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用HttpSessionListener 。
package demo.listener;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class SessionCounter implements HttpSessionListener {
???? public void sessionCreated(HttpSessionEvent event) {
???????? ServletContext ctx = event.getSession( ).getServletContext( );
???????? Integer numSessions = (Integer) ctx.getAttribute("numSessions");
???????? if (numSessions == null) {
???????????? numSessions = new Integer(1);
???????? }
???????? else {
???????????? int count = numSessions.intValue( );
???????????? numSessions = new Integer(count + 1);
???????? }
???????? ctx.setAttribute("numSessions", numSessions);
???? }
???? public void sessionDestroyed(HttpSessionEvent event) {
???????? ServletContext ctx = event.getSession( ).getServletContext( );
???????? Integer numSessions = (Integer) ctx.getAttribute("numSessions");
???????? if (numSessions == null) {
???????????? numSessions = new Integer(0);
???????? }
???????? else {
???????????? int count = numSessions.intValue( );
???????????? numSessions = new Integer(count - 1);
???????? }
???????? ctx.setAttribute("numSessions", numSessions);
???? }
}
在這個解決方案中，任何一個Session被創建或者銷毀時，都會通知SessionCounter 這個類，當然通知的原因是必須在web.xml文件中做相關的配置工作。如下面的配置代碼：
<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE web-app PUBLIC
???? "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
???? "http://java.sun.com/dtd/web-app_2_3.dtd">
????
<web-app>
?? <display-name>Struts Examples</display-name>
??
?? <listener>
?????? <listener-class>demo.listener.SessionCounter
?????? </listener-class>
?? </listener>
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