1、创建 CharEncodingFilter.java

package com.by.filter;
import javax.servlet.*;
import java.io.IOException;
public class CharEncodingFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain) throws IOException, ServletException {
        //设置编码格式
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("text/html;charset=utf-8");

        //放行，即调用servlet
        filterChain.doFilter(req,resp);
    }
    @Override
    public void destroy() {
    }
}

2、创建 LoginFilter.java

package com.by.filter;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class LoginFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        //直接放行: “login.jsp”和“/”
        String uri = request.getRequestURI();
        if(uri.contains("login") || uri.endsWith("/")){
            System.out.println("uri:"+uri+"中包含login或以/结尾 --> 直接放行");
            //放行
            chain.doFilter(req,resp);
            //结束
            return;
        }
        //1、从session获取user
        Object user = request.getSession().getAttribute("user");
        //2、获取到user，则放行
        if(user != null){
            System.out.println("用户已登录 --> 直接放行");
            //放行
            chain.doFilter(req,resp);
            //结束
            return;
        }else{
            System.out.println("用户未登录 --> 重定向到 login.jsp");
            //3、没有获取到user，则重定向到login.jsp
            response.sendRedirect("login.jsp");
        }
    }
    @Override
    public void destroy() {

    }
}

2、 配置 web.xml

3、结果