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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.
- If an instance of the servlet does not exist, the Web container
- Loads the servlet class.
- Creates an instance of the servlet class.
- Initializes the servlet instance by calling the
init
method. Initialization is covered in Initializing a Servlet.- Invokes the
service
method, passing a request and response object. Service methods are discussed in Writing Service Methods.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.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 the listener class and specify the listener class.
Defining The Listener Class
You define a listener class as an implementation of a listener interface. Servlet Life Cycle Events 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 anHttpSessionEvent
, which contains anHttpSession
.
Table 12-3 Servlet Life Cycle Events Object
Event
Listener Interface and Event Class
Web context(See Accessing the Web Context)
Initialization and destruction
javax.servlet.ServletContextListener
and
ServletContextEvent
Attribute added, removed, or replaced
javax.servlet.ServletContextAttributeListener
and
ServletContextAttributeEvent
Session(See Maintaining Client State)
Creation, invalidation, and timeout
javax.servlet.http.HttpSessionListener
and
HttpSessionEvent
Attribute added, removed, or replaced
javax.servlet.http.HttpSessionAttributeListener
and
HttpSessionBindingEvent
The
listeners.ContextListener
class creates and removes the database helper and counter objects used in 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.BookDB; import javax.servlet.*; import util.Counter; public final class ContextListener implements ServletContextListener { private ServletContext context = null; public void contextInitialized(ServletContextEvent event) { context = event.getServletContext(); try { BookDB bookDB = new BookDB(); context.setAttribute("bookDB", bookDB); } catch (Exception ex) { System.out.println( "Couldn't create database: " + ex.getMessage()); } Counter counter = new Counter(); context.setAttribute("hitCounter", counter); context.log("Created hitCounter" + counter.getCounter()); counter = new Counter(); context.setAttribute("orderCounter", counter); context.log("Created orderCounter" + counter.getCounter()); } public void contextDestroyed(ServletContextEvent event) { context = event.getServletContext(); BookDB bookDB = context.getAttribute( "bookDB"); bookDB.remove(); context.removeAttribute("bookDB"); context.removeAttribute("hitCounter"); context.removeAttribute("orderCounter"); } }Specifying Event Listener Classes
To specify an event listener class, you add a
listener
element to the Web application deployment descriptor. Here is thelistener
element for the Duke's Bookstore application:<listener> <listener-class>listeners.ContextListener</listener-class> </listener>You specify a listener class for a WAR in the
deploytool
Event Listeners inspector (see Event Listeners).Handling Errors
Any number of exceptions can occur when a servlet is executed. The Web container will generate a default page containing the message
A Servlet Exception Has Occurred
when an exception occurs, but you can also specify that the container should return a specific error page for a given exception. To specify such a page, you add anerror-page
element to the Web application deployment descriptor. These elements map the exceptions returned by the Duke's Bookstore application toerrorpage.html
:<error-page> <exception-type> exception.BookNotFoundException </exception-type> <location>/errorpage.html</location> </error-page> <error-page> <exception-type> exception.BooksNotFoundException </exception-type> <location>/errorpage.html</location> </error-page> <error-page> <exception-type>exception.OrderException</exception-type> <location>/errorpage.html</location> </error-page>You specify error pages for a WAR in the
deploytool
File Refs inspector (see Error Mappings).
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This tutorial contains information on the 1.0 version of the Java Web Services Developer Pack.
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