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> ## Maintaining Client State

> Many applications require that a series of requests from a client be associated with one another. For example, the Duke's Bookstore application saves the state of a user's shopping cart across requests. Web-based applications are responsible for maintaining such state, called a session, because HTTP is stateless. To support applications that need to maintain state, Java servlet technology provides an API for managing sessions and allows several mechanisms for implementing sessions.

> ### Accessing a Session

> Sessions are represented by an `HttpSession` object. You access a session by calling the `getSession` method of a request object. This method returns the current session associated with this request, or, if the request does not have a session, it creates one.

> ### Associating Objects with a Session

> You can associate object-valued attributes with a session by name. Such attributes are accessible by any web component that belongs to the same web context and is handling a request that is part of the same session.

> The Duke's Bookstore application stores a customer's shopping cart as a session attribute. This allows the shopping cart to be saved between requests and also allows cooperating servlets to access the cart. `CatalogServlet` adds items to the cart; `ShowCartServlet` displays, deletes items from, and clears the cart; and `CashierServlet` retrieves the total cost of the books in the cart.

>     public class CashierServlet extends HttpServlet {
>       public void doGet (HttpServletRequest request,
>         HttpServletResponse response)
>         throws ServletException, IOException {

>         // Get the user's session and shopping cart
>         HttpSession session = request.getSession();
>         ShoppingCart cart =
>           (ShoppingCart)session.
>             getAttribute("cart");
>         ...
>         // Determine the total price of the user's books
>         double total = cart.getTotal();

> #### Notifying Objects That Are Associated with a Session

> Recall that your application can notify web context and session listener objects of servlet life-cycle events (Handling Servlet Life-Cycle Events). You can also notify objects of certain events related to their association with a session such as the following:

> - When the object is added to or removed from a session. To receive this notification, your object must implement the `javax.servlet.http.HttpSessionBindingListener` interface`.`
> - When the session to which the object is attached will be passivated or activated. A session will be passivated or activated when it is moved between virtual machines or saved to and restored from persistent storage. To receive this notification, your object must implement the `javax.servlet.http.HttpSessionActivationListener` interface.

> ### Session Management

> Because there is no way for an HTTP client to signal that it no longer needs a session, each session has an associated timeout so that its resources can be reclaimed. The timeout period can be accessed by using a session's `[get|set]MaxInactiveInterval` methods. You can also set the timeout period in the deployment descriptor by setting an integer value with the `session-timeout` element, which is a child element of a `session-config` element. The integer value represents the number of minutes of inactivity that must pass before the session times out.

> To ensure that an active session is not timed out, you should periodically access the session via service methods because this resets the session's time-to-live counter.

> When a particular client interaction is finished, you use the session's `invalidate` method to invalidate a session on the server side and remove any session data. The bookstore application's `ReceiptServlet` is the last servlet to access a client's session, so it has the responsibility to invalidate the session:

>     public class ReceiptServlet extends HttpServlet {
>       public void doPost(HttpServletRequest request,
>               HttpServletResponse response)
>               throws ServletException, IOException {
>         // Get the user's session and shopping cart
>         HttpSession session = request.getSession();
>         // Payment received -- invalidate the session
>         session.invalidate();
>         ...

> ### Session Tracking

> A web container can use several methods to associate a session with a user, all of which involve passing an identifier between the client and the server. The identifier can be maintained on the client as a cookie, or the web component can include the identifier in every URL that is returned to the client.

> If your application uses session objects, you must ensure that session tracking is enabled by having the application rewrite URLs whenever the client turns off cookies. You do this by calling the response's `encodeURL(URL)` method on all URLs returned by a servlet. This method includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged.

> The `doGet` method of `ShowCartServlet` encodes the three URLs at the bottom of the shopping cart display page as follows:

>     out.println("<p> &nbsp; <p><strong><a href=\"" +
>       response.encodeURL(request.getContextPath() +
>         "/bookcatalog") +
>         "\">" + messages.getString("ContinueShopping") +
>         "</a> &nbsp; &nbsp; &nbsp;" +
>         "<a href=\"" +
>       response.encodeURL(request.getContextPath() +
>         "/bookcashier") +
>         "\">" + messages.getString("Checkout") +
>         "</a> &nbsp; &nbsp; &nbsp;" +
>         "<a href=\"" +
>       response.encodeURL(request.getContextPath() +
>         "/bookshowcart?Clear=clear") +
>         "\">" + messages.getString("ClearCart") +
>         "</a></strong>");

> If cookies are turned off, the session is encoded in the Check Out URL as follows:

>     http://localhost:8080/bookstore1/cashier;
>       jsessionid=c0o7fszeb1

> If cookies are turned on, the URL is simply

>     http://localhost:8080/bookstore1/cashier