java - Jackson deserialize object or array

Question

I have a Jackson Question.

Is there a way to deserialize a property that may have two types, for some objects it appears like this

"someObj" : { "obj1" : 5, etc....}

then for others it appears as an empty array, i.e.

"someObj" : []

Any help is appreciated!

Thanks!

Answer 1

Edit: Since Jackson 2.5.0, you can use DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_EMPTY_OBJECT to resolve your problem.

The solution Bruce provides has a few problems/disadvantages:

• you'll need to duplicate that code for each type you need to deserialize that way
• ObjectMapper should be reused since it caches serializers and deserializers and, thus, is expensive to create. See http://wiki.fasterxml.com/JacksonBestPracticesPerformance
• if your array contains some values, you probably want let jackson to fail deserializing it because it means there was a problem when it got encoded and you should see and fix that asap.

Here is my "generic" solution for that problem:

public abstract class EmptyArrayAsNullDeserializer<T> extends JsonDeserializer<T> {

  private final Class<T> clazz;

  protected EmptyArrayAsNullDeserializer(Class<T> clazz) {
    this.clazz = clazz;
  }

  @Override
  public T deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
    ObjectCodec oc = jp.getCodec();
    JsonNode node = oc.readTree(jp);
    if (node.isArray() && !node.getElements().hasNext()) {
      return null;
    }
    return oc.treeToValue(node, clazz);
  }
}

then you still need to create a custom deserializer for each different type, but that's a lot easier to write and you don't duplicate any logic:

public class Thing2Deserializer extends EmptyArrayAsNullDeserializer<Thing2> {

  public Thing2Deserializer() {
    super(Thing2.class);
  }
}

then you use it as usual:

@JsonDeserialize(using = Thing2Deserializer.class)

If you find a way to get rid of that last step, i.e. implementing 1 custom deserializer per type, I'm all ears ;)