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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);
}
}
在这个解决方案中，任何一个Session被创建或者销毁时，都会通知SessionCounter 这个类，当然通知的原因是必须在web.xml文件中做相关的配置工作。如下面的配置代码：
<?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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