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Servlet and JSP Application Events Framework

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In this sample chapter, Marty Hall discusses the general event-handling strategy. He covers: monitoring servlet context initialization and shutdown, setting application-wide values, detecting changes in attributes of the servlet context, recognizing creation and destruction of HTTP sessions, analyzing overall session usage, watching for changes in session attributes, tracking purchases at an e-commerce site, using multiple cooperating listeners, and packaging listeners in JSP tag libraries.
This sample chapter is excerpted from More Servlets and JavaServer Pages (JSP), by Marty Hall.
This chapter is from the book

Developers have many tools at their disposal for handling the life cycle of individual servlets or JSP pages. The servlet init method (Section 2.3) fires when a servlet is first instantiated. JSP pages use the nearly identical jspInit method (Section 3.3). Both methods can use initialization parameters that are specified with the init-param subelement of the web.xml servlet element (Section 5.5). Requests are handled with service and _jspService, and destruction is handled with destroy and jspDestroy.

This is all fine for individual resources. But what if you want to respond to major events in the life cycle of the Web application itself? What if you want to create application-wide connection pools, locate resources, or set up shared network connections? For example, suppose you want to record the email address of the support group at your company, an address that will be used by many different servlets and JSP pages. Sure, you can use the following to store the information:

context.setAttribute("supportAddress", "balmer@microsoft.com"); 

Better yet, you could use the web.xml context-param element (Section 5.5) to designate the address, then read it with the getInitParameter method of ServletContext. Fine. But which servlet or JSP page should perform this task? Or you could read the address from a database. Fine. But which servlet or JSP page should establish the database connection? There is no good answer to this question; you don't know which resources will be accessed first, so the code that performs these tasks would have to be repeated many different places. You want more global control than any one servlet or JSP page can provide. That's where application life-cycle event listeners come in.

There are four kinds of event listeners that respond to Web application life-cycle events.

  • Servlet context listeners. These listeners are notified when the servlet context (i.e., the Web application) is initialized and destroyed.

  • Servlet context attribute listeners. These listeners are notified when attributes are added to, removed from, or replaced in the servlet context.

  • Session listeners. These listeners are notified when session objects are created, invalidated, or timed out.

  • Session attribute listeners. These listeners are notified when attributes are added to, removed from, or replaced in any session.

Using these listeners involves six basic steps. I'll give a general outline here, then provide listener-specific details in the following sections.

  1. Implement the appropriate interface. Use ServletContext-Listener, ServletContextAttributeListener, Http-SessionListener, or HttpSessionAttributeListener. The first two interfaces are in the javax.servletpackage; the second two are in javax.servlet.http.

  2. Override the methods needed to respond to the events of interest. Provide empty bodies for the other methods in the interface. For example, the ServletContextListenerinterface defines two methods: contextInitialized(the Web application was just loaded and the servlet context was initialized) and contextDestroyed(the Web application is being shut down and the servlet context is about to be destroyed). If you wanted to define an application-wide servlet context entry, you could provide a real implementation for contextInitializedand an empty body for contextDestroyed.

  3. Obtain access to the important Web application objects. There are six important objects that you are likely to use in your event-handling methods: the servlet context, the name of the servlet context attribute that changed, the value of the servlet context attribute that changed, the session object, the name of the session attribute that changed, and the value of the session attribute that changed.

  4. Use these objects. This process is application specific, but there are some common themes. For example, with the servlet context, you are most likely to read initialization parameters (getInitParameter), store data for later access (setAttribute), and read previously stored data (getAttribute).

  5. Declare the listener. You do this with the listenerand listener-classelements of the general Web application deployment descriptor (web.xml) or of a tag library descriptor file.

  6. Provide any needed initialization parameters. Servlet context listeners commonly read context initialization parameters to use as the basis of data that is made available to all servlets and JSP pages. You use the context-paramweb.xml element to provide the names and values of these initialization parameters.

If servlet and JSP filters are the most important new feature in version 2.3 of the servlet specification, then application life-cycle events are the second most important new capability. Remember, however, that these event listeners work only in servers that are compliant with version 2.3 of the servlet specification. If your Web application needs to support older servers, you cannot use life-cycle listeners.

Core Warning

Application life-cycle listeners fail in servers that are compliant only with version 2.2 or earlier versions of the servlet specification.

10.1 Monitoring Creation and Destruction of the Servlet Context

The ServletContextListener class responds to the initialization and destruction of the servlet context. These events correspond to the creation and shutdown of the Web application itself. The ServletContextListener is most commonly used to set up application-wide resources like database connection pools and to read the initial values of application-wide data that will be used by multiple servlets and JSP pages. Using the listener involves the following six steps.

  1. Implement the ServletContextListenerinterface. This interface is in the javax.servletpackage.

  2. Override contextInitialized and contextDestroyed. The first of these (contextInitialized) is triggered when the Web application is first loaded and the servlet context is created. The two most common tasks performed by this method are creating application-wide data (often by reading context initialization parameters) and storing that data in an easily accessible location (often in attributes of the servlet context). The second method (contextDestroyed) is triggered when the Web application is being shut down and the servlet context is about to be destroyed. The most common task performed by this method is the releasing of resources. For example, context-Destroyed can be used to close database connections associated with a now-obsolete connection pool. However, since the servlet context will be destroyed (and garbage collected if the server itself continues to execute), there is no need to use contextDestroyed to remove normal objects from servlet context attributes.

  3. Obtain a reference to the servlet context. The context-Initializedand contextDestroyedmethods each take a ServletContextEventas an argument. The ServletContext-Eventclass has a getServletContextmethod that returns the servlet context.

  4. Use the servlet context. You read initialization parameters with getInitParameter, store data with setAttribute, and make log file entries with log.

  5. Declare the listener. Use the listenerand listener-class elements to simply list the fully qualified name of the listener class, as below.

    <listener>
    <listener-class>somePackage.SomeListener</listener-class>
    </listener>

    For now, assume that this declaration goes in the web.xml file (immediately before the servlet element). However, in Section 10.5 you'll see that if you package listeners with tag libraries, you can use the identical declaration within the TLD (tag library descriptor) file of the tag library.

  6. Provide any needed initialization parameters. Once you have a reference to the servlet context (see Step 3), you can use the get-InitParametermethod to read context initialization parameters as the basis of data that will be made available to all servlets and JSP pages. You use the context-paramweb.xml element to provide the names and values of these initialization parameters, as follows.

    <context-param>
      <param-name>name</param-name>
      <param-value>value</param-value>
    </context-param>
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