Jackson serialize simple one attribute ValueObject like Enum - without nesting

Is it possible to configure jackson to serialize simple value objects, wrapping only one attribute, to be serialized like enums?

public final class ErrorCode {

  private final String value;

  public ErrorCode(@JsonProperty("value") final String value) {
    this.value = value;
  }

  public String getValue() {
    return value;
  }
}

Now it is serialized as ..., "errorCode":{"value":"invalid.login"}, ... but I would like to have, as if it were enum, ..., "errorCode":"invalid.login", .... It is possible to but only via @JsonUnwrapped() in each surrounding class

SorroundingClass {
  @JsonUnwrapped()
  private ErrorCode errorCode;  
  ...
}

I would like to configure It only in one place, best in ErrorCode itself.

Looking at flattening-nested-attributes-in-jackson it seems to me impossible, but I want to make sure.

Solution

You can achieve this by implementing custom com.fasterxml.jackson.databind.JsonSerializer serializer:

class SingleValueJsonSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        if (value == null) {
            gen.writeNull();
            return;
        }
        final Field[] fields = value.getClass().getDeclaredFields();
        if (fields.length != 1) {
            throw new IOException("Do not use this serialiser for the class " + value.getClass().getSimpleName() + " which has" + fields.length + " fields!");
        }
        final Field first = fields[0];
        try {
            final Object fieldValue = first.get(value);
            gen.writeObject(fieldValue);
        } catch (IllegalAccessException e) {
            throw new IOException(e);
        }
    }
}

You can define it in required class:

class Wrapper {

    @JsonSerialize(using = SingleValueJsonSerializer.class)
    private ErrorCode error;
    // other fields
}

or register it globally by defining on the class level:

@JsonSerialize(using = SingleValueJsonSerializer.class)
class ErrorCode {