Java™ Application Development on Linux - Dator

414Chapter 18Servlets: Java Pressed into Serviceconnection is gone. 2 About the best one can hope for is that you’ll use what,in our postal analogy, would be like a supplied reply envelope. This allows theservlet engine of the Web server to track requests from the same user and providea session capability across requests. It will use your browsers cookie mechanismto store this session’s ID used to track your session. If you don’t havesessions on, it will need to use URL rewriting, whereby the URLs generatedwill have an added parameter, the session ID.Unlike the early days in the life of the Web, nowadays virtually everyonehas cookies enabled in their browsers—anyone who shops at amazon.com, atleast. This makes session tracking so much easier for the servlet developer. TheWeb server handles all that automatically, and you only need to make a fewcalls to the session-related methods of the HttpRequest.To get a session for a user, ask for one from the HttpRequest:HttpSession session = request.getSession(true);The boolean parameter says whether (true) or not to create a session ifone does not yet exist for this user. Once you have a session, you can storeobjects associated with that session:session.setAttribute("cart", shopCart);where shopCart is any serializable Object and "cart" could be any Stringthat you want to use to later identify and retrieve this object, for example:Basket myCart = (Basket) session.getAttribute("cart");Notice that we need to explicitly cast the object type returned bygetAttribute(), because it returns a generic Object.18.6.1 CookiesFor any information that you want to save for longer than the duration of asession, you may want to investigate cookies—little bits of data (4K max; typicallyonly a few bytes) sent to the browser for it to store and send back at a latertime. You make a cookie thus:2. You can go to another page, just be staring at the page for a long long time, or you mighthave shut down your browser completely—and the server-side servlet will never know.