原理就是很简单，就是利用HttpSessionListener 监听session的创建和销毁，然后定义个静态变量存储在线人数的变化。

说两种方式，第一种是使用配置类，第二种是使用@WebListener注解，都差不多

方式一：使用配置类

1.创建session监听器

1.  
   package com.sdsft.pcweb.common.listener;
2.  
    
3.  
   import javax.servlet.http.HttpSessionEvent;
4.  
   import javax.servlet.http.HttpSessionListener;
5.  
    
6.  
   public class MyHttpSessionListener implements HttpSessionListener {
7.  
    
8.  
   public static int online = 0;
9.  
   @Override
10.  
    public void sessionCreated(HttpSessionEvent se) {
11.  
    System.out.println("创建session");
12.  
    online ++;
13.  
    }
14.  
     
15.  
    @Override
16.  
    public void sessionDestroyed(HttpSessionEvent se) {
17.  
    System.out.println("销毁session");
18.  
    online --;
19.  
    }
20.  
     
21.  
    }

2.配置类

1.  
   package com.sdsft.pcweb.common.config;
2.  
    
3.  
   import com.sdsft.pcweb.common.listener.MyHttpSessionListener;
4.  
   import org.springframework.boot.web.servlet.ServletListenerRegistrationBean;
5.  
   import org.springframework.context.annotation.Bean;
6.  
   import org.springframework.context.annotation.Configuration;
7.  
   import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
8.  
    
9.  
   @Configuration
10.  
    public class MywebConfig implements WebMvcConfigurer {
11.  
    @Bean
12.  
    public ServletListenerRegistrationBean listenerRegist() {
13.  
    ServletListenerRegistrationBean srb = new ServletListenerRegistrationBean();
14.  
    srb.setListener(new MyHttpSessionListener());
15.  
    System.out.println("listener");
16.  
    return srb;
17.  
    }
18.  
    }

3.控制器

1.  
   package com.sdsft.pcweb.controller;
2.  
    
3.  
   import com.sdsft.pcweb.common.entity.CommonResult;
4.  
   import com.sdsft.pcweb.common.enums.ResultEnum;
5.  
   import com.sdsft.pcweb.common.listener.MyHttpSessionListener;
6.  
   import com.sdsft.pcweb.service.LoginService;
7.  
   import org.apache.commons.lang3.StringUtils;
8.  
   import org.slf4j.Logger;
9.  
   import org.slf4j.LoggerFactory;
10.  
    import org.springframework.beans.factory.annotation.Autowired;
11.  
    import org.springframework.web.bind.annotation.PostMapping;
12.  
    import org.springframework.web.bind.annotation.RequestMapping;
13.  
    import org.springframework.web.bind.annotation.ResponseBody;
14.  
    import org.springframework.web.bind.annotation.RestController;
15.  
     
16.  
    import javax.servlet.http.HttpServletRequest;
17.  
    import javax.servlet.http.HttpSession;
18.  
     
19.  
    /**
20.  
    * LoginController class
21.  
    * @author zcz
22.  
    * @date 2018/09/20
23.  
    */
24.  
    @RequestMapping("/userInfo")
25.  
    @RestController
26.  
    public class LoginController {
27.  
    private static Logger logger = LoggerFactory.getLogger(LoginController.class);
28.  
     
29.  
    /**
30.  
    * 登录
31.  
    */
32.  
    @PostMapping("/Login")
33.  
    public void getUserByUserNameAndPassword(String username, String password, HttpSession session) {
34.  
    logger.info("用户【"+username+"】登陆开始！");
35.  
    if("admin".equals(username) && "123456".equals(password)){
36.  
    session.setAttribute("loginName",username);
37.  
    logger.info("用户【"+username+"】登陆成功！");
38.  
    }else{
39.  
    logger.info("用户【"+username+"】登录失败！");
40.  
    }
41.  
    }
42.  
    /**
43.  
    *查询在线人数
44.  
    */
45.  
    @RequestMapping("/online")
46.  
    public Object online() {
47.  
    return "当前在线人数：" + MyHttpSessionListener.online + "人";
48.  
    }
49.  
    /**
50.  
    * 退出登录
51.  
    */
52.  
    @RequestMapping("/Logout")
53.  
    public CommonResult Logout( HttpServletRequest request) {
54.  
    logger.info("用户退出登录开始！");
55.  
    HttpSession session = request.getSession(false);//防止创建Session
56.  
    if(session != null){
57.  
    session.removeAttribute("loginName");
58.  
    session.invalidate();
59.  
    }
60.  
    logger.info("用户退出登录结束！");
61.  
    return new CommonResult(ResultEnum.SUCCESS.getCode(), "退出成功！");
62.  
    }
63.  
     
64.  
     
65.  
    /**
66.  
    * 判断session是否有效
67.  
    * @param httpServletRequest
68.  
    * @return
69.  
    */
70.  
    @RequestMapping("/getSession")
71.  
    public String getSession(HttpServletRequest httpServletRequest) {
72.  
    HttpSession session = httpServletRequest.getSession();
73.  
    String loginName = (String) session.getAttribute("loginName");
74.  
    if (StringUtils.isNotBlank(loginName)) {
75.  
    return "200";
76.  
    }
77.  
    return "";
78.  
    }
79.  
     
80.  
    }

测试一下吧

方式二：使用@WebListener注解

1.创建监听器，加注解@WebListener

1.  
   package com.sdsft.pcweb.common.listener;
2.  
    
3.  
   import javax.servlet.http.HttpSessionEvent;
4.  
   import javax.servlet.http.HttpSessionListener;
5.  
   @WebListener
6.  
   public class MyHttpSessionListener implements HttpSessionListener {
7.  
    
8.  
   public static int online = 0;
9.  
   @Override
10.  
    public void sessionCreated(HttpSessionEvent se) {
11.  
    System.out.println("创建session");
12.  
    online ++;
13.  
    }
14.  
     
15.  
    @Override
16.  
    public void sessionDestroyed(HttpSessionEvent se) {
17.  
    System.out.println("销毁session");
18.  
    online --;
19.  
    }
20.  
     
21.  
    }

2.控制器

1.  
   package com.sdsft.pcweb.controller;
2.  
    
3.  
   import com.sdsft.pcweb.common.entity.CommonResult;
4.  
   import com.sdsft.pcweb.common.enums.ResultEnum;
5.  
   import com.sdsft.pcweb.common.listener.MyHttpSessionListener;
6.  
   import com.sdsft.pcweb.service.LoginService;
7.  
   import org.apache.commons.lang3.StringUtils;
8.  
   import org.slf4j.Logger;
9.  
   import org.slf4j.LoggerFactory;
10.  
    import org.springframework.beans.factory.annotation.Autowired;
11.  
    import org.springframework.web.bind.annotation.PostMapping;
12.  
    import org.springframework.web.bind.annotation.RequestMapping;
13.  
    import org.springframework.web.bind.annotation.ResponseBody;
14.  
    import org.springframework.web.bind.annotation.RestController;
15.  
     
16.  
    import javax.servlet.http.HttpServletRequest;
17.  
    import javax.servlet.http.HttpSession;
18.  
     
19.  
    /**
20.  
    * LoginController class
21.  
    * @author zcz
22.  
    * @date 2018/09/20
23.  
    */
24.  
    @RequestMapping("/userInfo")
25.  
    @RestController
26.  
    public class LoginController {
27.  
    private static Logger logger = LoggerFactory.getLogger(LoginController.class);
28.  
     
29.  
    /**
30.  
    * 登录
31.  
    */
32.  
    @PostMapping("/Login")
33.  
    public void getUserByUserNameAndPassword(String username, String password, HttpSession session) {
34.  
    logger.info("用户【"+username+"】登陆开始！");
35.  
    if("admin".equals(username) && "123456".equals(password)){
36.  
    session.setAttribute("loginName",username);
37.  
    logger.info("用户【"+username+"】登陆成功！");
38.  
    }else{
39.  
    logger.info("用户【"+username+"】登录失败！");
40.  
    }
41.  
    }
42.  
    /**
43.  
    *查询在线人数
44.  
    */
45.  
    @RequestMapping("/online")
46.  
    public Object online() {
47.  
    return "当前在线人数：" + MyHttpSessionListener.online + "人";
48.  
    }
49.  
    /**
50.  
    * 退出登录
51.  
    */
52.  
    @RequestMapping("/Logout")
53.  
    public CommonResult Logout( HttpServletRequest request) {
54.  
    logger.info("用户退出登录开始！");
55.  
    HttpSession session = request.getSession(false);//防止创建Session
56.  
    if(session != null){
57.  
    session.removeAttribute("loginName");
58.  
    session.invalidate();
59.  
    }
60.  
    logger.info("用户退出登录结束！");
61.  
    return new CommonResult(ResultEnum.SUCCESS.getCode(), "退出成功！");
62.  
    }
63.  
     
64.  
     
65.  
    /**
66.  
    * 判断session是否有效
67.  
    * @param httpServletRequest
68.  
    * @return
69.  
    */
70.  
    @RequestMapping("/getSession")
71.  
    public String getSession(HttpServletRequest httpServletRequest) {
72.  
    HttpSession session = httpServletRequest.getSession();
73.  
    String loginName = (String) session.getAttribute("loginName");
74.  
    if (StringUtils.isNotBlank(loginName)) {
75.  
    return "200";
76.  
    }
77.  
    return "";
78.  
    }
79.  
     
80.  
    }

3.启动类加@ServletComponentScan注解，这样才能在程序启动时将对应的bean加载进来

1.  
   package com.sdsft.pcweb;
2.  
   import org.springframework.boot.SpringApplication;
3.  
   import org.springframework.boot.autoconfigure.SpringBootApplication;
4.  
   import org.springframework.boot.web.servlet.ServletComponentScan;
5.  
    
6.  
   @SpringBootApplication
7.  
   @ServletComponentScan
8.  
   public class PcwebApplication {
9.  
   public static void main(String[] args) {
10.  
    SpringApplication.run(PcwebApplication.class, args);
11.  
    }
12.  
    }

开始测试吧