bootstrap 汉堡按钮无法显示

<nav class="navbar navbar-default" role="navigation">
<div class="container-fluid">
<div class="navbar-header">
<button type="button" class="navbar-toggle" data-toggle="collapse"
data-target="#example-navbar-collapse">
<span class="sr-only">切换导航</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">菜鸟教程</a>
</div>
<div class="collapse navbar-collapse" id="example-navbar-collapse">
<ul class="nav navbar-nav">
<li class="active"><a href="#">iOS</a></li>
<li><a href="#">SVN</a></li>
<li class="dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown">
Java <b class="caret"></b>
</a>
<ul class="dropdown-menu">
<li><a href="#">jmeter</a></li>
<li><a href="#">EJB</a></li>
<li><a href="#">Jasper Report</a></li>
<li class="divider"></li>
<li><a href="#">分离的链接</a></li>
<li class="divider"></li>
<li><a href="#">另一个分离的链接</a></li>
</ul>
</li>
</ul>
</div>
</div>
</nav>