关于bootstrap书写警告框

代码如下：

<div class="fade in alert alert-danger col-md-3">
    <button type="button" class="close" data-dismiss="alert" aria-label="Close">&times;</button>
<p>警告警告警告！！！</p>
<button type="button" class="btn btn-danger " data-dismiss="alert" ></button>
</div>