How to apply a Jackson SerializationFeature on one field?

Question

Given the following POJO, I would like to apply SerializationFeature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED on the links field ONLY.

public class HalRepresentation {

   @JsonProperty("_links")
   @JsonInclude(JsonInclude.Include.NON_DEFAULT)
   private final Map<String, List<Link>> links = new HashMap<String, List<Link>>();

    @JsonProperty("_embedded")
    @JsonInclude(JsonInclude.Include.NON_DEFAULT)
    private final Map<String, Object> embedded = new HashMap<String, Object>();

    protected HalRepresentation() {
    }

    public Map<String, List<Link>> getLinks() {
        return links;
    }
    public Map<String, Object> getEmbedded() {
        return embedded;
    }
}

I tried to serialize it as follows:

ObjectMapper objectMapper = new ObjectMapper()
    .enable(SerializationFeature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED);
try {
    outputStream.write(objectMapper.writeValueAsBytes(halRepresentation));
    outputStream.flush();
} catch (JsonProcessingException e) {
    throw new IllegalStateException(e);
}

But when I do this the unwrap feature is also applied on the embedded field. I tried to find an equivalent annotation for WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED, but I can't find one. Do you have an idea for this using Jackson ?

Answer 1

It appears additional functionality was added to Jackson 2.6 to allow you to apply this feature to a single field using the @JsonFormat annotation:

    @JsonProperty("_links")
    @JsonInclude(JsonInclude.Include.NON_DEFAULT)
    @JsonFormat(with = {Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY,Feature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED})
   private final Map<String, List<Link>> links = new HashMap<String, List<Link>>();