What is the cause for an HTTP 405 Error in GAE application?

I'm using eclipse to develop a Google App Engine application. I am on a login page, but I click login, then it shows "Error 405 HTTP method POST is not supported by this URL"?

Here is my servlet code:

my LgServlet.java:

package com.lan; import java.io.IOException; import javax.servlet.http.*; public class LgServlet extends HttpServlet { /** * */ private static final long serialVersionUID = 1L; @Override public void doPost(HttpServletRequest req,HttpServletResponse resp) throws IOException { check(req,resp); } @Override public void doGet(HttpServletRequest req,HttpServletResponse resp) throws IOException { check(req,resp); } public void check(HttpServletRequest req,HttpServletResponse resp) throws IOException { if(req.getAttribute("userID").equals("admin")&&req.getAttribute("userPassword").equals("123456")){ resp.setContentType("text/plain"); resp.getWriter().println("Hello Admin"); } } } 

my web.xml:

<?xml version="1.0" encoding="utf-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5"> <servlet> <servlet-name>lg</servlet-name> <servlet-class>com.lan.LgServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>lg</servlet-name> <url-pattern>/lg</url-pattern> </servlet-mapping> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list> </web-app> 

part of index.jsp:

<form action="lg" method="post"> 

I hope someone can help explain to me why I'm getting this error?

Thanks!