调用servlet时出错

index.jsp文件：
<%@ page language="java" import="java.util.*" pageEncoding="gb2312"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">    
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
<script type="text/javascript">
function search(){
with(document.biao){
action='../servlet/SelectServlet';
method='post';
target='_blank';
submit();
}
}
</script>
  </head>
  
  <body>
    <form name='biao'>
     <input type='button' name='btn' value='查询'onClick='search()'/>
    </form>
  </body>
</html>SelectServlet文件：
package com.feifei.servlet;import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.sql.DataSource;public class SelectServlet extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException { response.setContentType("text/html");
PrintWriter out = response.getWriter();

 Context ctx = null;
 DataSource ds = null;
 Connection conn = null;
 Statement st = null;
 PreparedStatement prst = null;
 ResultSet rs = null;
 ResultSetMetaData meta = null;
 String sql = "SELECT * FROM feifei";
 
 try {
ctx = new InitialContext();
ds = (DataSource) ctx.lookup("java:comp/env/jdbc/feifei");
conn = ds.getConnection();
st = conn.createStatement();
rs = st.executeQuery(sql);

out.print("<table border='1' bgcolor='black'>");
while(rs.next()){

out.print("<tr>");
out.print("<td bgcolor='white'>");
out.print(rs.getString(1));
out.print("</td>");
out.print("<td bgcolor='white'>");
out.print(rs.getString(2));
out.print("</td>");
out.print("<td bgcolor='white'>");
out.print(rs.getInt(3));
out.print("</td>");
out.print("</tr>");
}
    out.print("</table>");

} catch (NamingException e) {
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

out.flush();
out.close();
}}web.xml文件：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
xmlns="http://java.sun.com/xml/ns/javaee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>SelectServlet</servlet-name>
    <servlet-class>com.feifei.servlet.SelectServlet</servlet-class>
  </servlet>  <servlet-mapping>
    <servlet-name>SelectServlet</servlet-name>
    <url-pattern>/servlet/SelectServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
   <resource-ref>
<res-ref-name>jdbc/feifei</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
  </resource-ref>
</web-app>问题：在点击“查询”按钮的时候，在调用servlet时出现HTTP 400错误，而且地址栏是：http://localhost:8080/server/SelectServer,这是什么原因。。
（本人刚学J2EE，请大侠们多多指教）