package com.test.filter;
  
import java.io.IOException;
import java.io.PrintWriter;
  
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
  
  
public class LoginFilter implements Filter {
    public static final String login_page = "/test/admin/index.jsp";
 public static final String logout_page = "/test/admin/Public/login.jsp";
 public void destroy(){
  
 }
 public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)throws ServletException, IOException {
 HttpServletRequest request = (HttpServletRequest)servletRequest;
 HttpServletResponse response = (HttpServletResponse)servletResponse;
 String currentURL = request.getRequestURI();
 String ctxPath = request.getContextPath();
 //除掉项目名称时访问页面当前路径
 String targetURL = currentURL.substring(ctxPath.length());
 HttpSession session = request.getSession(false);
 //对当前页面进行判断，如果当前页面不为登录页面
 if(!("/admin/Public/login.jsp".equals(targetURL))){
  System.out.println("1"+targetURL+"ctxPath:"+ctxPath+"currentURL:"+currentURL);
  //在不为登陆页面时，再进行判断，如果不是登陆页面也没有session则跳转到登录页面，
  if(session == null || session.getAttribute("admin") == null){
  response.sendRedirect(logout_page);
  return;
  }else{
  //这里表示正确，会去寻找下一个链，如果不存在，则进行正常的页面跳转
  filterChain.doFilter(request, response);
  return;
  }
 }else{
  //这里表示如果当前页面是登陆页面，跳转到登陆页面
  filterChain.doFilter(request, response);
  return;
 }
  
 }
 public void init(FilterConfig filterConfig)throws ServletException{
  
 }
  
}

<filter>
 <filter-name>LoginFilter</filter-name>
 <filter-class>com.test.filter.LoginFilter</filter-class>
 </filter>
 <filter-mapping>
 <filter-name>LoginFilter</filter-name>
 //这里表示对所有的以jsp后缀的文件有效，其它的无效
 <url-pattern>*.jsp</url-pattern>
</filter-mapping>