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7.2. Hidden Form Fields

One way to support anonymous session tracking is to use hidden form fields. As the name implies, these are fields added to an HTML form that are not displayed in the client's browser. They are sent back to the server when the form that contains them is submitted. You include hidden form fields with HTML like this:

<FORM ACTION="/servlet/MovieFinder" METHOD="POST">
...
<INPUT TYPE=hidden NAME="zip" VALUE="94040">
<INPUT TYPE=hidden NAME="level" VALUE="expert">
...
</FORM>

In a sense, hidden form fields define constant variables for a form. To a servlet receiving a submitted form, there is no difference between a hidden field and a visible field.

With hidden form fields, we can rewrite our shopping cart servlets so that users can shop anonymously until check-out time. Example 7-1 demonstrates the technique with a servlet that displays the user's shopping cart contents and lets the user choose to add more items or check out. An example screen for a bookworm is shown in Figure 7-1.

Example 7-1. Session tracking using hidden form fields

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ShoppingCartViewerHidden extends HttpServlet {

  public void doGet(HttpServletRequest req, HttpServletResponse res)
                               throws ServletException, IOException {
    res.setContentType("text/html");
    PrintWriter out = res.getWriter();

    out.println("<HEAD><TITLE>Current Shopping Cart Items</TITLE></HEAD>");
    out.println("<BODY>");

    // Cart items are passed in as the item parameter.
    String[] items = req.getParameterValues("item");

    // Print the current cart items.
    out.println("You currently have the following items in your cart:<BR>");
    if (items == null) {
      out.println("<B>None</B>");
    }
    else {
      out.println("<UL>");
      for (int i = 0; i < items.length; i++) {
        out.println("<LI>" + items[i]);
      }
      out.println("</UL>");
    }

    // Ask if the user wants to add more items or check out.
    // Include the current items as hidden fields so they'll be passed on.
    out.println("<FORM ACTION=\"/servlet/ShoppingCart\" METHOD=POST>");
    if (items != null) {
      for (int i = 0; i < items.length; i++) {
        out.println("<INPUT TYPE=hidden NAME=item VALUE=\"" +
          items[i] + "\">");
      }
    }
    out.println("Would you like to<BR>");
    out.println("<INPUT TYPE=submit VALUE=\" Add More Items \">");
    out.println("<INPUT TYPE=submit VALUE=\" Check Out \">");
    out.println("</FORM>");

    out.println("</BODY></HTML>");
  }
}
figure

Figure 7-1. Shopping cart contents

This servlet first reads the items already in the cart using getParameterValues("item"). Presumably, the item parameter values were sent to this servlet using hidden fields. The servlet then displays the current items to the user and asks if he wants to add more items or check out. The servlet asks its question with a form that includes hidden fields, so the form's target (the ShoppingCart servlet) receives the current items as part of the submission.

As more and more information is associated with a client's session, it can become burdensome to pass it all using hidden form fields. In these situations, it's possible to pass on just a unique session ID that identifies a particular client's session. That session ID can be associated with complete information about the session that is stored on the server.

The advantages of hidden form fields are their ubiquity and support for anonymity. Hidden fields are supported in all the popular browsers, they demand no special server requirements, and they can be used with clients that haven't registered or logged in. The major disadvantage with this technique, however, is that it works only for a sequence of dynamically generated forms. The technique breaks down immediately with static documents, emailed documents, bookmarked documents, and browser shutdowns.



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