分兩步走：
（1）. 實現 javax.servlet.ServletContextListener 接口的兩個方法：contextInitialized()和contextDestroyed()
contextInitialized()：當Servlet容器啟動時會執行
contextDestroyed()：當Servlet容器停止時會執行
（2）. 在contextInitialized()中加入需要監聽的程序，並由 java.util.Timer 的 schedule() 方法來控制監聽程序執行的頻率
DEMO（這是我的一個短信回復監聽的程序原型，精簡了一下）
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ReplyListener.java
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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();
}
}
}
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ReplyTimer.java
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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();
}
}
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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();
}
}
將編譯好的class文件放入WEB-INF/classes中，最後別忘記了在Servlet容器中當前WEB應用的web.xml中加入監聽語句：
<listener>
<listener-class>com.hanweb.jcms.ReplyListener</listener-class>
</listener>
