JAXB解组：子元素为null

我试图解组XML文档：

<map>
   <room id="1" name="Stairway" east="2"/>
   <room id="2" name="Kitchen" north="4" south="3" west="1">
       <object name="Hat"/>
   </room>
   <room id="3" name="Hallway" east="1" south="4">
       <object name="Money"/>
   </room>
   <room id="4" name="Bathroom" south="2"/>
</map>

和XSD模式（由某些工具生成）：

<xsd:schema attributeFormDefault="unqualified" elementFormDefault="qualified" version="1.0" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:element name="map">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element minOccurs="1" maxOccurs="unbounded" name="room">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element minOccurs="0" maxOccurs="unbounded" name="object">
                <xsd:complexType>
                  <xsd:attribute name="name" type="xsd:string" />
                </xsd:complexType>
              </xsd:element>
            </xsd:sequence>
            <xsd:attribute name="id" type="xsd:int" />
            <xsd:attribute name="name" type="xsd:string" />
            <xsd:attribute name="north" type="xsd:int" />
            <xsd:attribute name="south" type="xsd:int" />
            <xsd:attribute name="west" type="xsd:int" />
            <xsd:attribute name="east" type="xsd:int" />
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>
</xsd:schema>

这些是重要的模型类：

Map.java

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {"rooms"})
@XmlRootElement(name = "map")
public class Map {

    /**
     * List of rooms contained in this map.
     */
    @XmlElement(required = true)
    protected List<Room> rooms;

    /**
     * Gets the value of the rooms property.
     *
     * <p>
     * This accessor method returns a reference to the live list,
     * not a snapshot. Therefore any modification you make to the
     * returned list will be present inside the JAXB objects.
     * This is why there is not a <CODE>set</CODE> method for the rooms property.
     *
     * <p>
     * For example, to add a new item, do as follows:
     * <pre>
     *    getRooms().add(newItem);
     * </pre>
     *
     *
     * <p>
     * Objects of the following type(s) are allowed in the list
     * {@link Room }
     *
     *
     */
    public List<Room> getRooms() {
        if (rooms == null) {
            rooms = new ArrayList<Room>();
        }
        return this.rooms;
    }

}

Room.java

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
        "objects"
})
public class Room {

    @XmlElement(required = true)
    protected List<Object> objects;
    @XmlAttribute(name = "id")
    protected Integer id;
    @XmlAttribute(name = "name")
    protected String name;
    @XmlAttribute(name = "north")
    protected Integer north;
    @XmlAttribute(name = "south")
    protected Integer south;
    @XmlAttribute(name = "west")
    protected Integer west;
    @XmlAttribute(name = "east")
    protected Integer east;

    /**
     * Gets the value of the objects property.
     * 
     * <p>
     * This accessor method returns a reference to the live list,
     * not a snapshot. Therefore any modification you make to the
     * returned list will be present inside the JAXB objects.
     * This is why there is not a <CODE>set</CODE> method for the objects property.
     * 
     * <p>
     * For example, to add a new item, do as follows:
     * <pre>
     *    getObjects().add(newItem);
     * </pre>
     * 
     * 
     * <p>
     * Objects of the following type(s) are allowed in the list
     * {@link Object }
     * 
     * 
     */
    public List<Object> getObjects() {
        if (objects == null) {
            objects = new ArrayList<Object>();
        }
        return this.objects;
    }

    /**
     * Ruft den Wert der id-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link Integer }
     *     
     */
    public Integer getId() {
        return id;
    }

    /**
     * Legt den Wert der id-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link Integer }
     *     
     */
    public void setId(Integer value) {
        this.id = value;
    }

    /**
     * Ruft den Wert der name-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link String }
     *     
     */
    public String getName() {
        return name;
    }

    /**
     * Legt den Wert der name-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link String }
     *     
     */
    public void setName(String value) {
        this.name = value;
    }

    /**
     * Ruft den Wert der north-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link Integer }
     *     
     */
    public Integer getNorth() {
        return north;
    }

    /**
     * Legt den Wert der north-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link Integer }
     *     
     */
    public void setNorth(Integer value) {
        this.north = value;
    }

    /**
     * Ruft den Wert der south-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link Integer }
     *     
     */
    public Integer getSouth() {
        return south;
    }

    /**
     * Legt den Wert der south-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link Integer }
     *     
     */
    public void setSouth(Integer value) {
        this.south = value;
    }

    /**
     * Ruft den Wert der west-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link Integer }
     *     
     */
    public Integer getWest() {
        return west;
    }

    /**
     * Legt den Wert der west-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link Integer }
     *     
     */
    public void setWest(Integer value) {
        this.west = value;
    }

    /**
     * Ruft den Wert der east-Eigenschaft ab.
     * 
     * @return
     *     possible objects is
     *     {@link Integer }
     *     
     */
    public Integer getEast() {
        return east;
    }

    /**
     * Legt den Wert der east-Eigenschaft fest.
     * 
     * @param value
     *     allowed objects is
     *     {@link Integer }
     *     
     */
    public void setEast(Integer value) {
        this.east = value;
    }

}

现在，当我尝试解组XMLmap.rooms始终为null时，应该有房间列表。

知道是什么原因造成的，以及如何解决？