JAXB Pre And Post-processing with Unmarshaller Listener

This post shows how to perform JAXB pre and post-processing with Unmarshaller Listener. For instance, you may want to compare the contents of an object before and after you unmarshal an XML. Another use-case is when you may need to store unmarshalled values to some list not mapped to any XML element. We can achieve it by creating a class that extends Unmarshaller Listener and overriding two methods. These methods are beforeUnmarshal and afterUnmarshal.

Software Requirements

Windows 10
Java 8 (Source Level = 1.8)
Eclipse Mars.2 Release (4.5.2)

JAXB Unmarshaller Listener

The Unmarshaller Listener is an abstract class available in java.xml.bind.package. It has two methods.

    public static abstract class Listener {
        /**
         * <p/>
         * Callback method invoked before unmarshalling into <tt>target</tt>.
         * <p/>
         * <p/>
         * This method is invoked immediately after <tt>target</tt> was created and
         * before the unmarshalling of this object begins. Note that
         * if the class of <tt>target</tt> defines its own <tt>beforeUnmarshal</tt> method,
         * the class specific callback method is invoked before this method is invoked.
         *
         * @param target non-null instance of JAXB mapped class prior to unmarshalling into it.
         * @param parent instance of JAXB mapped class that will eventually reference <tt>target</tt>.
         *               <tt>null</tt> when <tt>target</tt> is root element.
         */
        public void beforeUnmarshal(Object target, Object parent) {
        }

        /**
         * <p/>
         * Callback method invoked after unmarshalling XML data into <tt>target</tt>.
         * <p/>
         * <p/>
         * This method is invoked after all the properties (except IDREF) are unmarshalled into <tt>target</tt>,
         * but before <tt>target</tt> is set into its <tt>parent</tt> object.
         * Note that if the class of <tt>target</tt> defines its own <tt>afterUnmarshal</tt> method,
         * the class specific callback method is invoked before this method is invoked.
         *
         * @param target non-null instance of JAXB mapped class prior to unmarshalling into it.
         * @param parent instance of JAXB mapped class that will reference <tt>target</tt>.
         *               <tt>null</tt> when <tt>target</tt> is root element.
         */
        public void afterUnmarshal(Object target, Object parent) {
        }
    }

public static abstract class Listener {

/**

* <p/>

* Callback method invoked before unmarshalling into <tt>target</tt>.

* <p/>

* This method is invoked immediately after <tt>target</tt> was created and

* before the unmarshalling of this object begins. Note that

* if the class of <tt>target</tt> defines its own <tt>beforeUnmarshal</tt> method,

* the class specific callback method is invoked before this method is invoked.

* @param target non-null instance of JAXB mapped class prior to unmarshalling into it.

* @param parent instance of JAXB mapped class that will eventually reference <tt>target</tt>.

* <tt>null</tt> when <tt>target</tt> is root element.

public void beforeUnmarshal(Object target, Object parent) {

}

/**

* <p/>

* Callback method invoked after unmarshalling XML data into <tt>target</tt>.

* <p/>

* This method is invoked after all the properties (except IDREF) are unmarshalled into <tt>target</tt>,

* but before <tt>target</tt> is set into its <tt>parent</tt> object.

* Note that if the class of <tt>target</tt> defines its own <tt>afterUnmarshal</tt> method,

* the class specific callback method is invoked before this method is invoked.

* @param target non-null instance of JAXB mapped class prior to unmarshalling into it.

* @param parent instance of JAXB mapped class that will reference <tt>target</tt>.

* <tt>null</tt> when <tt>target</tt> is root element.

public void afterUnmarshal(Object target, Object parent) {

}

Pre And Post-Processing Person Unmarshal Listener

The PersonUnmarshallListener is our listener implementation that prints some texts and stores the name value to a list. For instance, the afterUnmarshal prints out “AFTER UNMARSHAL”, the object’s content, and then holds the person’s name on a list. For the beforeUnmarshal method, it prints out “BEFORE UNMARSHAL” and the contents of the initial object.

package com.turreta.jaxb.unmarshaller.listener;

import javax.xml.bind.Unmarshaller.Listener;
public class PersonUnmarshallListener extends Listener {
	
    /**
     * Access the created object after its fields have been set to 
     * some values 
     * 
     * @param target
     * @param parent
     */
    @Override
    public void afterUnmarshal(Object target, Object parent) {
        super.afterUnmarshal(target, parent);
        System.out.println("AFTER UNMARSHAL");
        if(target instanceof Person) {
            Person p = (Person) target;
            p.getSomeList().add(p.getName());
        }
    }
	
    /**
     * Access the created initial object 
     *  
     * @param target
     * @param parent
     */
    @Override
    public void beforeUnmarshal(Object target, Object parent) {
        super.beforeUnmarshal(target, parent);
        System.out.println("BEFORE UNMARSHAL");
        if(target instanceof Person) {
            Person p = (Person) target;
            System.out.println(p);
        }
    }
}

package com.turreta.jaxb.unmarshaller.listener;

import javax.xml.bind.Unmarshaller.Listener;

public class PersonUnmarshallListener extends Listener {

/**

* Access the created object after its fields have been set to

* some values

* @param target

* @param parent

@Override

public void afterUnmarshal(Object target, Object parent) {

super.afterUnmarshal(target, parent);

System.out.println("AFTER UNMARSHAL");

if(target instanceof Person) {

Person p = (Person) target;

p.getSomeList().add(p.getName());

}

/**

* Access the created initial object

* @param target

* @param parent

@Override

public void beforeUnmarshal(Object target, Object parent) {

super.beforeUnmarshal(target, parent);

System.out.println("BEFORE UNMARSHAL");

if(target instanceof Person) {

Person p = (Person) target;

System.out.println(p);

}

JAXB Model and Test XML For Pre and Post-Processing

Our Java JAXB class represents the contents of our test XML. Then, we have a list field that we annotated with @XmlTransient. As a result, this field will not be to any element in the test XML, and we will use it to store the name of the Person object.

package com.turreta.jaxb.unmarshaller.listener;

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlTransient;

@XmlRootElement
public class Person {

    private String name;
	
    @XmlTransient
    private List<String> someList = new ArrayList<>();

    public List<String> getSomeList() {
        return someList;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", someList=" + someList + "]";
    }
}

package com.turreta.jaxb.unmarshaller.listener;

import java.util.ArrayList;

import java.util.List;

import javax.xml.bind.annotation.XmlRootElement;

import javax.xml.bind.annotation.XmlTransient;

@XmlRootElement

public class Person {

private String name;

@XmlTransient

private List<String> someList = new ArrayList<>();

public List<String> getSomeList() {

return someList;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

@Override

public String toString() {

return "Person [name=" + name + ", someList=" + someList + "]";

}

Below is our test XML.

<?xml version="1.0" encoding="UTF-8"?>
<person>
    <name>Jane Doe</name>
</person>

<?xml version="1.0" encoding="UTF-8"?>

</person>

First, we must instantiate our listener and set it as the unmarshaller’s listener.

package com.turreta.jaxb.unmarshaller.listener;

import java.io.FileInputStream;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamReader;

public class DemoApp {
    public static void main(String[] args) throws Exception { 
        JAXBContext jc = JAXBContext.newInstance(Person.class);
        XMLInputFactory xif = XMLInputFactory.newFactory();
        FileInputStream xml = new FileInputStream("src/main/resources/person.xml");
        XMLStreamReader xsr = xif.createXMLStreamReader(xml);

        Unmarshaller unmarshaller = jc.createUnmarshaller();

        // Instantiate our listener
        PersonUnmarshallListener pul = new PersonUnmarshallListener();

        // Set the unmarshaller lister to this object
        unmarshaller.setListener(pul);

        Person person = (Person) unmarshaller.unmarshal(xsr);

        System.out.println(person);
    }
}

package com.turreta.jaxb.unmarshaller.listener;

import java.io.FileInputStream;

import javax.xml.bind.JAXBContext;

import javax.xml.bind.Unmarshaller;

import javax.xml.stream.XMLInputFactory;

import javax.xml.stream.XMLStreamReader;

public class DemoApp {

public static void main(String[] args) throws Exception {

JAXBContext jc = JAXBContext.newInstance(Person.class);

XMLInputFactory xif = XMLInputFactory.newFactory();

FileInputStream xml = new FileInputStream("src/main/resources/person.xml");

XMLStreamReader xsr = xif.createXMLStreamReader(xml);

Unmarshaller unmarshaller = jc.createUnmarshaller();

// Instantiate our listener

PersonUnmarshallListener pul = new PersonUnmarshallListener();

// Set the unmarshaller lister to this object

unmarshaller.setListener(pul);

Person person = (Person) unmarshaller.unmarshal(xsr);

System.out.println(person);

}

When we run the codes, we get the following sample output. We can see the content of the JAXB object after pre and post-processing using the Unmarshaller Listener.