Servlet错误

servlet文件 package zhangchao.com;import java.io.IOException;
import java.io.PrintWriter;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;public class Hello extends HttpServlet { /**
 * Constructor of the object.
 */
public Hello() {
super();
} /**
 * Destruction of the servlet. <br>
 */
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
} /**
 * The doGet method of the servlet. <br>
 *
 * This method is called when a form has its tag value method equals to get.
 * 
 * @param request the request send by the client to the server
 * @param response the response send by the server to the client
 * @throws ServletException if an error occurred
 * @throws IOException if an error occurred
 */
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
                doPost(request,response);

} /**
 * The doPost method of the servlet. <br>
 *
 * This method is called when a form has its tag value method equals to post.
 * 
 * @param request the request send by the client to the server
 * @param response the response send by the server to the client
 * @throws ServletException if an error occurred
 * @throws IOException if an error occurred
 */
public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException 
{
response.setContentType("text/html;charset=gb2312");
String name=request.getParameter("username");
String password=request.getParameter("password");
PrintWriter out=response.getWriter();
    out.println("<html>");
    out.println("<body>");
    out.println(name);
    out.println(password);
    out.println("</body>");
    out.println("</html>");

} public void init() throws ServletException {
// Put your code here
}}
web.xml 文件
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" 
xmlns="http://java.sun.com/xml/ns/j2ee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
  <servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>zhangchao.com.Hello</servlet-class>
  </servlet>  <servlet-mapping>
    <servlet-name>Hello</servlet-name>
    <url-pattern>/servlet/Hello</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>
jsp 文件<%@ page language="java" import="java.util.*" pageEncoding="GB2312"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <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">
-->
  </head>
  
  <body>
    <form action="/servlet/Hello">
    <table>
    <tr><td colspan="2" align="center">userlogin</td></tr>
    <tr><td>userName:</td><td><input type="text" name="username" size="10"></td></tr>
    <tr><td>Password:</td><td><input type="password" name="password" size="10"></td></tr>
    <tr><td><input type="submit" value="submit"></td><input type="reset" value="reset"></tr>
    </table>
    
    
    
    
    </form>
  </body>
</html>错误页面
HTTP Status 404 - /web/servlet/Hello--------------------------------------------------------------------------------type Status reportmessage /web/servlet/Hellodescription The requested resource (/web/servlet/Hello) is not available.
--------------------------------------------------------------------------------Apache Tomcat/6.0.20总是这个错误，在线等。谢谢了