开发者学堂课程【Java Web项目实战2：图书商城：前台登录过滤器】学习笔记，与课程紧密联系，让用户快速学习知识。
课程地址：
https://developer.aliyun.com/learning/course/35/detail/805
前台登录过滤器
1．从 session 中获取用户信息
2．判断如题 session 中存在用户信息,放行!
3．否则，保存错误信息，转发到 login.jsp 显示
package cn.itcast.bookstore.user.web.filter;
import java.io.IOException;
public class LoginFilter implements Filter {
public void destroy() {
}
public void doFilter(ServletRequest request,ServletResponse response,
FilterChain chain) throws IOException，servletException {
/*
*1．从 session 中获取用户信息
*2．判断如题 session 中存在用户信息,放行!
*3．否则，保存错误信息，转发到 login.jsp 显示
*/
HttpservletRequest httpRequest =(HttpservletRequest) request;
User user = (User)httpRequest.getsession ( ).getattribute ("session_user");
if(user != null){
chain.doFilter (request, response) ;
} else {
httpRequest.setattribute ( "msg"，"您还没有登录!");
httpRequest.getRequestDispatcher("/jsps/user/login.jsp")
.forward (httpRequest, response) ;
