Servlet Life Cycle

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

1.	If an instance of the servlet does not exist, the web container a. Loads the servlet class. b. Creates an instance of the servlet class. c. Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet (page 70).
2.	Invokes the service method, passing request and response objects. Service methods are discussed in Writing Service Methods (page 71).

If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's destroy method. Finalization is discussed in Finalizing a Servlet (page 92).

Handling Servlet Life-Cycle Events

You can monitor and react to events in a servlet's life cycle by defining listener objects whose methods get invoked when life-cycle events occur. To use these 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 32 lists the events that can be monitored and the corresponding interface that must be implemented. When a listener method is invoked, it is passed an event that contains information appropriate to the event. For example, the methods in the HttpSessionListener interface are passed an HttpSessionEvent, which contains an HttpSession.

Table 32. Servlet Life-Cycle Events

Object	Event	Listener Interface and Event Class
Web context (see Accessing the Web Context, page 88)	Initialization and destruction	javax.servlet.ServletContextListener and ServletContextEvent
	Attribute added, removed, or replaced	javax.servlet.ServletContextAttributeListener and ServletContextAttributeEvent
Session (See Maintaining Client State, page 89)	Creation, invalidation, activation, passivation, and timeout	javax.servlet.http.HttpSessionListener, javax.servlet.http.HttpSessionActivationListener, and HttpSessionEvent
	Attribute added, removed, or replaced	javax.servlet.http.HttpSessionAttributeListener and HttpSessionBindingEvent
Request	A servlet request has started being processed by web components	javax.servlet.ServletRequestListener and ServletRequestEvent
	Attribute added, removed, or replaced	javax.servlet.ServletRequestAttributeListener and ServletRequestAttributeEvent

The listeners.ContextListener class creates and removes the database access and counter objects used in the Duke's Bookstore application. The methods retrieve the web context object from ServletContextEvent 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 the listener element of the deployment descriptor. Review The Example Servlets (page 60) for information on how to specify the ContextListener listener class. You can specify an event listener using the deployment descriptor editor of NetBeans 5.5 by doing the following:

1.	Expand your application's project node.
2.	Expand the project's Web Pages and WEB-INF nodes.
3.	Double-click web.xml.
4.	Click General at the top of the web.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 Errors

Any number of exceptions can occur when a servlet executes. When an exception occurs, 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 error page for a given exception. Review the deployment descriptor file included with the example to learn how to map the exceptions exception.BookNotFound, exception.BooksNotFound, and exception.OrderException returned by the Duke's Bookstore application to errorpage.html.

See Mapping Errors to Error Screens (page 51) for instructions on how to specify error pages using NetBeans 5.5.