Using jackson deserialising a property which can be List of object or the object

Question

My lib is calling an API which can return either of the following JSON structure -

{
  "key_is_same" : {
     "inner" : "val"
  } 
}

-or-

 {
  "key_is_same" : [{
     "inner" : "val1"
    },
    {
     "inner" : "val2"
    }
  ]
}

Is there any annotation in jakson which can handle this and deserializ it into respective type

Answer 1

Looks like you are looking for the ACCEPT_SINGLE_VALUE_AS_ARRAY deserialization feature.

Feature that determines whether it is acceptable to coerce non-array (in JSON) values to work with Java collection (arrays, java.util.Collection) types. If enabled, collection deserializers will try to handle non-array values as if they had "implicit" surrounding JSON array. This feature is meant to be used for compatibility/interoperability reasons, to work with packages (such as XML-to-JSON converters) that leave out JSON array in cases where there is just a single element in array.

Feature is disabled by default.

It could be enabled either in ObjectMapper:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

Or via the @JsonFormat annotation:

@JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
private List<Foo> oneOrMany;

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For illustration purposes, consider the following JSON documents:

{
  "oneOrMany": [
    {
      "value": "one"
    },
    {
      "value": "two"
    }
  ]
}

{
  "oneOrMany": {
    "value": "one"
  }
}

It could be the deserialized to the following classes:

@Data
public class Foo {
    private List<Bar> oneOrMany;
}

@Data
public class Bar {
    private String value;
}

Just ensure the feature is enabled in your ObjectMapper or your field is annotated with @JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY).

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And in case you are looking for the equivalent feature for serialization, refer to WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED.