Deserialize JSON object/map as generic Collection with Jackson - json

Question

I have JSON maps like this:
"things": {"foo": {"name": "foo", ...}, "bar": {"name": "bar", ...}}
I want to deserialize them as if they were arrays:
"things": [{"name": "foo", ...}, {"name": "bar", ...}]
(to match XML/JAXB deserialization behavior):
<things><thing name="foo">...</thing><thing name="bar">...</thing></things>
into a collection such as this:
#XmlElementWrapper
#XmlElement(name = "thing")
#JsonDeserialize(using = MapToCollectionDeserializer.class)
Collection<Thing> things;
Note that I have collections with various element types -- not just Thing -- so I need a generic mechanism.
However, when writing a custom deserializer, what's the right way to access the type information of the context?
public class MapToCollectionDeserializer extends StdDeserializer<Object>
{
#Override
public Object deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException
{
Preconditions.checkState(jp.getCurrentToken() == JsonToken.START_OBJECT);
final LinkedList<Object> result = new LinkedList<>();
JsonToken tok;
while ((tok = jp.nextToken()) != JsonToken.END_OBJECT)
{
Preconditions.checkState(tok == JsonToken.FIELD_NAME);
// How to get the collection element type for deserialization?
result.add(...);
}
return result;
}
}
My approach so far is using ContextualDeserializer, which can provide a BeanProperty (which contains type information) to the deserializer. However, a JsonDeserializer must still have a no-arg constructor, so I end up constructing a broken object at first:
public class MapToCollectionDeserializer extends StdDeserializer<Object>
implements ContextualDeserializer<Object>
{
private final BeanProperty property;
public MapToCollectionDeserializer()
{
super(Collection.class);
property = null; // YUCK: BROKEN!!!
}
private MapToCollectionDeserializer(BeanProperty property)
{
super(property.getType());
this.property = property;
}
#Override
public JsonDeserializer<Object> createContextual(DeserializationConfig config,
BeanProperty property) throws JsonMappingException
{
return new MapToCollectionDeserializer(property);
}
#Override
public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException,
JsonProcessingException
{
Preconditions.checkState(jp.getCurrentToken() == JsonToken.START_OBJECT);
final JavaType elementType = property.getType().containedType(0);
final LinkedList<Object> result = new LinkedList<>();
JsonToken tok;
while ((tok = jp.nextToken()) != JsonToken.END_OBJECT)
{
Preconditions.checkState(tok == JsonToken.FIELD_NAME);
jp.nextToken();
final JsonDeserializer<Object> valueDeser = ctxt.getDeserializerProvider()
.findValueDeserializer(ctxt.getConfig(), elementType, property);
result.add(valueDeser.deserialize(jp, ctxt));
}
return result;
}
}
Is there a better/simpler way to do this?

Answer 1

It looks like you stopped using Jackson, but for anyone with a similar problem, you can turn on DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY. With that setting enabled, when Jackson finds an object in the JSON but is supposed to deserialize to a collection, it will create a collection and put the object into the collection which seems like what you want here.