package filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.*;

import com.tarena.crm.domain.admin.Manager;

public class LoginFilter  extends HttpServlet  implements Filter{

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req=(HttpServletRequest)request;
        HttpServletResponse res=(HttpServletResponse) response;
        if(req.getSession().getAttribute("manager")==null){
            res.sendRedirect("/crm/core/login.jsp");
        }else
        chain.doFilter(request, response);        
    }
    public void init(FilterConfig arg0) throws ServletException {
        
    }
}

------解决方案--------------------
不会过滤器的话就用JSP代码写吧。  
首先你在用户登录以后把用户的ID写到session里。
session.setAttribute("userid",userid);后边的userid是变量；
然后建一个safe.jsp，代码如下
HTML code

<%
 if(session.getAttribute("userid")==null)
{
%>
     <script>
          alert("SORRY,LOGIN PLEASE!");
         window.location.href="login.jsp";<!--这里是你的登录页面 -->
     </script>
     <%
}
%>