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Using JAXB to Generate Java Objects from XML Document

01.29.2013
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Quite sometime back I had written about Using JAXB to generate XML from the Java, XSD. And now I am writing how to do the reverse of it i.e generating Java objects from the XML document. There was one comment mentioning that JAXB reference implementation and hence in this article I am making use of the reference implementation that is shipped with the JDK.

Firstly the XML which I am using to generate the java objects are:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 
<expenseReport>
  <user>
    <userName>Sanaulla</userName>
  </user>
  <items>
    <item>
      <itemName>Seagate External HDD</itemName>
      <purchasedOn>August 24, 2010</purchasedOn>
      <amount>6776.5</amount>
    </item>
    <item>
      <itemName>Benq HD Monitor</itemName>
      <purchasedOn>August 24, 2012</purchasedOn>
      <amount>15000</amount>
    </item>
  </items>
</expenseReport>

And the XSD to which it conforms to is:

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="expenseReport" type="ExpenseT" />
  <xs:complexType name="ExpenseT">
    <xs:sequence>
      <xs:element name="user" type="UserT" />
      <xs:element name="items" type="ItemListT" />
    </xs:sequence>
  </xs:complexType>
 
  <xs:complexType name="UserT">
    <xs:sequence>
      <xs:element name="userName" type="xs:string" />
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="ItemListT">
    <xs:sequence>
      <xs:element name="item" type="ItemT" maxOccurs="unbounded" />
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="ItemT">
    <xs:sequence>
      <xs:element name="itemName" type="xs:string" />
      <xs:element name="purchasedOn" type="xs:string" />
      <xs:element name="amount" type="xs:decimal" />
    </xs:sequence>
  </xs:complexType>
</xs:schema>

The XSD has already been explained and one can find out more by reading this article.

I create a class: XmlToJavaObjects which will drive the unmarshalling operation and before I generate the JAXB Classes from the XSD, the directory structure is:
DirStruct

I go ahead and use the xjc.exe to generate JAXB classes for the given XSD:

$> xjc expense.xsd

and I now refresh the directory structure to see these generated classes as shown below:
DirStruct2
With the JAXB Classes generated and the XML data available, we can go ahead with the Unmarshalling process.

Unmarshalling the XML:

To unmarshall:

  1. We need to create JAXContext instance.
  2. Use JAXBContext instance to create the Unmarshaller.
  3. Use the Unmarshaller to unmarshal the XML document to get an instance of JAXBElement.
  4. Get the instance of the required JAXB Root Class from the JAXBElement.

Once we get the instance of the required JAXB Root class, we can use it to get the complete XML data in Java objects. The code to unmarshal the XML data is given below:

package problem;
 
import generated.ExpenseT;
import generated.ItemListT;
import generated.ItemT;
import generated.ObjectFactory;
import generated.UserT;
 
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBElement;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;
 
public class XmlToJavaObjects {
 
  /**
   * @param args
   * @throws JAXBException 
   */
  public static void main(String[] args) throws JAXBException {
    //1. We need to create JAXContext instance
    JAXBContext jaxbContext = JAXBContext.newInstance(ObjectFactory.class);
     
    //2. Use JAXBContext instance to create the Unmarshaller.
    Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
 
    //3. Use the Unmarshaller to unmarshal the XML document to get an instance of JAXBElement.
    JAXBElement<ExpenseT> unmarshalledObject = 
        (JAXBElement<ExpenseT>)unmarshaller.unmarshal(
            ClassLoader.getSystemResourceAsStream("problem/expense.xml"));
 
    //4. Get the instance of the required JAXB Root Class from the JAXBElement.
    ExpenseT expenseObj = unmarshalledObject.getValue();
    UserT user = expenseObj.getUser();
    ItemListT items = expenseObj.getItems();
 
    //Obtaining all the required data from the JAXB Root class instance.
    System.out.println("Printing the Expense for: "+user.getUserName());
    for ( ItemT item : items.getItem()){
      System.out.println("Name: "+item.getItemName());
      System.out.println("Value: "+item.getAmount());
      System.out.println("Date of Purchase: "+item.getPurchasedOn());
    }   
  }
 
}

And the output would be:
Output

Do drop in your queries/feedback as comments and I will try to address them at the earliest.



Published at DZone with permission of Mohamed Sanaulla, author and DZone MVB. (source)

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