首先，引入Bootstarp的CDN，
<button type="button"class="btn dropdown-toggle"id="dropdownMenu1"data-toggle="dropdown">
主题<span class="caret"></span>
</button>
<ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
<li role="presentation">
<a role="memuitem" tabindex="-1" a href="#">Java</a>
</li>
<li role="presentation">
<a role="menuitem" tabindex="-1" a href="#">数据挖掘</a>
</li>
<li role="presentation">
<a role = "menuitem" tabindex="-1" a href=""#>数据通信</a>
</li>
<li role="presentation">
<a role = "menuitem" tabindex="-1" a href="#">分离的链接</a>
</li>
</ul>
