Tomcat数据源 数据库连接池问题

<%@ page contentType="text/html" pageEncoding="gbk"%>

<%@ page import="java.sql.*,javax.sql.*,javax.naming.*"%>



<%

	 String DSNAME="java:comp/env/jdbc/mydatabase" ;

		Context ctx = new InitialContext() ;

		DataSource ds = (DataSource) ctx.lookup(DSNAME);

		Connection conn = ds.getConnection() ;

	

%>

<%=conn%>

连接成功并关闭

<%

	conn.close() ;//将连接放回池中

%>

Tomcat安装目录下的server.xml和web-inf下的web.xml都正确配置了，而且这个jsp页面正常输出了conn连接的路径
但是下面的：

package sjy.xg.myc.datasource;



import java.sql.*;//Connection

import javax.sql.*;//DataSource

import javax.naming.*;//Context

public class DatabaseConnection{

	private Connection conn = null ;

	private String DSNAME="java:comp/env/jdbc/mydatabase" ;

	public DatabaseConnection() throws Exception{

		Context ctx = new InitialContext() ;

		DataSource ds = (DataSource) ctx.lookup(DSNAME);

		this.conn = ds.getConnection() ;

	}

	

	public Connection getConnection(){

		return this.conn ;

	}

	

	public void close() throws Exception {

		if(this.conn != null){

			try{

				conn.close() ;

			}catch(Exception e){

				throw e ;

			}	

		}

	}



        public static void main(String[] args) throws Exception{

	   Connection conn = new DatabaseConnection().getConnection();

            System.out.println("sucess"+conn);

        }

	

}

老出现错误：
Exception in thread "main" javax.naming.NoInitialContextException: Need to specify class name in env
ironment or system property, or as an applet parameter, or in an application resource file: java.na
ming.factory.initial
at javax.naming.spi.NamingManager.getInitialContext(NamingManager.java:645)
at javax.naming.InitialContext.getDefaultInitCtx(InitialContext.java:288)
at javax.naming.InitialContext.getURLOrDefaultInitCtx(InitialContext.java:325)
at javax.naming.InitialContext.lookup(InitialContext.java:392)
at sjy.xg.myc.datasource.DatabaseConnection.<init>(DatabaseConnection.java:11)
at sjy.xg.myc.datasource.DatabaseConnection.main(DatabaseConnection.java:30)

怎么解决？？？？？