Conexão com banco Oracle via Data Source - erro

<Context path="/ga" reloadable="true" docBase="C:\work\ga" workDir="C:\work\ga\work" 
        		debug="5" crossContext="true">

  				<Logger className="org.apache.catalina.logger.FileLogger"
            		prefix="localhost_ga_log." suffix=".txt"
             			timestamp="true"/>

  				<Resource name="jdbc/OracleAmbientalDB"
              		auth="Container"
              	 	type="javax.sql.DataSource"/>

  				<ResourceParams name="jdbc/OracleAmbientalDB">
    				<parameter>
      					<name>factory</name>
      					<value>org.apache.commons.dbcp.BasicDataSourceFactory</value>
   					</parameter>


    				<parameter>
       					<name>driverClassName</name>
       					<value>oracle.jdbc.driver.OracleDriver</value>
    				</parameter>
    		
    				
    			
    				
    			
    				<parameter>
      					<name>maxWait</name>
      					<value>-1</value>
    				</parameter>


    				<parameter>
     					<name>username</name>
     					<value>ga</value>
    				</parameter>
    				<parameter>
     					<name>password</name>
     					<value>ga</value>
    				</parameter>
    
    				<parameter>
      					<name>url</name>
      					<value>jdbc:oracle:thin:@192.168.0.2:1521:desenv</value>
    				</parameter>
    
    				
        			<parameter>
              			<name>removeAbandoned</name>
              			<value>true</value>
            		</parameter>

    				<parameter>
              			<name>removeAbandonedTimeout</name>
              			<value>60</value>
            		</parameter>
	

					<parameter>
              			<name>logAbandoned</name>
              			<value>true</value>
            		</parameter>
				</ResourceParams>
			</Context>

<resource-ref>
 	<description>Oracle Datasource</description>
	<res-ref-name>jdbc/OracleAmbientalDB</res-ref-name>
	<res-type>javax.sql.DataSource</res-type>
	<res-auth>Container</res-auth>
  </resource-ref>

import java.sql.Connection;
import java.sql.SQLException;

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

public class ConnectionFactory {
    
    private static Connection conn;
    
    public static Connection getConnection(){
        try {
            Context initContext = new InitialContext();
            Context envContext  = (Context)initContext.lookup("java:/comp/env"); // ESSA É LINHA 18!!!
            DataSource ds = (DataSource)envContext.lookup("jdbc/OracleAmbientalDB");
            conn = ds.getConnection();
        }catch (NamingException ne){
            ne.printStackTrace();
        }catch (SQLException se){
            se.printStackTrace();
        }

        return conn;
        
    }
    
    public static void main(String[] args) {
        getConnection();
    }
}

javax.naming.NoInitialContextException: Need to specify class name in environment or system property, or as an applet parameter, or in an application resource file:  java.naming.factory.initial
	at javax.naming.spi.NamingManager.getInitialContext(NamingManager.java:640)
	at javax.naming.InitialContext.getDefaultInitCtx(InitialContext.java:243)
	at javax.naming.InitialContext.getURLOrDefaultInitCtx(InitialContext.java:280)
	at javax.naming.InitialContext.lookup(InitialContext.java:347)
	at br.com.gaea.util.ConnectionFactory.getConnection(ConnectionFactory.java:18) - ////Esta marcada acima qual é !!
	at br.com.gaea.util.ConnectionFactory.main(ConnectionFactory.java:32)