读取 xml文件数据到对应的 java类，但是有一个节点下的所有数据一直都是null。

Question

@XmlRootElement(name="Waybill",namespace = "iata:waybill:1")

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "WaybillType",propOrder = {
    "messageHeaderDocument",
    "businessHeaderDocument",
    "masterConsignment"
})
public class WaybillType {

    @XmlElement(name = "MessageHeaderDocument", required = true)
    protected MessageHeaderDocumentType messageHeaderDocument;
    @XmlElement(name = "BusinessHeaderDocument", required = true)
    protected BusinessHeaderDocumentType businessHeaderDocument;
    @XmlElement(name = "MasterConsignment")
    protected MasterConsignmentType masterConsignment;

我有点佛了，MessageHeaderDocument节点下的所有节点，一直都是null，这是为何