Java validation de Schéma XML: préfixe qui n'est pas lié

J'ai suivi ce tutoriel pour la validation des fichiers XML. Mais je suis la réception d'exception lors de la validation du fichier XML. Ce que je fais de mal? Mes codes:
Schéma XML:

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

<!-- definition of simple elements -->
<xs:element name="first_name" type="xs:string" />
<xs:element name="last_name" type="xs:string" />
<xs:element name="phone" type="xs:string" />

<!-- definition of attributes -->
<xs:attribute name="type" type="xs:string" use="required"/>
<xs:attribute name="date" type="xs:date" use="required"/>

<!-- definition of complex elements -->

<xs:element name="reporter">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="first_name" />
            <xs:element ref="last_name" />
            <xs:element ref="phone" />
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="report">
    <xs:complexType>
        <xs:attribute ref="type"/>
        <xs:attribute ref="date" />
        <xs:sequence>
            <xs:element ref="reporter" />
        </xs:sequence>
    </xs:complexType>
</xs:element>

Fichier XML pour valider:

<?xml version="1.0" encoding="utf-8" ?>
<report type="5" date="2012-12-14">
    <reporter>
        <first_name>FirstName</firstname>
        <last_name>Lastname</lastname>
        <phone>+xxxxxxxxxxxx</phone>
    </reporter>
</report>

Source Java pour la validation:

import javax.xml.XMLConstants;
import javax.xml.transform.Source;
import javax.xml.transform.stream.StreamSource;
import javax.xml.validation.*;
import org.xml.sax.SAXException;
import java.io.*;

public class ProtocolValidator
{
    public static void main(String [] args) throws Exception
    {
        Source schemaFile = new StreamSource(new File("schema.xsd"));
        Source xmlFile = new StreamSource(new File("test_xml.xml"));

        SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
        Schema schema = schemaFactory.newSchema(schemaFile);
        Validator validator = schema.newValidator();

        try{
            validator.validate(xmlFile);
            System.out.println(xmlFile.getSystemId() + " is valid");
        }
        catch (SAXException e) 
        {
            System.out.println(xmlFile.getSystemId() + " is NOT valid");
            System.out.println("Reason: " + e.getLocalizedMessage());
        }
    }
}

Exception que je reçois:

Exception in thread "main" org.xml.sax.SAXParseException; systemId: file:/root/test/schema.xsd; lineNumber: 4; columnNumber: 50; The prefix "xs" for element "xs:element" is not bound.
    at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:198)...

OriginalL'auteur torayeff | 2012-12-14