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.

Answer 2

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.