Question

I have a class which extends the HttpServlet class. I know that Java it’s multithreaded and different threads (read: HTTP requests) can make use of the same instance. I want to instantiate an Object on the first request and use it until the end of session.

How should I proceed?

For example : I want to open a file on the first request and close it at the end of session.

Answer 1

You could either just instantiate it yourself in one of the servlet’s doXxx() methods depending on its presence in the session

SomeObject someObject = (SomeObject) request.getSession().getAttribute("someObject");

if (someObject == null) {
    someObject = new SomeObject();
    request.getSession().setAttribute("someObject", someObject);
}

// Use someObject.

Or you could instantiate it yourself in a HttpSessionListener

@WebListener
public class SomeObjectInitializer implements HttpSessionListener {

    @Override
    public void sessionCreated(HttpSessionEvent event) {
        event.getSession().setAttribute("someObject", new SomeObject());
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent event) {
        // NOOP.
    }

}

Either way, the SomeObject itself could implement HttpSessionBindingListener so that you can perform some logic during binding and unbinding to the session.

public class SomeObject implements HttpSessionBindingListener {

    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        // Perform job during adding to the session.
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        // Perform job during removal from the session.
    }

}