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216

Chapter 9 Session Tracking

use the same session from accessing the data concurrently, while still allowing

simultaneous requests from different users to proceed. Figures 9 5 and 9 6

show some typical results. 

Listing 9.5 OrderPage.java 

package coreservlets;

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

import java.util.*;

import java.text.NumberFormat;

/** Shows all items currently in ShoppingCart. Clients

 *  have their own session that keeps track of which

 *  ShoppingCart is theirs. If this is their first visit

 *  to the order page, a new shopping cart is created.

 *  Usually, people come to this page by way of a page

 *  showing catalog entries, so this page adds an additional

 *  item to the shopping cart. But users can also

 *  bookmark this page, access it from their history list,

 *  or be sent back to it by clicking on the "Update Order"

 *  button after changing the number of items ordered.

 */

public class OrderPage extends HttpServlet {

  public void doGet(HttpServletRequest request,

                    HttpServletResponse response)

      throws ServletException, IOException {

    HttpSession session = request.getSession(true);

    ShoppingCart cart;

    synchronized(session) {

      cart = (ShoppingCart)session.getValue("shoppingCart");

      // New visitors get a fresh shopping cart.

      // Previous visitors keep using their existing cart.

      if (cart == null) {

        cart = new ShoppingCart();

        session.putValue("shoppingCart", cart);

      }

      String itemID = request.getParameter("itemID");

      if (itemID != null) {

        String numItemsString =

          request.getParameter("numItems");

        if (numItemsString == null) {

          // If request specified an ID but no number,

          // then customers came here via an "Add Item to Cart"

          // button on a catalog page.

          cart.addItem(itemID);

        } else {

          // If request specified an ID and number, then

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