Erro na configuração do Pool Tomcat

java.lang.Exception: Cannot create JDBC driver of class '' for connect URL 'null'

<?xml version="1.0" encoding="UTF-8"?>  
	<Context>   
    	<Resource  
         	name="jdbc/HELLOTALK"
         	type="javax.sql.DataSource"  
          	maxActive="4"  
          	maxIdle="2"  
         	maxWait="5000"  
          	username="sa"  
         	password="xxxxxxx"  
         	driverClassName="net.sourceforge.jtds.jdbc.Driver"
        	url="jdbc:jtds:sqlserver://localhost:1433/HELLOTALK"
         	auth="Container"
         	removeAbandoned="true"  
         	removeAbandonedTimeout="60"  
         />  
        
 </Context>

<resource-ref>  
       <description>Conexão Banco de Dados</description>   
       <res-ref-name>jdbc/HELLOTALK</res-ref-name>  
       <res-type>javax.sql.DataSource</res-type>  
       <res-auth>Container</res-auth>  
</resource-ref>

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.sql.DataSource;

public class conPoolTomcat {

	private java.sql.Connection conexao;
	
	public static Connection getConexao(String JNDINome) throws Exception {
        Connection con = null;

        InitialContext contexto = new InitialContext();  
        DataSource ds = (DataSource)contexto.lookup("java:comp/env/jdbc/HELLOTALK");  
         
        try {

        	return ds.getConnection(); 
        } catch (SQLException e) {

        	throw new Exception(e.getMessage());
        }
        
    }

}