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<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <title>Servlet Life Cycle</title> <link rel="StyleSheet" href="document.css" type="text/css" media="all" /> <link rel="StyleSheet" href="catalog.css" type="text/css" media="all" /> <link rel="Table of Contents" href="J2EETutorialTOC.html" /> <link rel="Previous" href="Servlets3.html" /> <link rel="Next" href="Servlets5.html" /> <link rel="Index" href="J2EETutorialIX.html" /> </head> <body> <table width="550" summary="layout" id="SummaryNotReq1"> <tr> <td align="left" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/download.html#tutorial" target="_blank">Download</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/faq.html" target="_blank">FAQ</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/history.html" target="_blank">History</a> </td> <td align="center" valign="center"><a accesskey="p" href="Servlets3.html"><img id="LongDescNotReq1" src="images/PrevArrow.gif" width="26" height="26" border="0" alt="Prev" /></a><a accesskey="c" href="J2EETutorialFront.html"><img id="LongDescNotReq1" src="images/UpArrow.gif" width="26" height="26" border="0" alt="Home" /></a><a accesskey="n" href="Servlets5.html"><img id="LongDescNotReq3" src="images/NextArrow.gif" width="26" height="26" border="0" alt="Next" /></a><a accesskey="i" href="J2EETutorialIX.html"></a> </td> <td align="right" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/docs/api/index.html" target="_blank">API</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/search.html" target="_blank">Search</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/sendusmail.html" target="_blank">Feedback</a></font> </font> </td> </tr> </table> <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider"> <blockquote><a name="wp73579"> </a><h2 class="pHeading1">Servlet Life Cycle</h2><a name="wp64200"> </a><p class="pBody">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.</p><div class="pSmartList1"><ol type="1" class="pSmartList1"><a name="wp64202"> </a><div class="pSmartList1"><li>If an instance of the servlet does not exist, the Web container</li></div><div class="pSmartList2"><ol type="a" class="pSmartList2"><a name="wp64203"> </a><div class="pSmartList2"><li>Loads the servlet class.</li></div><a name="wp64204"> </a><div class="pSmartList2"><li>Creates an instance of the servlet class.</li></div><a name="wp64205"> </a><div class="pSmartList2"><li>Initializes the servlet instance by calling the <code class="cCode">init</code> method. Initialization is covered in <a href="Servlets6.html#wp64410">Initializing a Servlet</a>.</li></div></ol></div><a name="wp64209"> </a><div class="pSmartList1"><li>Invokes the <code class="cCode">service</code> method, passing a request and response object. Service methods are discussed in <a href="Servlets7.html#wp64424">Writing Service Methods</a>.</li></div></ol></div><a name="wp64213"> </a><p class="pBody">If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's <code class="cCode">destroy</code> method. Finalization is discussed in <a href="Servlets12.html#wp64798">Finalizing a Servlet</a>.</p><a name="wp64218"> </a><h3 class="pHeading2">Handling Servlet Life Cycle Events</h3><a name="wp64221"> </a><p class="pBody">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.</p><a name="wp64224"> </a><h4 class="pHeading3">Defining The Listener Class</h4><a name="wp64230"> </a><p class="pBody">You define a listener class as an implementation of a listener interface. <a href="Servlets4.html#wp71961">Servlet Life Cycle Events</a> 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 <code class="cCode">HttpSessionListener</code> interface are passed an <code class="cCode">HttpSessionEvent</code>, which contains an <code class="cCode">HttpSession</code>.</p><div align="left"><table border="1" summary="Servlet Life Cycle Events" id="wp71961"> <caption><a name="wp71961"> </a><div class="pTableTitle">Table 11-3 Servlet Life Cycle Events </div></caption> <tr align="center"> <th><a name="wp71967"> </a><div class="pCellHeading">Object</div></th> <th><a name="wp71969"> </a><div class="pCellHeading">Event</div></th> <th><a name="wp71971"> </a><div class="pCellHeading">Listener Interface and Event Class</div></th></tr> <tr align="left"> <td colspan="1" rowspan="2"><a name="wp71973"> </a><div class="pCellBody">Web context<br />(See <a href="Servlets10.html#wp64724">Accessing the Web Context</a>)</div></td> <td><a name="wp71978"> </a><div class="pCellBody">Initialization and destruction </div></td> <td><a name="wp71981"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContextListener.html" target="_blank">javax.servlet.<br />ServletContextListener</a></code> and </div><a name="wp71983"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContextEvent.html" target="_blank">ServletContextEvent</a></code></div></td></tr> <tr align="left"> <td><a name="wp71987"> </a><div class="pCellBody">Attribute added, removed, or replaced</div></td> <td><a name="wp71990"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContextAttributeListener.html" target="_blank">javax.servlet.<br />ServletContextAttributeListener</a></code> and</div><a name="wp71992"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContextAttributeEvent.html" target="_blank">ServletContextAttributeEvent</a></code></div></td></tr> <tr align="left"> <td colspan="1" rowspan="2"><a name="wp71994"> </a><div class="pCellBody">Session<br />(See <a href="Servlets11.html#wp64744">Maintaining Client State</a>)</div></td> <td><a name="wp71999"> </a><div class="pCellBody">Creation, invalidation, activation, passivation, and timeout</div></td> <td><a name="wp72002"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSessionListener.html" target="_blank">javax.servlet.http.<br />HttpSessionListener</a></code>, <br /><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSessionActivationListener.html" target="_blank">javax.servlet.http.<br />HttpSessionActivationListener</a></code><code class="cCode">,</code><code class="cCode"> </code>and</div><a name="wp75606"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSessionEvent.html" target="_blank">HttpSessionEvent</a></code></div></td></tr> <tr align="left"> <td><a name="wp72008"> </a><div class="pCellBody">Attribute added, removed, or replaced</div></td> <td><a name="wp72011"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSessionAttributeListener.html" target="_blank">javax.servlet.http.<br />HttpSessionAttributeListener</a></code> and</div><a name="wp72013"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSessionBindingEvent.html" target="_blank">HttpSessionBindingEvent</a></code></div></td></tr> <tr align="left"> <td colspan="1" rowspan="2"><a name="wp72015"> </a><div class="pCellBody">Request</div></td> <td><a name="wp72017"> </a><div class="pCellBody">A servlet request has started being processed by Web components</div></td> <td><a name="wp72032"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequestListener.html" target="_blank">javax.servlet.<br />ServletRequestListener</a></code> and </div><a name="wp72019"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequestEvent.html" target="_blank">ServletRequestEvent</a></code></div></td></tr> <tr align="left"> <td><a name="wp72023"> </a><div class="pCellBody">Attribute added, removed, or replaced</div></td> <td><a name="wp72039"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequestAttributeListener.html" target="_blank">javax.servlet.<br />ServletRequestAttributeListener</a></code> and</div><a name="wp72041"> </a><div class="pCellBody"><code class="cCode"><a href="http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequestAttributeEvent.html" target="_blank">ServletRequestAttributeEvent</a></code></div></td></tr></table></div><p class="pBody"></p><a name="wp64292"> </a><p class="pBody">The <code class="cCode"><a href="../examples/web/bookstore1/src/listeners/ContextListener.java" target="_blank">listeners.ContextListener</a></code> class creates and removes the database helper and counter objects used in the Duke's Bookstore application. The methods retrieve the Web context object from <code class="cCode">ServletContextEvent</code> and then store (and remove) the objects as servlet context attributes. </p><div class="pPreformattedRelative"><pre class="pPreformattedRelative">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); counter = new Counter(); context.setAttribute("orderCounter", counter); } public void contextDestroyed(ServletContextEvent event) { context = event.getServletContext(); BookDB bookDB = context.getAttribute( "bookDB"); bookDB.remove(); context.removeAttribute("bookDB"); context.removeAttribute("hitCounter"); context.removeAttribute("orderCounter"); }}<a name="wp64293"> </a></pre></div><a name="wp64294"> </a><h4 class="pHeading3">Specifying Event Listener Classes</h4><a name="wp81467"> </a><p class="pBody">You specify an event listener class in the Event Listener tab of the WAR inspector. Review step <a href="Servlets3.html#wp81327">11.</a> in <a href="Servlets3.html#wp63984">The Example Servlets</a> for the <code class="cCode">deploytool</code> procedure for specifying the <code class="cCode">ContextListener</code> listener class.</p><a name="wp76701"> </a><h3 class="pHeading2">Handling Errors</h3><a name="wp79832"> </a><p class="pBody">Any number of exceptions can occur when a servlet is executed. The Web container will generate a default page containing the message <code class="cCode">A Servlet Exception Has Occurred </code>when an exception occurs, but you can also specify that the container should return a specific error page for a given exception. Review step <a href="Servlets3.html#wp81336">12.</a> in <a href="Servlets3.html#wp63984">The Example Servlets</a> for <code class="cCode">deploytool</code> procedures for mapping the exceptions <code class="cCode">exception.BookNotFound</code>, <code class="cCode">exception.BooksNotFound</code>, and <code class="cCode">exception.OrderException</code> returned by the Duke's Bookstore application to <code class="cCode">errorpage.html</code>. </p> </blockquote> <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider"> <table width="550" summary="layout" id="SummaryNotReq1"> <tr> <td align="left" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/download.html#tutorial" target="_blank">Download</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/faq.html" target="_blank">FAQ</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/history.html" target="_blank">History</a> </td> <td align="center" valign="center"><a accesskey="p" href="Servlets3.html"><img id="LongDescNotReq1" src="images/PrevArrow.gif" width="26" height="26" border="0" alt="Prev" /></a><a accesskey="c" href="J2EETutorialFront.html"><img id="LongDescNotReq1" src="images/UpArrow.gif" width="26" height="26" border="0" alt="Home" /></a><a accesskey="n" href="Servlets5.html"><img id="LongDescNotReq3" src="images/NextArrow.gif" width="26" height="26" border="0" alt="Next" /></a><a accesskey="i" href="J2EETutorialIX.html"></a> </td> <td align="right" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/docs/api/index.html" target="_blank">API</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/search.html" target="_blank">Search</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/sendusmail.html" target="_blank">Feedback</a></font> </font> </td> </tr> </table> <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider"><p><font size="-1">All of the material in <em>The J2EE(TM) 1.4 Tutorial</em> is <a href="J2EETutorialFront2.html">copyright</a>-protected and may not be published in other workswithout express written permission from Sun Microsystems.</font> </body></html>⌨️ 快捷键说明
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