JasperReport java

Bonjour,
SVP si quelqu'un a déjà travaillé avec Ireport, JasperReport il me donne deux messages d'erreurs :
log4j:WARN No appenders could be found for logger (net.sf.jasperreports.engine.xml.JRXmlDigesterFactory).
log4j:WARN Please initialize the log4j system properly.

package Dao;

import java.util.HashMap;
import java.util.Map;
import net.sf.jasperreports.engine.JRException;
import net.sf.jasperreports.engine.JasperCompileManager;
import net.sf.jasperreports.engine.JasperExportManager;
import net.sf.jasperreports.engine.JasperFillManager;
import net.sf.jasperreports.engine.JasperPrint;
import net.sf.jasperreports.engine.JasperReport;
import net.sf.jasperreports.engine.design.JasperDesign;
import net.sf.jasperreports.engine.xml.JRXmlLoader;

/**
*
* @author Ayoub
*/
public class generatePdf {

public static void PDF() {
try {

//chargement et compilation du rapport
JasperDesign jasperDesign = JRXmlLoader.load("C:\\categorie.jrxml");
JasperReport jasperReport = JasperCompileManager.compileReport(jasperDesign);

Map parameters = new HashMap();
parameters.put("Titre", "1");

JasperPrint jasperPrint = JasperFillManager.fillReport(jasperReport, parameters, connexion.connectWP_CMS());

JasperExportManager.exportReportToPdfFile(jasperPrint, "C:\\categorie.pdf");

} catch (JRException e) {
e.getStackTrace();
}
}

public static void main(String[] args) {
PDF();
}
}
merci d'avance