JAXB to map Java objects to XML

JAXB is a java object to XML mapping framework. It makes it rather easy to convert Java objects to and from XML. I've previously used Java DOM trees to create and write XML or XPath expressions when only reading XML.

There are two approaches in to code JABX:

1. Start with Java objects
2. Start with XML definition (typically an XML schema)

If you would like to go for the second approach you create a XML Schema file mySchema.xsd:

<?xml version="1.0" encoding="UTF-8"?>
<schema
xmlns="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.lesc.se/blog/test/mySchemaV1.0"
xmlns:tns="http://www.lesc.se/blog/test/mySchemaV1.0"
elementFormDefault="qualified">

<element name="persons">
<complexType>
<sequence>
  <element name="person" maxOccurs="unbounded" minOccurs="0">
    <complexType>
      <sequence>
        <element name="name" type="string" />
        <element name="age" type="int" />
      </sequence>
    </complexType>
  </element>
</sequence>
</complexType>
</element>
</schema>

Then run this the xjc command (it is included in the JDK installation in the bin-directory) to generate Java classes:

xjc mySchema.xsd

I like to start with the Java objects since I'm a programmer and like to have control over the Java code. To do this create the Java objects and annotate the class. For example like this:

package se.lesc.blog.test.myschemav1;

import java.util.*;
import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType
@XmlRootElement(name = "persons")
public class Persons {

 @XmlElement(name = "person")
 protected List<Persons.Person> persons =  new LinkedList<Persons.Person>();

 public List<Persons.Person> getPersons() {
   return persons;
 }

 @XmlAccessorType(XmlAccessType.FIELD)
 @XmlType
 public static class Person {

   protected String name;
   protected int age;

   public String getName() {
     return name;
   }

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

   public int getAge() {
     return age;
   }

   public void setAge(int value) {
     this.age = value;
   }
 }
}

You then need to make JAXB aware of the two classes you've created in special file jaxb.index:

Persons
Persons$Person

You also need a package annotation (package-info.java):

@javax.xml.bind.annotation.XmlSchema(namespace = "http://www.lesc.se/blog/test/mySchemaV1.0", elementFormDefault = javax.xml.bind.annotation.XmlNsForm.QUALIFIED)
package se.lesc.blog.test.myschemav1;

Then it is easy to produce XML from this:

//Set up your model
Persons persons = new Persons();
Person someOne = new Person();
someOne.setName("A name");
someOne.setAge(40);
persons.getPersons().add(someOne);

JAXBContext jaxbContext = JAXBContext.newInstance("se.lesc.blog.test.myschemav1");
Marshaller marshaller = jaxbContext.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
marshaller.marshal(persons, System.out);

Or you can read from XML back into java objects:

String xml = "<?xml.... your xml here";
StringReader xmlReader = new StringReader(xml);
JAXBContext jaxbContext = JAXBContext.newInstance("se.lesc.blog.test.myschemav");
Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
Persons persons = (Persons) unmarshaller.unmarshal(xml);