源码

package javax.servlet;

import java.util.EventListener;

/**
 *  Implementations of this interface receive notifications 
 *  about changes to the servlet context of the  web
 *  application they are part of. To receive notification events,
 *  the implementation class must be configured in the
 *  deployment descriptor for the web application.
 * @see ServletContextEvent
 * @since v 2.3
 */

public interface ServletContextListener extends EventListener {

    /**
     ** Notification that the web application initialization process 
     *  is starting. All ServletContextListeners are notified of 
     *  context initialization before any filter or servlet in the 
     *  web application is initialized.
     * @param sce Information about the ServletContext that was 
     * initialized
     * 当Servlet 容器启动Web 应用时调用该方法。
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     ** Notification that the servlet context is about to be shut 
     *  down. All servlets and filters have been destroy()ed 
     *  before any ServletContextListeners are notified of context 
     *  destruction.
     * @param sce Information about the ServletContext that was 
     * destroyed
     * 当Servlet 容器终止Web 应用时调用该方法。
     */
    public void contextDestroyed(ServletContextEvent sce);
}

介绍

ServletContextListener本质上就是一个监听器，监听 Web 应用的生命周期

  在 Servlet API 中有一个 ServletContextListener 接口，它能够监听 ServletContext 对象的生命周期，实际上就是监听 Web 应用的生命周期。

  当Servlet 容器启动或终止Web 应用时，会触发ServletContextEvent 事件，该事件由ServletContextListener 来处理。在 ServletContextListener 接口中定义了处理ServletContextEvent 事件的两个方法。

项目代码实现

定义一个监听器

public class OperationReplyListener implements ServletContextListener {

    private static final Log logger = LogFactory.getLog(OperationReplyListener.class);
    final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);  

    public void contextDestroyed(ServletContextEvent sce) {

    }

    public void contextInitialized(ServletContextEvent sce) {
        logger.info("定时器启动...");
        OperationReplyTask operationReplyTask = new OperationReplyTask();
        scheduler.scheduleAtFixedRate(operationReplyTask, 2, 5, TimeUnit.SECONDS);
    }
}

web.xml中配置监听器

<listener>
    <listener-class>com.sto.rookie.timer.OperationReplyListener</listener-class>
</listener>