JAXB中怎么构建对父对象的链接

还是以在第一节介绍JAXB的schema为例：

<?xml version="1.0" encoding="UTF-8"?>  
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:tns="http://www.liulutu.com/students/"  
    targetNamespace="http://www.liulutu.com/students/">  
    <element name="students">  
        <complexType>  
            <sequence>  
                <element name="student" type="tns:StudentType" maxOccurs="unbounded" />  
            </sequence>  
        </complexType>  
    </element>  
    <simpleType name="SexType">  
        <restriction base="string">  
            <enumeration value="Male"></enumeration>  
            <enumeration value="Female"></enumeration>  
        </restriction>  
    </simpleType>  
  
    <complexType name="StudentType">  
        <attribute name="sex" type="tns:SexType"></attribute>  
        <attribute name="name" type="string"></attribute>  
    </complexType>  
</schema>  

 生成的类文件如下：

 

从Schema定义里可以看出，Students里包含有多个StudentType对象：

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
    "student"
})
@XmlRootElement(name = "students")
public class Students {

    @XmlElement(required = true)
    protected List<StudentType> student;

    /**
     * Gets the value of the student 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 object.
     * This is why there is not a <CODE>set</CODE> method for the student property.
     * 
     * <p>
     * For example, to add a new item, do as follows:
     * <pre>
     *    getStudent().add(newItem);
     * </pre>
     * 
     * 
     * <p>
     * Objects of the following type(s) are allowed in the list
     * {@link StudentType }
     * 
     * 
     */
    public List<StudentType> getStudent() {
        if (student == null) {
            student = new ArrayList<StudentType>();
        }
        return this.student;
    }

}

现在的问题是，从StudentType对象里，怎么能访问到它的父对象也就是上面的students对象呢？StudentType类如下：

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "StudentType")
public class StudentType {

    @XmlAttribute(name = "sex")
    protected SexType sex;
    @XmlAttribute(name = "name")
    protected String name;
    @XmlAttribute(name = "birthday")
    @XmlJavaTypeAdapter(Adapter1 .class)
    @XmlSchemaType(name = "date")
    protected Calendar birthday;

    /**
     * Gets the value of the sex property.
     * 
     * @return
     *     possible object is
     *     {@link SexType }
     *     
     */
    public SexType getSex() {
        return sex;
    }

    /**
     * Sets the value of the sex property.
     * 
     * @param value
     *     allowed object is
     *     {@link SexType }
     *     
     */
    public void setSex(SexType value) {
        this.sex = value;
    }

    /**
     * Gets the value of the name property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getName() {
        return name;
    }

    /**
     * Sets the value of the name property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setName(String value) {
        this.name = value;
    }

    /**
     * Gets the value of the birthday property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public Calendar getBirthday() {
        return birthday;
    }

    /**
     * Sets the value of the birthday property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setBirthday(Calendar value) {
        this.birthday = value;
    }

}

从类定义可以看出，其中没有任何描述Students对象的信息。

解决方法：

首先，假设要解析的文件内容如下：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:students xmlns:ns2="http://www.liulutu.com/students/">
	<student sex="Female" name="Bob" birthday="2003-11-27+08:00" />
	<student sex="Male" name="Lisa" birthday="2003-10-26+08:00" />
</ns2:students>

（如果手头没有文件，也可以用以上模型快速的生成一个，例如：）

		StudentType studentType = new StudentType();
		studentType.setBirthday(Calendar.getInstance());
		studentType.setName("Bob");
		studentType.setSex(SexType.FEMALE);

		Students students = new Students();
		students.getStudent().add(studentType);

		JAXBContext context = JAXBContext.newInstance(Students.class);
		Marshaller marshaller = context.createMarshaller();
		marshaller.marshal(students, new File("a.xml")); 

方法一：使用afterUnmarshal(Unmarshaller u, Object parent)方法

首先修改一下StudentType类，增加一个用来存取父对象的变量：

	private Students parent;
	
	public Students getParent() {
		return parent;
	}

因为模型是由JAXB直接读取和生成的，因此我们不能通过简单的setParent()来设置parent对象。

 

可以在生成的模型类中添加一个public void afterUnmarshal(Unmarshaller u, Object parent) 方法，这个方法在每次构建完模型后都会调用（甚至我们什么接口实现，类继承都不需要声明就会自动调用），其中的parent参数就是我们期望的父模型对象，因此可以直接把这个parent对象设置成我们期望的parent对象：

	public void afterUnmarshal(Unmarshaller u, Object parent) {
		if (parent instanceof Students) {
			this.parent = (Students)parent;
		}
	}

可以检验一下：

		JAXBContext context = JAXBContext.newInstance(Students.class);
		Unmarshaller unmarshaller = context.createUnmarshaller();
		Students students = (Students) unmarshaller.unmarshal(new File("a.xml"));
		List<StudentType> student = students.getStudent();
		for(StudentType st: student){
			System.out.println(st.getName());
			System.out.println(st.getParent());
		}

方法二：使用Binder

方法一简单易用，但是缺点就是需要修改模型，这里介绍的方法就不需要修改模型，只是在读取的时候需要有点不一样：

首先我们用普通的Dom方式读取文件内容：

		DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
		factory.setNamespaceAware(true);
		Document document = factory.newDocumentBuilder().parse(new File("a.xml"));

然后创建JAXBContext对象和Binder对象：

		JAXBContext context = JAXBContext.newInstance(Students.class);
		Binder<Node> binder = context.createBinder();

最后用Binder对象去解析模型的和链接父子关系：

		Students students = (Students) binder.updateJAXB(document.getDocumentElement());
		List<StudentType> student = students.getStudent();
		for(StudentType st: student){
			System.out.println(st.getName());
			Node xmlNode = binder.getXMLNode(st);
			Object parent = binder.getJAXBNode(xmlNode.getParentNode());
			System.out.println(parent);
		}

方法三 使用Unmarshall.Listener监听

除了以上两种方法，还可以使用监听的方式，在marshall的过程中构建父子关系，有点类似于方法一

		JAXBContext context = JAXBContext.newInstance(Students.class);
		Unmarshaller unmarshaller = context.createUnmarshaller();
		
		final Map<StudentType, Students> map = new HashMap<StudentType, Students>();
		unmarshaller.setListener(new Listener() {
			public void afterUnmarshal(Object target, Object parent) {
				super.afterUnmarshal(target, parent);
				if(target instanceof StudentType && parent instanceof Students){
					map.put((StudentType)target, (Students) parent);
				}
				
			}
		});
		Students students = (Students) unmarshaller.unmarshal(new File("a.xml"));
		List<StudentType> student = students.getStudent();
		for(StudentType st: student){
			System.out.println(st.getName());
			System.out.println(map.get(st));
		}