murach’s java servlets/jsp (2 nd ed.), c19© 2008, mike murach & associates, inc.slide 1

Murach’s Java Servlets/JSP (2nd Ed.), C19 © 2008, Mike Murach & Associates, Inc. Slide 1

Chapter 19

How to work with listeners


Objectives

Applied

Create, register, and use a listener class.

Knowledge

Describe the use of a listener class.


A listener class that implements the ServletContextListener interface package util; import javax.servlet.*; import java.util.*; import business.*; import data.*; public class CartContextListener implements ServletContextListener { public void contextInitialized(ServletContextEvent event) { ServletContext sc = event.getServletContext(); // initialize the customer service email address String custServEmail = sc.getInitParameter("custServEmail"); sc.setAttribute("custServEmail", custServEmail);


A listener class (cont.) // initialize the current year that's used in the // copyright notice GregorianCalendar currentDate = new GregorianCalendar(); int currentYear = currentDate.get(Calendar.YEAR); sc.setAttribute("currentYear", currentYear); // initialize the path for the products text file String productsPath = sc.getRealPath("WEB-INF/products.txt"); sc.setAttribute("productsPath", productsPath); // initialize the list of products ArrayList<Product> products = new ArrayList<Product>(); products = ProductIO.getProducts(productsPath); sc.setAttribute("products", products); }


A listener class (cont.) public void contextDestroyed(ServletContextEvent event) { // no cleanup necessary } }


How to use a ServletContextListener A listener is a class that listens for various events that can occur in

an application and provides methods that respond to those events when they occur.

To code the class for a listener, you must implement one of the listener interfaces that are stored in the javax.servlet and javax.servlet.http packages.

A class that implements a listener interface must override the methods of that interface.


A web.xml file that includes a listener element <?xml version="1.0" encoding="UTF-8"?> <web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation= "http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"> <servlet> <servlet-name>CartServlet</servlet-name> <servlet-class>cart.CartServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>CartServlet</servlet-name> <url-pattern>/cart</url-pattern> </servlet-mapping> <listener> <listener-class>util.CartContextListener </listener-class> </listener>


A web.xml file that includes a listener element (cont.) <context-param> <param-name>custServEmail</param-name> <param-value>[email protected]</param-value> </context-param> <session-config> <session-timeout>30</session-timeout> </session-config> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list> </web-app>


The listener elements

Element Description

listener Adds a listener to the application.

listener-class Specifies the fully qualified name of a class that implements a listener.


A JSP file that uses attributes set by a listener <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <title>Murach's Java Servlets and JSP</title> </head> <body> <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %> <h1>CD list</h1> <table cellpadding="5" border=1> <tr valign="bottom"> <td align="left"><b>Description</b></td> <td align="left"><b>Price</b></td> <td align="left"></td> </tr>


A JSP file that uses attributes set by a listener (cont.) <c:forEach var="product" items="${products}"> <tr valign="top"> <td>${product.description}</td> <td>${product.priceCurrencyFormat}</td> <td><a href= "<c:url value='/cart?productCode=${product.code}' />"> Add To Cart</a></td> </tr> </c:forEach> </table> <p> For customer service, please send an email to ${custServEmail}. </p>


The ServletContext interfaces

Interface Provides methods that are executed when…

ServletContextListener The ServletContext object is initialized and destroyed (that is, when the application is started and stopped).

SerlvetContextAttributeListener Attributes are added to, removed from, or replaced in the ServletContext object.


The HttpSession interfaces


HttpSessionListener The session object is created and destroyed for a user (every time a new user accesses an application or the user session is destroyed).

HttpSessionAttributeListener Attributes are added to, removed from, or replaced in the session object.

HttpSessionBindingListener An object is bound to or unbound from the session.

HttpSessionActivationListener The session is activated or deactivated (that is, when the session is migrating to another JVM).


The ServletRequest interfaces


ServletRequestListener A request object is initialized and destroyed (every time the server receives and processes a request).

ServletRequestAttributeListener Attributes are added to, removed from, or replaced in the request object.


The ServletContext interfaces The ServletContextListener interface contextInitialized(ServletContextEvent e) contextDestroyed(ServletContextEvent e)

The ServletContextAttributeListener interface attributeAdded(ServletContextAttributeEvent e) attributeRemoved(ServletContextAttributeEvent e) attributeReplaced(ServletContextAttributeEvent e)


The HttpSession interfaces The HttpSessionListener interface sessionCreated(HttpSessionEvent e) sessionDestroyed(HttpSessionEvent e)

The HttpSessionAttributeListener interface attributeAdded(HttpSessionBindingEvent e) attributeRemoved(HttpSessionBindingEvent e) attributeReplaced(HttpSessionBindingEvent e)

The HttpSessionBindingListener interface valueBound(HttpSessionBindingEvent e) valueUnbound(HttpSessionBindingEvent e)

The HttpSessionActivationListener interface sessionDidActivate(HttpSessionEvent e) sessionWillPassivate(HttpSessionEvent e)


The ServletRequest interfaces The ServletRequestListener interface requestInitialized(ServletRequestEvent e) requestDestroyed(ServletRequestEvent e)

The ServletRequestAttributeListener interface attributeAdded(ServletRequestAttributeEvent e) attributeRemoved(ServletRequestAttributeEvent e) attributeReplaced(ServletRequestAttributeEvent e)


The ServletContextEvent class

Method Description

getServletContext() Returns the ServletContext object that was initialized or destroyed.

The ServletContextAttributeEvent class

Method Description

getName() Returns a string for the name of the attribute that was added to, removed from, or replaced in the ServletContext object.

getValue() Returns an object for the value of the attribute that was added to, removed from, or replaced in the ServletContext object.


The HttpSessionEvent class

Method Description

getSession() Returns the HttpSession object that was changed.

The HttpSessionBindingEvent class

Method Description

getName() Returns a string for the name of the attribute that was added to, removed from, or replaced in the HttpSession object.

getValue() Returns an object for the value of the attribute that was added to, removed from, or replaced in the HttpSession object.


The ServletRequestEvent class

Method Description

getServletRequest() Returns the ServletRequest object that was initialized or destroyed.

The ServletRequestAttributeEvent class

Method Description

getName() Returns a string for the name of the attribute that was added to, removed from, or replaced in the ServletRequest object.

getValue() Returns an object for the value of the attribute that was added to, removed from, or replaced in the ServletRequest object.

murach’s java servlets/jsp (2 nd ed.), c19© 2008, mike murach & associates, inc.slide 1

Documents