使用filter过滤器实现对用户登录的检查，如果未登录跳转至登录页面

        对于前几天写的一个简单的学生管理系统，想要实现对于用户登陆检查的功能，就上网自学了filter知识

        第一步先在项目目录中创建包filter，在此包下面创建filter名为LoginFilter，代码如下

package com.student.filter;

import com.student.entity.Users;
import com.sun.deploy.net.HttpRequest;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;

/**
 * @Description
 * @Author 单继重
 * @Date 2016/10/24 10:19
 */
@WebFilter(filterName = "LoginFilter")
public class LoginFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        //chain.doFilter(req, resp);
        HttpServletRequest request = (HttpServletRequest)req;
        HttpServletResponse response = (HttpServletResponse)resp;
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        HttpSession session = request.getSession();
        PrintWriter out = response.getWriter();
        Users user = (Users)session.getAttribute("user");
        if(user != null){
            chain.doFilter(request,response);
        } else{
            out.println("您还未登陆，三秒钟后跳转至登录页面");
            //out.println("<script language='javascript'>alert('你还未登录');");
            response.setHeader("refresh","3;/pages/users/login.jsp");
            //response.sendRedirect("/pages/users/login.jsp");
            //request.getRequestDispatcher("/pages/users/login.jsp").forward(request,response);
        }
    }

    public void init(FilterConfig config) throws ServletException {

    }

}

        待解决问题：JS的alert方法无法实现，不知道原因。

        未登录跳页使用response.setHeader实现定时跳页功能。

         第二步在web.xml文件中配置相应文件，部分代码如下

<filter>
        <filter-name>LoginFilter</filter-name>
        <filter-class>com.student.filter.LoginFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/pages/student/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/pages/users/update.jsp</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/index.jsp</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/servlet/StudentServlet</url-pattern>
    </filter-mapping>

         然后在以上配置中的所有页面检查session中是否有登陆后存入的信息，如果没有，则跳转至登录页面