1. JavaServer Pages (JSP)
Lifecycle Listeners
Primary Reference:
Marty Hall, Larry Brown, and Yaakov Chaikin, Core Servlets and JavaServer Pages, Volume 2: Advanced Technologies (Second Edition), Prentice Hall, 2008.
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2. JavaServer Pages (JSP)
Motivation
Respond to events in the overall application lifecycle.
notice when certain attributes are removed from the servlet context
Perform tasks more general than any one servlet or JSP page has responsibility for.
insert application-wide data into the servlet context before any servlets or JSP pages are accessed
keep track of certain session attributes, regardless of which servlet or JSP page added the attributes
Lifecycle listeners respond to
web application lifecycle events
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3. Lifecycle Listeners
A listener is a class that “listens” for and can respond to various events that can occur during the lifecycle of a web application.
A listener must extend one of the “listener” interfaces in package javax.servlet or package javax.servlet.http.
Listeners must be configured in the deployment descriptor for the web application.
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4. Example Listener Interfaces
ServletContextListener
void contextInitialized(ServletContextEvent sce)
void contextDestroyed(ServletContextEvent sce)
ServletRequestListener
void requestInitialized(ServletRequestEvent sre)
void requestDestroyed(ServletRequestEvent sre)
HttpSessionListener
void sessionCreated(HttpSessionEvent se)
void sessionDestroyed(HttpSessionEvent se)
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5. Kinds of Lifecycle Listeners
Servlet context listeners – notified when the servlet context (i.e., the web application) is initialized and destroyed.
Servlet context attribute listeners – notified when attributes are added to, removed from, or replaced in the servlet context
Session listeners – notified when session objects are created, invalidated, or timed out
Session attribute listeners – notified when attributes are added to, removed from, or replaced in any session
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6. Kinds of Lifecycle Listeners (continued)
Session migration listeners – notified when session objects are serialized and deserialized by the container
Session object binding listeners – notified when the implementing object is added to or removed from the session object
Request listeners – notified when request objects are initialized or destroyed
Request attribute listeners – notified when attributes are added to, removed from, or replaced in any request
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7. Implementing a Lifecycle Listener
Implement the appropriate interface
– ServletContextListener – ServletContextAttributeListener
– HttpSessionListener – HttpSessionAttributeListener
– HttpSessionBindingListener − HttpSessionActivationListener
– ServletRequestLIstener – ServletRequestAttributeListener
Implement the methods needed to respond to the events of interest
Provide empty bodies for the other methods in the interface.
Obtain access to the important Web application objects
– name of the servlet context attribute that changed
– value of the servlet context attribute that changed
– request /session object
– name of the session attribute that changed
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8. Implementing a Lifecycle Listener (continued)
Use the objects. For example, use the servlet context to
read initialization parameters, method getInitParameter()
store data for later access, method setAttribute()
read previously stored data, method getAttribute()
Declare the listener.
Add listener and listener-class elements to the application deployment descriptor (web.xml) or of a tag library descriptor file.
Provide any needed initialization parameters.
Add context-param elements to the application deployment descriptor (web.xml) to provide initialization parameters for servlet context listeners
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9. Example: ContextListener
package com.softmoore.servlet; import com.softmoore.process.DatabaseUpdateThread; import com.softmoore.util.Log; import javax.servlet.ServletContextEvent; import javax.servlet.ServletContextListener; public class ContextListener implements ServletContextListener { private DatabaseUpdateThread updateThread; public ContextListener() super(); updateThread = new DatabaseUpdateThread(); }
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10. Example: ContextListener (continued)
@Override public void contextInitialized(ServletContextEvent event) { updateThread.start(); Log log = Log.getInstance(); log.logMessage("DatabaseUpdateThread started"); } public void contextDestroyed(ServletContextEvent event) updateThread.interrupt(); log.logMessage("DatabaseUpdateThread terminated");
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11. Declaring a Listener in the Deployment Descriptor
<?xml version="1.0" encoding="ISO "?> <web-app xmlns=" xmlns:xsi=" xsi:schemaLocation=" version="2.5"> ... <listener> <listener-class> edu.citadel.cis.servlet.ContextListener </listener-class> </listener> </web-app>
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12. Using Annotations to Declare Listeners
Introduced in servlet API version 3.0, the can be used to register a class as a listener of a web application.
description)
Example
@WebListener
public class ContextListener implements ServletContextListener {
... }
Note that the class must still implement one of the listener interfaces.
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13. Using Annotations to Declare/Map Filters (continued)
replaces the equivalent <listener> configuration element in the deployment descriptor file (web.xml)
Servlet containers process the annotated classes when the application is deployed.
Note: Several additional annotations were
also introduced in servlet API 3.0; e.g.,
@WebServlet
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