How to notify an object that the session is invalid or has ended?

To notify an object about the invalidation or completion of an HTTP session in Java web applications, session listeners (Session Listeners) defined by the Servlet API specification are used.

Classes implementing the javax.servlet.http.HttpSessionListener interface receive notifications about session creation and destruction.

import javax.servlet.http.HttpSessionEvent;  
import javax.servlet.http.HttpSessionListener;  
import javax.servlet.http.HttpSession;  

public class MySessionListener implements HttpSessionListener {  

    @Override  
    public void sessionCreated(HttpSessionEvent se) {  
        // Logic upon session creation  
        HttpSession session = se.getSession();  
        System.out.println("Session created: " + session.getId());  
    }  

    @Override  
    public void sessionDestroyed(HttpSessionEvent se) {  
        // Logic upon session destruction  
        HttpSession session = se.getSession();  
        System.out.println("Session destroyed: " + session.getId());  

        // Here you can access session attributes and notify objects  
        MySessionObject myObject = (MySessionObject) session.getAttribute("myObjectKey");  
        if (myObject != null) {  
            myObject.notifySessionInvalidated(); // Call a method on your object  
        }  
    }  
}  

To use the listener, it must be registered in the deployment descriptor (web.xml) or via annotations (@WebListener):

Registration in web.xml:

<web-app ...>  
    <listener>  
        <listener-class>com.example.MySessionListener</listener-class>  
    </listener>  
</web-app>  

Registration using @WebListener (since Servlet 3.0):

import javax.servlet.annotation.WebListener;  
import javax.servlet.http.HttpSessionEvent;  
import javax.servlet.http.HttpSessionListener;  

@WebListener  
public class MySessionListener implements HttpSessionListener {  
    // ... implementation of sessionCreated and sessionDestroyed methods
}

Within the sessionDestroyed method, you can access session-stored objects (attributes) and notify them of session termination by calling specific methods on these objects if they are designed to handle such notifications.

If an object needs to know when its session ends, it can implement the javax.servlet.http.HttpSessionBindingListener interface. Objects of this type receive notifications when they are added to or removed from a session.

import javax.servlet.http.HttpSessionBindingEvent;  
import javax.servlet.http.HttpSessionBindingListener;  

public class MySessionBoundObject implements HttpSessionBindingListener {  

    @Override  
    public void valueBound(HttpSessionBindingEvent event) {  
        // Object added to session  
        System.out.println("Object bound to session: " + event.getSession().getId());  
    }  

    @Override  
    public void valueUnbound(HttpSessionBindingEvent event) {  
        // Object removed from session or session destroyed  
        System.out.println("Object unbound from session: " + event.getSession().getId());  

        // Logic for notification when object is removed from session  
        // (including session destruction)  
        notifySessionInvalidated(); // For example, call an internal method  
    }  

    public void notifySessionInvalidated() {  
        System.out.println("MySessionBoundObject notified of session invalidation.");  
        // Your logic for handling session end for this object  
    }  
}  

When MySessionBoundObject is added to a session (session.setAttribute("myObjectKey", myObject)), valueBound is called. When the session is destroyed or the object is removed (session.removeAttribute("myObjectKey")), valueUnbound is called. Thus, valueUnbound can serve as a notification for the object about the session ending.