在后台用httpclient来进行跨域，测试过可行，可是，当访问用CAS的系统时，响应的500，response的是html的，上面有CAS出现，猜测是CAS导致的。
（测试用的是一个没有CAS的简单系统，所以。。。）
直接上代码，请路过的大大们帮忙看看，是不是CAS的问题。该怎么解决，谢谢！
httpclient跨域的：
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpGet = new HttpGet(url);
CloseableHttpResponse response = null;
response = httpclient.execute(httpGet);
StatusLine statusLine = response.getStatusLine();
String result = "";
HttpEntity entity = response.getEntity();
result = EntityUtils.toString(entity);
System.out.println(statusLine.getStatusCode());
System.out.println(result);
result的结果：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
<title>CAS – Central Authentication Service</title>
<link type="text/css" rel="stylesheet" href="/cas/css/cas.css;jsessionid=F6CA81B83A103D740DA8EEDA39E306F4" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<link rel="icon" href="/cas/favicon.ico;jsessionid=F6CA81B83A103D740DA8EEDA39E306F4" type="image/x-icon" />
</head>
<body id="cas" class="fl-theme-iphone">
<div class="flc-screenNavigator-view-container">
<div class="fl-screenNavigator-view">
<div id="header" class="flc-screenNavigator-navbar fl-navbar fl-table">
<h1 id="company-name">Jasig</h1>
<h1 id="app-name" class="fl-table-cell">Central Authentication Service (CAS)</h1>
</div>
<div id="content" class="fl-screenNavigator-scroll-container">
<div id="welcome">
<h2>CAS is Unavailable</h2>
<p>
There was an error trying to complete your request. Please notify your support desk or try again.
</p>
</div>
</div>
<div id="footer" class="fl-panel fl-note fl-bevel-white fl-font-size-80">
<a id="jasig" href="http://www.jasig.org" title="go to Jasig home page"></a>
<div id="copyright">
<p>Copyright © 2005 - 2012 Jasig, Inc. All rights reserved.</p>
<p>Powered by <a href="http://www.jasig.org/cas">Jasig Central Authentication Service 3.5.2</a></p>
</div>
</div>
</div>
</div>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/jquery-ui.min.js"></script>
<script type="text/javascript" src="/cas/js/cas.js;jsessionid=F6CA81B83A103D740DA8EEDA39E306F4"></script>
</body>
</html>
