分两步走：
（1）. 实现 javax.servlet.ServletContextListener 接口的两个方法：contextInitialized()和contextDestroyed()
contextInitialized()：当Servlet容器启动时会执行
contextDestroyed()：当Servlet容器停止时会执行
（2）. 在contextInitialized()中加入需要监听的程序，并由 java.util.Timer 的 schedule() 方法来控制监听程序执行的频率
DEMO（这是我的一个短信回复监听的程序原型，精简了一下）
----------------------------------------------------------------
ReplyListener.java
----------------------------------------------------------------
package com.hanweb.jcms;
import javax.servlet.*;
public class ReplyListener implements ServletContextListener {
private ReplyTimer rt = null;
public void contextInitialized(ServletContextEvent event) {
String status = "[SYS] SMS reply listener start .";
event.getServletContext().log(status);
System.out.println(status);
rt = new ReplyTimer(1);
rt.start();
}
public void contextDestroyed(ServletContextEvent event) {
String status = "[SYS] SMS reply listener stop .";
event.getServletContext().log(status);
System.out.println(status);
if (rt != null) {
rt.stop();
}
}
}
----------------------------------------------------------------
ReplyTimer.java
----------------------------------------------------------------
package com.hanweb.jcms;
import java.util.*;
public class ReplyTimer {
private final Timer timer = new Timer();
private final int min;
public ReplyTimer(int minutes) {
min = minutes;
}
public void start() {
Date date = new Date();
timer.schedule(new ReplyTask(), date, min * 60 * 1000);
}
public void stop() {
timer.cancel();
}
}
----------------------------------------------------------------
ReplyTask.java
----------------------------------------------------------------
package com.hanweb.jcms;
import java.util.*;
public class ReplyTask extends TimerTask {
public void doSomething() {
System.out.println("[SYS] SMS reply listener running ");
}
public void run() {
doSomething();
}
}
将编译好的class文件放入WEB-INF/classes中，最后别忘记了在Servlet容器中当前WEB应用的web.xml中加入监听语句：
<listener>
<listener-class>com.hanweb.jcms.ReplyListener</listener-class>
</listener>
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