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, cont         


        

Answer 1

I also couldn't find a Jackson property or annotation for this. So I'll have to answer no to the first question. But I would recommend a simple setter instead of the special deserializer :

public class Entity {

    @JsonDeserialize(contentAs = HomeImpl.class)
    private Collection<Home> homes = new ArrayList<>();

    public void setHomes(List<Home> homes) {
        if (homes != null)
            this.homes = homes;
    }
}

This is generic as it only uses the Home interface instead of HomeImpl. You don't need @JsonProperty as Jackson will associate setHomes and homes.

Answer 2

What worked for me was simply to remove the setter and make the attribute final. jackson 2 will then use the getter to modify the list.

public class Entity {

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

  public List<Home> getHomes() {
     return homes;
  }
}

The responsible feature is USE_GETTERS_AS_SETTERS which is turned on by default: https://github.com/FasterXML/jackson-databind/wiki/Mapper-Features

Answer 3

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.