Message "log4j:WARN No appenders could be found for logger"

bonjour tout le monde

j' ai fais une application web j' ai un message d 'erreur voici :

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log4j:WARN No appenders could be found for logger (com.vente.implDao.UserImpl).
log4j:WARN Please initialize the log4j system properly.

voici le code de la classe

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package com.vente.implDao;
 
import java.util.ArrayList;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import com.vente.beans.User;
import com.vente.dao.UserDao;
 
public class UserImpl implements UserDao {
	private static final Log log = LogFactory.getLog(UserImpl.class);
 
    @PersistenceContext
	 private EntityManager en=null;
 
	@Override
	public void add(User obj) {
		try{
			log.debug("User :"+obj);
			en.persist(obj);
			en.getTransaction().commit();
	    }catch(Exception e){log.error("add user  failed :"+e);};	
	}
 
	@Override
	public ArrayList<User> findAll() {
		ArrayList<User> result=null;
		try{
		   result =(ArrayList<User>)en.createQuery("select from user").getResultList();
		}catch(Exception e){log.error("find user failed :" +e);};
 
		return result;
	}
 
	@Override
	public User findByid(long id) {
		// TODO Auto-generated method stub
		return null;
	}
 
	@Override
	public void remove(User obj) {
		try{
			en.remove(obj);
	    }catch(Exception e){log.error("add adresse failed", e);};
 
	}
 
	@Override
	public void update(User obj) {
		// TODO Auto-generated method stub
 
	}
 
}