具有相同类型的递归数据结构/子元素的EclipseLink MOXy

Question

我使用的是EclipseLink MOXy，并且有一个具有相同数据类型的子元素的数据结构。现在我不想序列化具有无限深度的数据结构，而只想序列化第一级。

以下是数据结构的一些示例代码：

package test;
import java.util.Collection;
import java.util.Vector;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElementRef;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;
@XmlAccessorType(XmlAccessType.PROPERTY)
@XmlRootElement
public class MyClass {
    private int id;
    private String details;
    private Collection<MyClass> children = new Vector<MyClass>();
    public MyClass() {
    }
    public MyClass(int id, String details) {
        this.id = id;
        this.details = details;
    }
    @XmlElementWrapper
    @XmlElementRef
    public Collection<MyClass> getChildren() {
        return children;
    }
    public void addChild(MyClass child) {
        children.add(child);
    }
    public String getDetails() {
        return details;
    }
    @XmlAttribute
    public int getId() {
        return id;
    }
    public void setChildren(Collection<MyClass> children) {
        this.children = children;
    }
    public void setDetails(String details) {
        this.details = details;
    }
    public void setId(int id) {
        this.id = id;
    }
}

我的测试程序：

package test;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
public class Test {
    public static void main(String[] args) throws Exception {
        MyClass l1 = new MyClass(1, "Level 1");
        MyClass l2 = new MyClass(2, "Level 2");
        l1.addChild(l2);
        MyClass l3 = new MyClass(3, "Level 3");
        l2.addChild(l3);
        JAXBContext jc = JAXBContext.newInstance(MyClass.class);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(l1, System.out);
    }
}

生成以下XML：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<myClass id="1">
    <children>
        <myClass id="2">
            <children>
                <myClass id="3">
                    <children/>
                    <details>Level 3</details>
                </myClass>
            </children>
            <details>Level 2</details>
        </myClass>
    </children>
    <details>Level 1</details>
</myClass>

然而，我也希望xml看起来像：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<myClass id="1">
    <children>
        <myClass id="2">
            <details>Level 2</details>
        </myClass>
    </children>
    <details>Level 1</details>
</myClass>

谢谢。

Answer 1

为了完成这个用例，我们将利用JAXB的两个概念：XmlAdapter和Marshaller.Listener.

MyClassAdapter

我们将利用默认的JAXB行为，即不为null值编组元素。为此，我们将实现一个XmlAdapter，它在达到指定级别后返回null。为了计算级别，我们将创建一个Marshaller.Listener。

package forum11769758;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class MyClassAdapter extends XmlAdapter<MyClass, MyClass>{
    private int levels;
    private MyMarshallerListener marshallerListener;
    public MyClassAdapter() {
    }
    public MyClassAdapter(int levels) {
        this.levels = levels;
    }
    public Marshaller.Listener getMarshallerListener() {
        if(null == marshallerListener) {
            marshallerListener = new MyMarshallerListener();
        }
        return marshallerListener;
    }
    @Override
    public MyClass marshal(MyClass myClass) throws Exception {
        if(null == marshallerListener || marshallerListener.getLevel() < levels) {
            return myClass;
        }
        return null;
    }
    @Override
    public MyClass unmarshal(MyClass myClass) throws Exception {
        return myClass;
    }
    static class MyMarshallerListener extends Marshaller.Listener {
        private int level = 0;
        public int getLevel() {
            return level;
        }
        @Override
        public void afterMarshal(Object object) {
            if(object instanceof MyClass) {
                level--;
            }
        }
        @Override
        public void beforeMarshal(Object object) {
            if(object instanceof MyClass) {
                level++;
            }
        }
    }
}

MyClass

@XmlJavaTypeAdapter注释用于指定应使用XmlAdapter。

package forum11769758;
import java.util.*;
import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
@XmlAccessorType(XmlAccessType.PROPERTY)
@XmlRootElement
public class MyClass {
    private int id;
    private String details;
    private Collection<MyClass> children = new Vector<MyClass>();
    public MyClass() {
    }
    public MyClass(int id, String details) {
        this.id = id;
        this.details = details;
    }
    @XmlElementWrapper
    @XmlElementRef
    @XmlJavaTypeAdapter(MyClassAdapter.class)
    public Collection<MyClass> getChildren() {
        return children;
    }
    public void addChild(MyClass child) {
        children.add(child);
    }
    public String getDetails() {
        return details;
    }
    @XmlAttribute
    public int getId() {
        return id;
    }
    public void setChildren(Collection<MyClass> children) {
        this.children = children;
    }
    public void setDetails(String details) {
        this.details = details;
    }
    public void setId(int id) {
        this.id = id;
    }
}

测试

由于我们需要以有状态的方式使用XmlAdapter，我们将在Marshaller上设置它的一个实例，我们还将设置我们在Marshaller上创建的Marshaller.Listener的实例。

package forum11769758;
import javax.xml.bind.*;
public class Test {
    public static void main(String[] args) throws Exception {
        MyClass l1 = new MyClass(1, "Level 1");
        MyClass l2 = new MyClass(2, "Level 2");
        l1.addChild(l2);
        MyClass l3 = new MyClass(3, "Level 3");
        l2.addChild(l3);
        JAXBContext jc = JAXBContext.newInstance(MyClass.class);
        Marshaller marshaller = jc.createMarshaller();
        MyClassAdapter myClassAdapter = new MyClassAdapter(2);
        marshaller.setAdapter(myClassAdapter);
        marshaller.setListener(myClassAdapter.getMarshallerListener());
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(l1, System.out);
    }
}

输出

<?xml version="1.0" encoding="UTF-8"?>
<myClass id="1">
   <children>
      <myClass id="2">
         <children/>
         <details>Level 2</details>
      </myClass>
   </children>
   <details>Level 1</details>
</myClass>

有关更多信息

以下文章对本答案中讨论的主题进行了扩展：

• http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html
• http://blog.bdoughan.com/2011/09/mixing-nesting-and-references-with.html