filter拦截后，登录也跳转回login页面了。

Listener

package com.imooc.web.listener;

import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

import com.imooc.domain.User;

@WebListener
public class InitServletContextListener implements ServletContextListener {


    public void contextDestroyed(ServletContextEvent sce)  { 
    }

    public void contextInitialized(ServletContextEvent sce)  { 
    	System.out.println("项目中数据初始化了....");
    	// 创建一个用于保存用户信息的List集合
    	List<User> userList = new ArrayList<User>();
    	User user = new User();
    	user.setUsername("m0001");
    	user.setPassword("123123");
    	userList.add(user);
    	// 将List集合存入到ServletContext域中
    	sce.getServletContext().setAttribute("userList", userList);
    }
	
}

loginServlet

package com.imooc.servlet;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.imooc.domain.User;
import com.imooc.service.UserService;
import com.imooc.serviceimpl.UserServiceImpl;

@WebServlet("/login.do")
public class LoginServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		System.out.println("进入登录servlet");
		// 完成验证码的校验:
		// 获得session中保存的验证码的信息
		String code1 = (String) request.getSession()
				.getAttribute(com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY);
		// 接收前台表单提交的验证码的信息
		String code2 = request.getParameter("verifyCode");
		if (code2 == null || !code2.equalsIgnoreCase(code1)) {
			request.setAttribute("msg", "验证码输入不正确!");
			request.getRequestDispatcher("/login.jsp").forward(request, response);
			return;
		}
		// 接收数据
		String username = request.getParameter("username");
		String password = request.getParameter("password");
		// 封装数据
		User user = new User();
		user.setPassword(password);
		user.setUsername(username);
		UserService userService = new UserServiceImpl();
		// 获取用户列表
		List<User> userList = (List<User>) getServletContext().getAttribute("userList");
		User existUser = userService.login(userList, user);
		if (existUser == null) {
			// 登录失败
			request.setAttribute("msg", "账号或密码输入错误！");
			request.getRequestDispatcher("login.jsp").forward(request, response);
		} else {
			// 登录成功
			request.setAttribute("msg", "登录成功！");
			//存储登录用户信息
			request.setAttribute("existUser", existUser);
			response.sendRedirect(request.getContextPath() + "/categoryList.jsp");
		}
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
	 *      response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

filter

package com.imooc.filter;

import java.io.IOException;
import java.util.List;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.imooc.domain.User;
@WebFilter(filterName = "loginFilter",urlPatterns = "*.jsp")
public class LoginFilter implements Filter {

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("注解形式过滤器已生效");
		HttpServletRequest Httprequest = (HttpServletRequest)request;
		HttpServletResponse Httpresponse = (HttpServletResponse)response;
		User user = (User) Httprequest.getAttribute("existUser");
		String uri = Httprequest.getRequestURI();
		if(user!=null) {
			//用户已经登录
			chain.doFilter(request, response);
			return;
		}if(uri!=null&&uri.indexOf("login")==-1){
			//用户没有登录且uri不是登录页面
			Httprequest.setAttribute("msg", "尚未登录，请先登录！");
			Httpresponse.sendRedirect(Httprequest.getContextPath()+"/login.jsp");
		}else {
			chain.doFilter(request, response);
		}
	}

}