判断session是否登陆

package com.abc.zp.filter;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class LoginFilter implements Filter
{
public void init(FilterConfig filterConfig)
          throws ServletException
{
//System.out.println("过滤器初始化*****");
}
public void doFilter(ServletRequest request,
              ServletResponse response,
              FilterChain chain)
              throws IOException,
                     ServletException
{
//Session属于Http范畴,所以ServletRequest对象需要先转换成HttpServletRequest对象
HttpServletRequest req = (HttpServletRequest)request;
HttpSession session = req.getSession();
//如果session不为空,则可以浏览其他页面
if(session.getAttribute("username")!=null)
{
chain.doFilter(request,response);
}
else
{
request.getRequestDispatcher("login.jsp").forward(request,response);
}
}
public void destroy()
{
System.out.println("过滤器销毁******");
}
}