javax.servlet.ServletException: Class servlets.LoginServlet is not a Servlet

package servlets;

import javax.servlet.*;
import javax.servlet.http.*;

import beans.*;
import java.io.*;


public class LoginServlet extends HttpServlet{
public void doGet(HttpServletRequest request,HttpServletResponse response) throws IOException,ServletException
{
response.setContentType("text/html;charset=gb2312");
request.setCharacterEncoding("GB2312");

String username=request.getParameter("username");
String password=request.getParameter("password");
String userType=request.getParameter("userType");

//创建模型对象
UserBean user=new UserBean();
//调用业务方法进行验证
boolean b=user.validate(username, password,userType);
//要转向的文件
String forward=null;
String info=null;
try{
if(b)
{
request.getSession().setAttribute("username", username);
request.getSession().setAttribute("userType", userType);
forward="login.jsp";
}
else
{
forward="index.jsp";
info="您输入的信息不正确，请重新输入";
}
}catch(Exception e)
{
info="数据库异常！";
}
request.setAttribute("info",info);
//获取Dispatcher对象
RequestDispatcher dispatcher=request.getRequestDispatcher(forward);
//完成跳转
dispatcher.forward(request,response);
}
public void doPost(HttpServletRequest request,HttpServletResponse response)throws IOException,ServletException
{
doGet(request,response);
}
}源代码也没啥问题啊
一运行就出现这样异常求解
exception 

javax.servlet.ServletException: Class servlets.LoginServlet is not a Servlet
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
java.lang.Thread.run(Thread.java:619)


root cause 

java.lang.ClassCastException: servlets.LoginServlet cannot be cast to javax.servlet.Servlet
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
java.lang.Thread.run(Thread.java:619)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.13 logs.
--------------------编程问答-------------------- 我运行了一下，没有任何问题
package servlets;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class LoginServlet extends HttpServlet 
{
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html;charset=gb2312");
request.setCharacterEncoding("GB2312");
response.getWriter().print("ssssssssssssssssssssssssssssss");

}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
doGet(request, response);
}
}
  <servlet>
   <servlet-name>LoginServlet</servlet-name>
   <servlet-class>servlets.LoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
   <servlet-name>LoginServlet</servlet-name>
   <url-pattern>/user/login</url-pattern>
  </servlet-mapping>

--------------------编程问答-------------------- 可能是配置文件配置错了，把LoginServlet配置成filter之类的。 --------------------编程问答-------------------- 可能是配置文件配置错了，把LoginServlet错误配置成了filter之类的。 --------------------编程问答-------------------- 你的 web.xml内容是什么？

补充：Java ,  Java EE