我想将以下对象存储到marklogic中。

@XmlRootElement(name = "trainModel")
@XmlAccessorType(XmlAccessType.FIELD)
public class TrainedModel implements Serializable {

    private static final long serialVersionUID = 1L;

    private String modelName;
    private CrossValidatorModel crossValidatorModel;

    public String getModelName() {
        return modelName;
    }

    public void setModelName(String modelName) {
        this.modelName = modelName;
    }

    public CrossValidatorModel getCrossValidatorModel() {
        return crossValidatorModel;
    }

    public void setCrossValidatorModel(CrossValidatorModel crossValidatorModel) {
        this.crossValidatorModel = crossValidatorModel;
    }

    @Override
    public String toString() {
        return "TrainedModel [modelName=" + modelName + ", crossValidatorModel=" + crossValidatorModel + "]";
    }
}

我正在使用jaxbhandle来完成我的需求，但是在将trainedmodel对象存储到marklogic时，crossvalidatormodel变空了。
crossvalidatormodel是由提供的外部对象 org.apache.spark.ml.tuning.CrossValidatorModel .
我试过下面的代码，

CrossValidatorModel crossValidatorModel = createDataFrame(null);
    TrainedModel trainedModel = new TrainedModel();
    trainedModel.setModelName("sample");
    trainedModel.setCrossValidatorModel(crossValidatorModel);

    JAXBContext context = JAXBContext.newInstance(TrainedModel.class);  
    JAXBHandle<TrainedModel> jaxbHandle = new JAXBHandle<TrainedModel>(context);
    jaxbHandle.set(trainedModel);

    GenericDocumentManager docMgr = client.newDocumentManager();
    docMgr.writeAs("/shivling.xml", null, jaxbHandle);

这是我得到的xml结果，

<?xml  version="1.0" encoding="UTF-8"?>
<trainModel>
    <modelName>sample</modelName>
    <crossValidatorModel>
    </crossValidatorModel>
</trainModel>

crossvalidatormodel在此为空，请帮助我解决此问题。