Servlet Life Cycle

Handling Servlet Life-Cycle Events

Defining the Listener Class

Specifying Event Listener Classes

Handling Servlet Errors

Servlet Life Cycle

The life cycle of a servlet is controlled by the container inwhich the servlet has been deployed. When a request is mapped to aservlet, the container performs the following steps.

1. If an instance of the servlet does not exist, the web container

   1. Loads the servlet class.

   2. Creates an instance of the servlet class.

   3. Initializes the servlet instance by calling theinit method. Initialization is covered inInitializing a Servlet.

2. Invokes theservice method, passing request and response objects. Service methods are discussed inWriting Service Methods.

If the container needs to remove the servlet, it finalizes the servlet bycalling the servlet’sdestroy method. Finalization is discussed inFinalizing a Servlet.

Handling Servlet Life-Cycle Events

You can monitor and react to events in a servlet’s life cycle bydefining listener objects whose methods get invoked when life-cycle events occur. To usethese listener objects you must define and specify the listener class.

Defining the Listener Class

You define a listener class as an implementation of a listener interface.Table 4-2lists the events that can be monitored and the corresponding interface that mustbe implemented. When a listener method is invoked, it is passed an eventthat contains information appropriate to the event. For example, the methods in theHttpSessionListener interface are passed anHttpSessionEvent, which contains anHttpSession.

Table 4-2 Servlet Life-Cycle Events

Thetut-install/javaeetutorial5/examples/web/bookstore1/src/java/com/sun/bookstore1/listeners/ContextListener class creates and removes the database access and counter objects usedin the Duke’s Bookstore application. The methods retrieve the web context object fromServletContextEvent and then store (and remove) the objects as servlet context attributes.

import database.BookDBAO;import javax.servlet.*;import util.Counter;import javax.ejb.*;import javax.persistence.*;public final class ContextListener    implements ServletContextListener {    private ServletContext context = null;    @PersistenceUnit    EntityManagerFactory emf;    public void contextInitialized(ServletContextEvent event) {        context = event.getServletContext();        try {            BookDBAO bookDB = new BookDBAO(emf);            context.setAttribute("bookDB", bookDB);        } catch (Exception ex) {            System.out.println(                "Couldn’t create database: " + ex.getMessage());        }        Counter counter = new Counter();        context.setAttribute("hitCounter", counter);        counter = new Counter();        context.setAttribute("orderCounter", counter);    }    public void contextDestroyed(ServletContextEvent event) {        context = event.getServletContext();        BookDBAO bookDB = context.getAttribute("bookDB");        bookDB.remove();        context.removeAttribute("bookDB");        context.removeAttribute("hitCounter");        context.removeAttribute("orderCounter");    }}

Specifying Event Listener Classes

You specify an event listener class using thelistener element of the deploymentdescriptor. ReviewThe Example Servlets for information on how to specify theContextListener listener class.

You can specify an event listener using the deployment descriptor editor of NetBeansIDE by doing the following:

1. Expand your application’s project node.

2. Expand the project’s Web Pages and WEB-INF nodes.

3. Double-clickweb.xml.

4. Click General at the top of theweb.xml editor.

5. Expand the Web Application Listeners node.

6. Click Add.

7. In the Add Listener dialog, click Browse to locate the listener class.

8. Click OK.

Handling Servlet Errors

Any number of exceptions can occur when a servlet executes. When an exceptionoccurs, the web container generates a default page containing the message

A Servlet Exception Has Occurred

But you can also specify that the container should return a specific errorpage for a given exception. Review the deployment descriptor file included with theexample to learn how to map the exceptionsexception.BookNotFound,exception.BooksNotFound, andexception.OrderExceptionreturned by the Duke’s Bookstore application toerrorpage.html.

SeeMapping Errors to Error Screens for instructions on how to specify error pages using NetBeans IDE.

Copyright © 2010, Oracle and/or its affiliates. All rights reserved.Legal Notices