Java中用Servlet容器实现程序监听

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>