Jackson deserialize single item into list

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.

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 :)

I think the anotation suits the code better.

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

@JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY) does not seem work with @JsonValue.

Meaning it won't work if you want to deserialize either of these as a list:

{
  "key": true
}

[
  {
    "key": true
  },
  {
    "key": false
  }
]

As a solution it's possible to create a custom deserializer like this:

@JsonDeserialize(using = CustomDeserializer.class)
public record ListHolder(List<Item> items) {}

public class CustomDeserializer extends JsonDeserializer<ListHolder> {

    @Override
    public ListHolder deserialize(JsonParser parser, DeserializationContext context) throws IOException {
        List<Item> items;

        if (parser.isExpectedStartArrayToken()) {
            items = parser.readValueAs(new TypeReference<>() {});
        } else {
            items = List.of(parser.readValueAs(Item.class));
        }

        return new ListHolder(items);
    }
}