初学监听器，报错找不到，请指正，谢谢。环境为eclipse

报错信息：

严重: Error configuring application listener of class com.imooc.listener.FirstListener

java.lang.ClassNotFoundException: com.imooc.listener.FirstListener

代码：

web.xml为：

<listener>

    <listener-class>com.imooc.listener.FirstListener</listener-class>

  </listener>

FirstListener监听器类的代码为：

package com.imooc.listener;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

public class FirstListener implements ServletContextListener {

public FirstListener() {

}

@Override

public void contextInitialized(ServletContextEvent sce) {

System.out.println("contextInitialized");

}

@Override

public void contextDestroyed(ServletContextEvent sce) {

System.out.println("contextDestroyed");

}

}