大家都知道jQuery是一个框架，它对JS进行了封装，使其更方便使用。前面两篇博文分别是用CSS样式和JS实现的，那么这篇就用jQuery来实现二级下拉式菜单。
使用JQuery实现需要用到的知识有：
1)使用$(function(){...})获取到想要作用的HTML元素。2)通过使用children()方法寻找子元素。3)通过使用show()方法来显示HTML元素。4)通过使用hide()方法来隐藏HTML元素。5)jQuery库引用方法:
第一种方法：将jQuery库下载到电脑上，然后引用，我下载的是jquery-1.7.1.min.js这个版本。
例如：
<script type="text/javascript" src="jquery/jquery-1.7.1.min.js"></script>
第二种方法：直接引用在线服务器上的jQuery库文件，比如谷歌服务器jQuery库，百度服务器jQuery库等。
例如：引用百度服务器上的jQuery库文件
<script type="text/javascript" src="jquery/1.9.0/jquery.js"></script>
接下来看看制作的流程：
1、调用jQuery库：编写代码，引用jquery库。由于谷歌已退出大陆，建议使用百度服务器的jQuery库。注意: 百度服务器的jQuery库地址:/jquery/1.9.0/jquery.js2 、编写显示子菜单函数，使用$,并通过class名获取一级菜单li，过children()找到li的孩子元素ul，使用show()方法,显示二级菜单。3、编写隐藏子菜单函数，使用$,并通过class名获取一级菜单li，过children()找到li的孩子元素ul，使用hide()方法, 隐藏二级菜单。4、做浏览器兼容性测试，至少五个浏览器。IE7,8,9，火狐，谷歌，2345浏览器等。
先来看看效果图：
最后我们来看看代码的情况，和前面的区别不大：
HTML代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "/TR/xhtml1/DTD/xhtml1-transitional.dtd"><htmlxmlns="/1999/xhtml"><head><metahttp-equiv="Content-Type"content="text/html; charset=gb2312" /><title>下拉菜单</title><linkrel="stylesheet"type="text/css"href="style.css" rel="external nofollow"/><!--引用百度服务器的jQuery库--><scriptsrc="/jquery/1.9.0/jquery.js"></script><scripttype="text/javascript"src="script.js"></script></head><body><divid="nav"class="nav"><ul><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >网站首页</a></li><liclass="navmenu"><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >课程大厅</a><ul><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >JavaScript</a></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >jQuery</a></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Ajax</a></li></ul></li><liclass="navmenu"><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >学习中心</a><ul><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >视频学习</a></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >案例学习</a></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >交流平台</a></li></ul></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >经典案例</a></li><li><ahref="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >关于我们</a></li></ul></div></body></html>
CSS样式表外部style.css文件代码：
/*CSS全局设置*/*{margin:0;padding:0;}.nav{background-color:#EEEEEE;height:40px;width:450px;margin:0auto;}ul{list-style:none;}ul li{float:left;line-height:40px;text-align:center;}a{text-decoration:none;color:#000000;display:block;width:90px;height:40px;}a:hover{background-color:#666666;color:#FFFFFF;}ul li ul li{float:none;background-color:#EEEEEE;}ul li ul{display:none;}/*为了兼容IE7写的CSS样式，但是必须写在a:hover前面*/ul li ul li a:link,ul li ul li a:visited{background-color:#EEEEEE;}ul li ul li a:hover{background-color:#009933;}
JS脚本外部script,js文件代码：
$(function(){$(".navmenu").mouseover(function(){$(this).children("ul").show();})$(".navmenu").mouseout(function(){$(this).children("ul").hide();})})
