I am using EclipseLink MOXy and have a data structure that has child elements of the same data type. Now I don't want to serialize the datastructure with infinite depth, but only the first level.
Here is some example code of the data structure:
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;
}
}
And my test program:
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);
}
}
The following XML is generated:
<?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>
However, I'd like the xml too look like:
<?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>
Thanks.
To accomplish this use case we will leverage two concepts from JAXB: XmlAdapter and Marshaller.Listener.
MyClassAdapter
We will leverage the default JAXB behaviour of not marshalling an element for a null value. To do this we will implement an XmlAdapter that returns null after a specified level has been reached. To count the levels we will create a 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
The @XmlJavaTypeAdapter annotation is used to specify that an XmlAdapter should be used.
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;
}
}
Test
Since we need to use the XmlAdapter in a stateful way, we will set an instance of it on the Marshaller, we will also set the instance of Marshaller.Listener we created on the Marshaller.
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);
}
}
Output
<?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>
For More Information
The following articles expand on the topics discussed in this answer: