(i).toString(), Item.class);
                items.add(item);
            }
        }

        return items;
    }

}

you need to tell jackson to use this to de-serialize items, like:

@JsonDeserialize(using = CustomDeserializer.class)
private List<Item> items;

After that it will work. Happy coding :)