1. com.stu.demo.Entity

package com.stu.com.stu.entity;

public class User {
private String uid;
private String pwd;

public String getUid() {
    return uid;
}

public void setUid(String uid) {
    this.uid = uid;
}

public String getPwd() {
    return pwd;
}

public void setPwd(String pwd) {
    this.pwd = pwd;
}

@Override
public String toString() {
    return "User{" +
                   "uid='" + uid + '\'' +
                   ", pwd='" + pwd + '\'' +
                   '}';
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    User user = (User) o;

    if (uid != null ? !uid.equals(user.uid) : user.uid != null) return false;
    return pwd != null ? pwd.equals(user.pwd) : user.pwd == null;

}

@Override
public int hashCode() {
    int result = uid != null ? uid.hashCode() : 0;
    result = 31 * result + (pwd != null ? pwd.hashCode() : 0);
    return result;
}
}

2. com.stu.demo.Login

package com.stu.demo;

import com.stu.com.stu.entity.User;

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 javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet(name = "LoginServlet")
public class LoginServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html; charset=utf-8");
    PrintWriter out = response.getWriter();
    HttpSession session = request.getSession();

    // 讀取session
    User user = (User) session.getAttribute("user");
    if (user != null) {
        System.out.println(user);
        out.print("<h1>已經(jīng)登陸</h1><br>");
        out.print("<a href='" + request.getContextPath() + "/doLogin'>退出登陸</a>");
    } else {
        out.print("<form action='/doLogin' method='post'>");
        out.print("<label>賬號(hào):<input type='text' placeholder='賬號(hào)' name='uid'></label><br>");
        out.print("<label>密碼:<input type='password' placeholder='密碼' name='pwd'></label><br>");
        out.print("<button>登陸</button><br>");
        out.print("</form>");
    }
}
}

3. com.stu.demo.DoLogin

package com.stu.demo;

import com.stu.com.stu.entity.User;

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 javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet(name = "DoLoginServlet")
public class DoLoginServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html; charset=utf-8");
    PrintWriter out = response.getWriter();
    HttpSession session = request.getSession();
    
    // 獲取賬號(hào)密碼
    String uid = request.getParameter("uid");
    String pwd = request.getParameter("pwd");
    if (uid.equals("123") && pwd.equals("321")) {
        // 登陸成功,將用戶信息保存到session
        User user = new User();
        user.setUid(uid);
        user.setPwd(pwd);
        session.setAttribute("user", user);
        session.setMaxInactiveInterval(60 * 15);
        out.print("<h1>登陸成功</h1>");
    } else {
        // 登陸失敗
        out.print("<h1>賬號(hào)或密碼不匹配</h1>");
        response.setHeader("refresh", "2;url=" + request.getContextPath() + "/login");
    }
}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html; charset=utf-8");
    PrintWriter out = response.getWriter();
    HttpSession session = request.getSession();
    
    // 清除session
    session.removeAttribute("user");
    // 或使用（刪除整個(gè)session）
    // session.invalidate();
    out.print("<h1>退出成功</h1>");
    response.setHeader("refresh", "2;url=" + request.getContextPath() + "/login");
}
}