帶有遞歸數據結構的EclipseLink MOXy /同一類型的子元素

Question

我正在使用EclipseLink MOXy並具有具有相同數據類型的子元素的數據結構。現在我不想以無限深度序列化數據結構，但只想到第一層。

這裏是數據結構的一些示例代碼：

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; 
    } 
} 

而我的測試程序：

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); 
    } 
} 

以下XML生成：

<?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，I」 d像xml一樣：

<?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> 

謝謝。

Answer 1

要完成此用例，我們將利用JAXB的兩個概念：XmlAdapter和Marshaller.Listener。

MyClassAdapter

我們將利用空值未編組的元素的默認JAXB行爲。爲此，我們將實現一個XmlAdapter，它在達到指定級別後返回null。爲了計算水平，我們將創建一個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的

的@XmlJavaTypeAdapter註解用於指定一個XmlAdapter應該被使用。

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; 
    } 

} 

測試

因爲我們需要一個有狀態的方式使用XmlAdapter，我們將設置它的實例在Marshaller，我們還將設置的Marshaller.Listener的情況下，我們的Marshaller創建。

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); 
    } 

} 

輸出

<?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> 

更多信息

以下文章對這個答案討論的議題展開：

• 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