//javax.servlet包下监听接口：  （或jakarta.servlet）
ServletContextListener    //监听ServletContext对象的状态
ServletContextAttributeListener //监听ContextAttribute中数据状态
ServetRequestListener     //监听ServetRequest的状态
ServletRequestAttributeListener //监听RequestAttribute中数据状态

//javax.servlet.http包下监听接口：
HttpSessionListener      //监听HttpSession会话的状态
HttpSessionAttributeListener //监听session域中的数据状态
HttpSessionBindingListener  (该监听器不需要配置xml或使用注解)//目标类实现该监听器，则监听该类是否有数据绑定
HttpSessionldListener //监听session的Id状态
HttpSessionActivationListener //监听session的存储状态（内存存储到硬盘, 硬盘到内存，又称钝化，活化）

所有的监听器中的方法，当指定时间发生后，则Web服务器调用该方法。

第一步：创建一个类，实现监听接口和接口方法。

public class MyListener implements ServletContextListener {
    //ServletContext被创建时调用
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        
    //代码--------, Web服务器调用该方法。
        
    }

    // ServletContext被销毁时调用
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        
    //代码--------, Web服务器调用该方法。
    }
}

第二步：web.xm文件配置或使用注解
<listener>
    <listener-class>com.javaweb.MyListener</listener-class>
</listener>

@WebListener

@WebListener

public class AttributeListenerDemo implements HttpSessionAttributeListener {

    //当向session域中存储数据的时，Web服务器执行该方法attributeAdded

    @Override

    public void attributeAdded(HttpSessionBindingEvent event) {

        System.out.println("Add");

    }

    //当向session域中删除数据的时，Web服务器执行该方法attributeRemoved

    @Override

    public void attributeRemoved(HttpSessionBindingEvent event) {

        System.out.println("Removed");

    }

    //当向session域中替换某一个数据的时，Web服务器执行该方法attributeReplaced

    @Override

    public void attributeReplaced(HttpSessionBindingEvent event) {

        System.out.println("Replaced");

    }

}

public class Student implements HttpSessionBindingListener {

    //valueBound绑定数据

    @Override

    public void valueBound(HttpSessionBindingEvent event) {

        System.out.println("valueBound");

    }

    //valueUnbound解绑数据

    @Override

    public void valueUnbound(HttpSessionBindingEvent event) {

        System.out.println("valueUnbound");

    }
    private int id;

    private String name;

    private int age;
    public Student() {

    }
    public Student(int id, String name, int age) {

        this.id = id;

        this.name = name;

        this.age = age;

    }
    public int getId() {

        return id;

    }
    public void setId(int id) {

        this.id = id;

    }
    public String getName() {

        return name;

    }
    public void setName(String name) {

        this.name = name;

    }
    public int getAge() {

        return age;

    }
    public void setAge(int age) {

        this.age = age;

    }

}

@WebServlet("/se/bind")

public class SessionBindingListenerDemo extends HttpServlet {

    @Override

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //获取session对象

        HttpSession session = request.getSession();

        Student student =new Student();

        student.setId(01);

        student.setName("张三");

        student.setAge(18);

        session.setAttribute("student",student);

    }

}

@WebListener
public class HttpSessionldListenerDemo implements HttpSessionIdListener {
  //当向session域中ID发生改变时，Web服务器执行该方法sessionIdChanged
    @Override
    public void sessionIdChanged(HttpSessionEvent httpSessionEvent, String s) {
        
    }
}

@WebListener
public class HttpSessionActivationListenerDemo implements HttpSessionActivationListener {
    @Override
    public void sessionWillPassivate(HttpSessionEvent se) {
        //钝化
    }

    @Override
    public void sessionDidActivate(HttpSessionEvent se) {
        //活化
    }
}

Web.xml
<!--    设置session超时时长，2分钟-->

<session-config>

    <session-timeout>2</session-timeout>

</session-config>

//此为演示案例，注意线程安全
@WebListener

public class HttpSessionListenerDemo implements HttpSessionListener {

    public static long k=0;
    @Override

    public void sessionCreated(HttpSessionEvent se) {

        System.out.println("sessionCreated");

        k++;

    }
    @Override

    public void sessionDestroyed(HttpSessionEvent se) {

        System.out.println("sessionDestroyed");

        k--;

    }
}

//此为演示案例，注意线程安全
User类

public class User implements HttpSessionBindingListener {
    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        //用户登入
        ServletContext servletContext = event.getSession().getServletContext();
        Object onLine = servletContext.getAttribute("onLine");
        if (onLine == null){
            servletContext.setAttribute("onLine",1);
        }else {
            Long count = (Long) onLine;
            count++;
            servletContext.setAttribute("onLine",count);
        }
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        //用户退出
        ServletContext servletContext = event.getSession().getServletContext();
        Object onLine = servletContext.getAttribute("onLine");
        Long count = (Long) onLine;
        count++;
        servletContext.setAttribute("onLine",count);
    }

    private String username;
    private String password;

    --有参无参构造方法 get和set方法--
}

Servlet类

//假设用户登入成功
User user =new User();
user.setUsername("张三");
session.setAttribute("user",user);
//假设用户登入失败 , 从session从删除User对象
session.removeAttribute("user");