java - Jackson deserializer - change null collection to empty one

Question

I have an entity that contains collection as attribute:

public class Entity {

    @JsonProperty(value="homes")
    @JsonDeserialize(as=HashSet.class, contentAs=HomeImpl.class)
    private Collection<Home> homes = new ArrayList<Home>();

}

If request contains null as request property:

{
  "homes": null
}

then homes is set to null. What I want to do is to set homes to empty list. Do I need to write special deserializer for this or is there one for collections? What I tried is this deserializer but it looks ugly (it's not generic and uses implementation instead of interface).

public class NotNullCollectionDeserializer extends JsonDeserializer<Collection<HomeImpl>> {

  @Override
  public Collection<HomeImpl> deserialize(final JsonParser jsonParser, final DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
    return jsonParser.readValueAs(new TypeReference<Collection<HomeImpl>>(){});
  }

  @Override
  public Collection<HomeImpl> getNullValue() {
    return Collections.emptyList();
  }
}

So few questions:

1. Is there some jackson property that changes null to empty collection during deserialization?
2. If no for the first point - do I need to write deserializer for this? If yes, can I write generic one?

Answer 1

As of Jackson 2.9, it looks like null-handling for specific properties can be configured with @JsonSetter, for example:

@JsonSetter(nulls = Nulls.AS_EMPTY)
public void setStrings(List<String> strings) {
    this.strings = strings;
}

Similar configuration can also be applied globally for collections:

ObjectMapper mapper = objectMapperBuilder()
    .changeDefaultNullHandling(n -> n.withContentNulls(Nulls.AS_EMPTY))
    .build();

Or by type:

ObjectMapper mapper = objectMapperBuilder()
    .withConfigOverride(List.class,
        o -> o.setNullHandling(JsonSetter.Value.forContentNulls(Nulls.AS_EMPTY)))
    .build();

I haven't been able to try the feature out, so this is based on the feature discussion and examination of unit tests. YMMV.