Android网络请求，在电脑浏览器上可以访问到服务器，手机却访问不到

以下是firstPost方法 ：

 public static String firstPost(String urlString) {
 StringBuffer resultData = new StringBuffer("");// 使用StringBuffer比String更高效
 try {
 URL url = new URL(urlString);// 新建URL对象
 HttpURLConnection urlConn = (java.net.HttpURLConnection) url
 .openConnection();// 新建网络连接对象
 urlConn.setRequestMethod("POST");
 // 避免出现乱码
 urlConn.setRequestProperty("Accept-Charset", "utf-8");
 urlConn.setRequestProperty("contentType", "utf-8");
 urlConn.setRequestProperty("Content-Type",
 "application/x-www-form-urlencoded");// POST方式必需
 // 获取sessionId
 java.lang.String cookieval = urlConn.getHeaderField("set-cookie");
 if (cookieval != null) {
 MyApplication.setSessionid(cookieval.substring(0,
 cookieval.indexOf(";")));
 Log.e("sessionId", MyApplication.getSessionid());
 }
 urlConn.connect();
 java.io.InputStreamReader in = new java.io.InputStreamReader(
 urlConn.getInputStream());// 新建IO对象
 if (urlConn.getContentLength() != -1) {// 响应头部返回了流的长度
 char[] buffer = new char[urlConn.getContentLength()];
 in.read(buffer, 0, buffer.length);
 resultData.append(buffer);
 } else {// 流的长度未知
 java.io.BufferedReader buffer = new java.io.BufferedReader(in);// 输出流
 java.lang.String inputLine = null;
 while (((inputLine = buffer.readLine()) != null))// 使用循环来读取获得的数据
 {
 resultData.append(inputLine + "\r\n");
 }
 }
 in.close();
 urlConn.disconnect();
 } catch (java.net.UnknownHostException e) {// 未连接网络
 e.printStackTrace();
 } catch (android.os.NetworkOnMainThreadException e) {// UI线程不支持网络操作
 e.printStackTrace();
 } catch (Exception e) {
 e.printStackTrace();
 }
 return resultData.toString();
 }