方法一：
<script src="http://lib.sinaapp.com/js/jquery11/1.8/jquery.min.js"></script>
<script>window.jQuery || document.write(unescape("%3Cscript src='/skin/mobile/js/jquery.min.js' type='text/javascript'%3E%3C/script%3E"))</script>
方法二：
<script type="text/javascript">
if (typeof jQuery == 'undefined') {
document.write(unescape("%3Cscript src='${ctx}/static/lib/jquery/jquery-1.9.1.min.js' type='text/javascript'%3E%3C/script%3E"));
}
</script>