检测session是否为空

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package cn.toher.filter;

import cn.toher.bean.User;
import java.io.IOException;
import java.io.PrintWriter;
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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 *
 * 
 * @date 2015-4-28 15:30:52
 */
public class LoginFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        
    }
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest servletRequest = (HttpServletRequest) request;
        HttpServletResponse servletResponse = (HttpServletResponse) response;
        HttpSession session = servletRequest.getSession();
        User user = (User)session.getAttribute("Suser");
//      过滤session是不是为空，为空则remove一下，然后跳到登录界面
        if(user!=null){
            chain.doFilter(request, response);
        }else{
            session.removeAttribute("Suser");
            PrintWriter out = servletResponse.getWriter();
            out.print("<script language='javascript'>");
            out.print("top.location.href='/index.jsp?error=timeout';");
            out.print("</script>");
        }
    }
    @Override
    public void destroy() {
        
    }

}

过滤*.do的文件