servlet实现登录功能【当用户当前未登陆，跳转登录页面才能访问，若已经登录了，才可以直接访问】

1. 前端

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body><form action="login" method="POST"><input type="text" name="username"><input type="password" name="password"><input type="submit" value="提交"></form>
</body>
</html>

2. 后端登录

package EnableUserLogin;import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpSession;
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 java.io.IOException;
@WebServlet("/login")
public class LoginServlet extends HttpServlet {@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {resp.setContentType("text/html; charset=utf-8");// 1. 获取到用户提交的用户名和密码String username = req.getParameter("username");String password = req.getParameter("password");// 2. 判定用户名密码是否正确if (!username.equals("admin") || !password.equals("123")) {// 登陆失败resp.getWriter().write("登陆失败");return;}// 登陆成功System.out.println("登陆成功");// 设置 SessionHttpSession session = req.getSession(true);session.setAttribute("username", "admin");session.setAttribute("loginCount", "0");resp.sendRedirect("index");}
}

3. 后端检验页面

package EnableUserLogin;import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpSession;
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 java.io.IOException;@WebServlet("/index")
public class IndexServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {resp.setContentType("text/html; charset=utf-8");// 1. 判定当前用户是否已经登陆HttpSession session = req.getSession(false);if (session == null) {// 用户没有登陆, 重定向到 login.htmlresp.sendRedirect("login.html");return;}// 2. 如果已经登陆, 则从 Session 中取出访问次数数据String userName = (String)session.getAttribute("username");String countString = (String)session.getAttribute("loginCount");int loginCount = Integer.parseInt(countString);loginCount += 1;session.setAttribute("loginCount", loginCount + "");// 3. 展示到页面上.StringBuilder html = new StringBuilder();html.append(String.format("<div>用户名: %s</div>", userName));html.append(String.format("<div>loginCount: %d</div>", loginCount));resp.getWriter().write(html.toString());}
}