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This chapter is from the book

10.11 Using Multiple Cooperating Listeners

Now, the listeners discussed in this chapter are all well and good. There are plenty of applications where one of them is useful. However, there are also plenty of applications where no single listener can, in isolation, accomplish the necessary tasks. Multiple listeners need to work together.

For example, suppose that your yacht-watching listener was so successful that you are asked to expand it. Rather than tracking buying patterns of a fixed item such as a yacht, you should track orders for the current daily special to let management discover if their specials are effective. Accomplishing this task requires three listeners to cooperate: a ServletContextListener to set up application-wide information about the session attributes that store daily specials, a ServletContext-AttributeListener to monitor changes to the attributes that store the information, and an HttpSessionAttributeListener to keep a running count of orders for the daily special.

The three listeners are described in more detail in the following subsections.

Tracking Orders for the Daily Special

As the first step in creating an order-tracking system, you need a servlet context listener to read initialization parameters that specify which session attributes correspond to orders and which items are the current daily specials. These values should be stored in the servlet context so other resources can determine what the daily specials are. Listing 10.31 shows this listener.

Second, you need a session attribute listener to keep a running count of orders for the daily special. The count will be incremented every time a designated attribute name is added with any of the daily specials as its value. The count will be decremented every time a designated attribute is replaced or removed and the previous value is one of the daily specials. Listing 10.32 shows this listener.

Listing 10.33 shows the deployment descriptor that registers the two listeners and sets up the servlet context initialization parameters that designate the names of order-related session attributes and the names of the daily specials.

Listing 10.34 shows a JSP page that prints the current order count. Figures 10–20 through 10–22 show some typical results.

Figures 10–20 Initial result of track-daily-specials.jsp.

Figures 10–21 Ordering the daily special.

Figures 10–22 Result of track-daily-specials.jsp after several clients placed orders.

Listing 10.31 DailySpecialRegistrar.java

package moreservlets.listeners;

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

/** Listener that records how to detect orders
 * of the daily special. It reads a list of attribute
 * names from an init parameter: these correspond to
 * session attributes that are used to record orders.
 * It also reads a list of item names: these correspond
 * to the names of the daily specials. Other listeners
 * will watch to see if any daily special names appear
 * as values of attributes that are hereby designated
 * to refer to orders.
 */

public class DailySpecialRegistrar
  implements ServletContextListener {

  /** When the Web application is loaded, record the
  * attribute names that correspond to orders and
  * the attribute values that are the daily specials.
  * Also set to zero the count of daily specials that have
  * been ordered.
  */

  public void contextInitialized(ServletContextEvent event) {
    ServletContext context = event.getServletContext();
    addContextEntry(context, "order-attribute-names");
    addContextEntry(context, "daily-special-item-names");
    context.setAttribute("dailySpecialCount", new Integer(0));
 }

  public void contextDestroyed(ServletContextEvent event) {}

  /** Read the designated context initialization parameter,
  * put the values into an ArrayList, and store the
  * list in the ServletContext with an attribute name
  * that is identical to the initialization parameter name.
  */
  private void addContextEntry(ServletContext context,
                       String initParamName) {
   ArrayList paramValues = new ArrayList();
   String attributeNames =
      context.getInitParameter(initParamName);
   if (attributeNames != null) {
      StringTokenizer tok = new StringTokenizer(attributeNames);
      String value;
      while(tok.hasMoreTokens()) {
        value = tok.nextToken();
        paramValues.add(value);
     }
     context.setAttribute(initParamName, paramValues);
   }
  }

  /** Returns a string containing the daily special
  * names. For insertion inside an HTML text area.
  */

  public static String dailySpecials(ServletContext context) {
   String attributeName = "daily-special-item-names";
   ArrayList itemNames =
     (ArrayList)context.getAttribute(attributeName);
   String itemString = "";
   for(int i=0; i<itemNames.size(); i++) {
     itemString = itemString + (String)itemNames.get(i) + "\n";
   }
   return(itemString);
  }

  /** Returns a UL list containing the daily special
  * names. For insertion within the body of a JSP page.
  */

  public static String specialsList(ServletContext context) {
   String attributeName = "daily-special-item-names";
   ArrayList itemNames =
     (ArrayList)context.getAttribute(attributeName);
   String itemString = "<UL>\n";
   for(int i=0; i<itemNames.size(); i++) {
     itemString = itemString + "<LI>" +
              (String)itemNames.get(i) + "\n";
   }
   itemString = itemString + "</UL>";
   return(itemString);
  }
}

Listing 10.32 DailySpecialWatcher.java

package moreservlets.listeners;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
/** Listener that keeps track of orders of the
* current daily special.
*/
public class DailySpecialWatcher
implements HttpSessionAttributeListener {
private static int dailySpecialCount = 0;
/** If the name of the session attribute that was added
* matches one of the stored order-attribute-names AND
* the value of the attribute matches one of the
* stored daily-special-item-names, then increment
* the count of daily specials ordered.
*/
public void attributeAdded(HttpSessionBindingEvent event) {
checkForSpecials(event, 1);
}
/** If the name of the session attribute that was removed
* matches one of the stored order-attribute-names AND
* the value of the attribute matches one of the
* stored daily-special-item-names, then decrement
* the count of daily specials ordered.
*/
public void attributeRemoved(HttpSessionBindingEvent event) {
checkForSpecials(event, -1);
}
/** If the name of the session attribute that was replaced
* matches one of the stored order-attribute-names AND
* the value of the attribute matches one of the
* stored daily-special-item-names, then increment
* the count of daily specials ordered. Note that the
* value here is the old value (the one being replaced);
* the attributeAdded method will handle the new value
* (the replacement).
*/
  public void attributeReplaced(HttpSessionBindingEvent event) {
  checkForSpecials(event, -1);
  }

  // Check whether the attribute that was just added or removed
  // matches one of the stored order-attribute-names AND
  // the value of the attribute matches one of the
  // stored daily-special-item-names. If so, add the delta
  // (+1 or -1) to the count of daily specials ordered.

  private void checkForSpecials(HttpSessionBindingEvent event,
                        int delta) {

   ServletContext context =
     event.getSession().getServletContext();
   ArrayList attributeNames =
     getList(context, "order-attribute-names");
   ArrayList itemNames =
     getList(context, "daily-special-item-names");
      synchronized(attributeNames) {
      for(int i=0; i<attributeNames.size(); i++) {
        String attributeName = (String)attributeNames.get(i);
        for(int j=0; j<itemNames.size(); j++) {
         String itemName = (String)itemNames.get(j);
         if (attributeName.equals(event.getName()) &&
            itemName.equals((String)event.getValue())) {
           dailySpecialCount = dailySpecialCount + delta;
         }
        }
      }
     }
     context.setAttribute("dailySpecialCount",
                    new Integer(dailySpecialCount));
   }

   // Get either the order-attribute-names or
   // daily-special-item-names list.

   private ArrayList getList(ServletContext context,
                     String attributeName) {
     ArrayList list =
      (ArrayList)context.getAttribute(attributeName);
     return(list);
   }

  /** Reset the count of daily specials that have
  * been ordered. This operation is normally performed
  * only when the daily special changes.
  */
  public static void resetDailySpecialCount() {
   dailySpecialCount = 0;
  }
}

Listing 10.33 web.xml (Excerpt for tracking daily special orders)

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC
   "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
   "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
  <!-- ... -->

  <!-- Register the listener that sets up the entries
     that will be used to monitor orders for the daily
     special. -->
  <listener>
   <listener-class>
     moreservlets.listeners.DailySpecialRegistrar
   </listener-class>
  </listener>

  <!-- Register the listener that counts orders for the daily
     special. -->
  <listener>
   <listener-class>
     moreservlets.listeners.DailySpecialWatcher
   </listener-class>
  </listener>

  <!-- ... -->
</web-app>

Listing 10.34 track-daily-specials.jsp

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Tracking Daily Special Orders</TITLE>
<LINK REL=STYLESHEET
     HREF="events-styles.css"
     TYPE="text/css">
</HEAD>
<BODY>
<CENTER>
<TABLE BORDER=5>
  <TR><TH CLASS="TITLE">Tracking Daily Special Orders
</TABLE>

<H2>Current Specials:</H2>
<%@ page import="moreservlets.listeners.*" %>
<%= DailySpecialRegistrar.specialsList(application) %>

<H2>Number of Orders:
<%= application.getAttribute("dailySpecialCount") %>
</H2>

</CENTER>
</BODY>
</HTML>

Resetting the Daily Special Order Count

The two listeners shown in the previous subsection are sufficient if you restart the server every time you change the daily specials.

However, if you change the daily specials while the server is running, you need a servlet context attribute listener to detect changes in the attribute that stores the names of the daily specials. In particular, when the daily specials change, you need to reset the running count of orders for the specials. Listing 10.35 shows this listener.

Listing 10.36 shows a JSP page that displays the current daily specials in a text area. It lets the user change the values and send them to a servlet (Listing 10.37) that records the changes in the servlet context. The JSP page is in the admin directory and the servlet is assigned a URL beginning with /admin (see the web.xml file in Listing 10.38), so the security restrictions discussed in Section 10.4 apply.

When an authorized user changes the names of the daily specials, the order count is reset. Figures 10–23 through 10–27 show some representative results.

Figures 10–23 Requests by unauthenticated users for change-daily-specials.jsp get sent to the login page (Listing 10.9).

Figures 10–24 Users who fail authentication are shown the login-failure page (Listing 10.10).

Figures 10–25 Users who pass authentication and are in the designated role (ceo) are shown the form for changing the daily specials (Listing 10.36). The current daily specials are displayed as the initial value of the text area.

Figures 10–26 Result of submitting the form for changing daily specials after yacht and chalet are entered in the text area.

Figures 10–27 When the daily specials are changed, the servlet context attribute listener (Listing 10.35) resets the order count.

Listing 10.35 ChangedDailySpecialListener.java

package moreservlets.listeners;

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

/** Listener that monitors changes to the names
 * of the daily specials (which are stored in
 * the daily-special-item-names attribute of
 * the servlet context). If the names change, the
 * listener resets the running count of the number
 * of daily specials being ordered.
 */

public class ChangedDailySpecialListener
   implements ServletContextAttributeListener {
  /** When the daily specials change, reset the
  * order counts.
  */

  public void attributeReplaced
               (ServletContextAttributeEvent event) {
   if (event.getName().equals("daily-special-item-names")) {
     ServletContext context = event.getServletContext();
     context.setAttribute("dailySpecialCount",
                   new Integer(0));
     DailySpecialWatcher.resetDailySpecialCount();
   }
  }
  public void attributeAdded
               (ServletContextAttributeEvent event) {}
  public void attributeRemoved
               (ServletContextAttributeEvent event) {}
}

Listing 10.36 change-daily-specials.jsp

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Changing Daily Specials</TITLE>
<LINK REL=STYLESHEET
     HREF="../events-styles.css"
     TYPE="text/css">
</HEAD>

<BODY>
<CENTER>
<TABLE BORDER=5>
  <TR><TH CLASS="TITLE">Changing Daily Specials
</TABLE>
<P>
<FORM ACTION="ChangeDailySpecial">
New specials:<BR>
<%@ page import="moreservlets.listeners.*" %>
<TEXTAREA NAME="newSpecials" ROWS=4 COLS=30>
<%= DailySpecialRegistrar.dailySpecials(application) %>
</TEXTAREA>
<P>
<INPUT TYPE="SUBMIT" VALUE="Submit Change">
</FORM>
</CENTER>
</BODY>
</HTML>

Listing 10.37 ChangeDailySpecial.java

package moreservlets;

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

/** Servlet that changes the daily specials. The web.xml
 * file specifies that only authenticated users in the
 * ceo role can access the servlet. A servlet context
 * attribute listener resets the count of daily special
 * orders when this servlet (or any other program) changes
 * the daily specials.
 */

public class ChangeDailySpecial extends HttpServlet {
  public void doGet(HttpServletRequest request,
               HttpServletResponse response)
      throws ServletException, IOException {
   String dailySpecialNames =
     request.getParameter("newSpecials");
   if ((dailySpecialNames == null) ||
     (dailySpecialNames.equals(""))) {
     dailySpecialNames = "MISSING-VALUE";
   }
   ArrayList specials = new ArrayList();
   StringTokenizer tok =
     new StringTokenizer(dailySpecialNames);
   while(tok.hasMoreTokens()) {
     specials.add(tok.nextToken());
   }
   ServletContext context = getServletContext();
   context.setAttribute("daily-special-item-names",
                  specials);
   response.setContentType("text/html");
   PrintWriter out = response.getWriter();
   String docType =
     "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
     "Transitional//EN\">\n";
   String title = "New Daily Specials";
   out.println
     (docType +
     "<HTML>\n" +
     "<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
     "<BODY BGCOLOR=\"#FDF5E6\">\n" +
     "<H2 ALIGN=\"CENTER\">" + title + "</H2>\n" +
     "<UL>");
   String special;
   for(int i=0; i<specials.size(); i++) {
     special = (String)specials.get(i);
     out.println("<LI>" + special);
   }
   out.println("</UL>\n" +
           "</BODY></HTML>");
  }
}

Listing 10.38 web.xml (Excerpt for resetting order counts)

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC
    "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
  <!-- ... -->

  <!-- Register the listener that resets the order counts
     when the names of the daily specials change. -->
  <listener>
   <listener-class>
     moreservlets.listeners.ChangedDailySpecialListener
   </listener-class>
  </listener>
  <!-- ... -->

  <!-- Assign the name ChangeDailySpecial to
     moreservlets.ChangeDailySpecial. -->
  <servlet>
   <servlet-name>ChangeDailySpecial</servlet-name>
   <servlet-class>
     moreservlets.ChangeDailySpecial
   </servlet-class>
   </servlet>
  <!-- ... -->

  <!-- Assign the URL /admin/ChangeDailySpecial to the
     servlet that is named ChangeDailySpecial.
  -->
  <servlet-mapping>
   <servlet-name>ChangeDailySpecial</servlet-name>
   <url-pattern>/admin/ChangeDailySpecial</url-pattern>
  </servlet-mapping>

  <!-- ... -->
</web-app>
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