连接池问题 Need to specify class name in envir

<Resource name="news" auth="Container" type="javax.sql.DataSource" maxActive="1000000" maxIdle="30" maxWait="10000" username="system" password="ok" driverClassName="oracle.jdbc.driver.OracleDriver" url="jdbc:oracle:thin:@localhost:1521:orcl"/>


<resource-ref>
<res-ref-name>news</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
运行：
public static void main(String[] args){

try {
Context ctx = new InitialContext();

Context envContext = (Context)ctx.lookup("java:/comp/env");
DataSource ds = (DataSource)envContext.lookup("news");

Connection conn = ds.getConnection();
conn.close();
System.out.print("连接成功");
} catch (NamingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
报错：
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: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 dao.ForumDao.main(ForumDao.java:33)用实例类进行测试时候遇到了上述问题，用Servlet或jsp页面却是成功的，没有错误。为什马呢？

浏览器访问test.jsp没问题，显示连接成功！
test.jsp：
<%@ page language="java" import="javax.naming.*,javax.sql.*,java.sql.*" pageEncoding="GBK"%>
<%
Context ctx = new InitialContext();
DataSource ds = (DataSource) ctx.lookup("java:comp/env/news");
Connection con = ds.getConnection();
out.print("连接成功");

%>