今天有人问如何实现用户不能重复登陆？
我认为是第二个人登录时，第一个用户掉线。

我的思路：

1、当用户登录时，记录下sessionId → key=username，value=session
2、用户再次登录时，检查key是否存在，再次登录是存在的，此时，把session换成再次登录的sessionId，原来的sessionid从httpsession中移除

需要考虑的情况：

1、系统重启
2、不能实现上一个用户登录了，第二个用户不能登录，只能是第二个用户登录使第一个掉线

项目代码：

https://github.com/xcocean/killsession
用到@ServletComponentScan和@WebListener
注意：Application启动入口需要添加@ServletComponentScan注解，否则监听失败

实现如下：

1、创建拦截器：

UserInterceptor

@Component // 给spring托管
public class UserInterceptor implements HandlerInterceptor {

  @Override
  public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
      throws Exception {
    if (request.getSession().getAttribute("username") == null) {
      response.sendRedirect("/login");//重定向到登录
      return false;
    }
    return true;
  }
}

2、配置拦截器

InterceptorConfig

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

@Configuration
public class InterceptorConfig implements WebMvcConfigurer {

  @Autowired
  private UserInterceptor userInterceptor;

  @Override
  public void addInterceptors(InterceptorRegistry registry) {
    //添加拦截器，拦截所有路径
    registry.addInterceptor(userInterceptor)
        .addPathPatterns("/**")
        .excludePathPatterns("/login");//排除登录的路径
  }
}

3、创建session监听

SessionListenerConfig

import com.lingkang.killsession.service.SessionService;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import org.springframework.beans.factory.annotation.Autowired;

/**
 *  session 监听事件
 */
@WebListener
public class SessionListenerConfig implements HttpSessionListener {

  @Autowired
  private SessionService sessionService;

  /* Session创建事件 */
  @Override
  public void sessionCreated(HttpSessionEvent event) {
    sessionService.create(event.getSession());
  }
}

4、创建SessionService

import javax.servlet.http.HttpSession;

public interface SessionService {

  void delete(String sessionId);

  void create(HttpSession httpSession);
}

实现如下：
SessionServiceImpl

import com.lingkang.killsession.service.SessionService;
import java.util.HashMap;
import java.util.Map;
import javax.servlet.http.HttpSession;
import org.springframework.stereotype.Service;

@Service
public class SessionServiceImpl implements SessionService {

  //用于存储全部的session，key=sessionid
  private Map<String, HttpSession> sessionMap = new HashMap<>();

  public void create(HttpSession httpSession) {
    sessionMap.put(httpSession.getId(), httpSession);
  }

  public void delete(String sessionId) {
    try {
      //移除会话id
      sessionMap.get(sessionId).invalidate();
    } catch (Exception e) {
    }
  }
}

5、创建controller

import com.lingkang.killsession.service.SessionService;
import java.util.HashMap;
import java.util.Map;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;

@Controller
public class UserController {

  @Autowired
  private HttpServletRequest request;
  @Autowired
  private SessionService sessionService;

  private static Map<String, String> sessionId = new HashMap<>();

  @GetMapping(value = {"/", "index"})
  public String index() {
    return "index";
  }

  @GetMapping("login")
  public String toLogin() {
    return "login";
  }

  @GetMapping("logout")
  public String logout(HttpServletResponse response) {
    request.getSession().removeAttribute("username");
    Cookie[] cookies = request.getCookies();
    for (Cookie c : cookies) {
      if (c.getName().equals("JSESSIONID")) {
        Cookie cookie = new Cookie("JSESSIONID", null);
        cookie.setMaxAge(0);
        response.addCookie(cookie);
        break;
      }
    }
    return "login";
  }

  @PostMapping("login")
  public String login() {
    String username = request.getParameter("username");
    String password = request.getParameter("password");
    if (username.equals("123") && password.equals("123")) {
      //登录成功！
      //实现只能一个用户登录
      //删除上一个session的username
      if (sessionId.get(username) != null) {
        sessionService.delete(sessionId.get(username));
      }
      sessionId.put(username, request.getSession().getId());
      request.getSession().setAttribute("username", "123");
      return "redirect:/index"; //使用重定向，
    }
    return "login";
  }
}

Application中

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;

@SpringBootApplication
@ServletComponentScan
public class KillsessionApplication {

  public static void main(String[] args) {
    SpringApplication.run(KillsessionApplication.class, args);
  }

}

6、页面：

index.html

<!DOCTYPE html>
<html xmlns:th="http://www.w3.org/1999/xhtml">
<head>
  <meta charset="UTF-8">
  <title>index</title>
</head>
<body>
欢迎您，<b th:text="${session.username}" style="color: red"></b>，您已经登录了<br>
<a href="/logout">注销</a>
</body>
</html>

login.html

<form action="/login" method="post">
  账号：<input name="username"><br>
  密码<input name="password" type="password"><br><br>
  <input value="sign in" type="submit">
</form>

7、效果如下

GIF.gif