检查网络地址是否有效

	public static boolean exists(String URLName)
	{
		HttpURLConnection con = null;
		boolean isTrue = false;
		try
		{
			HttpURLConnection.setFollowRedirects(false);
			con = (HttpURLConnection) new URL(URLName).openConnection();
			con.setRequestMethod("HEAD");
			isTrue = (con.getResponseCode() == HttpURLConnection.HTTP_OK);
		}
		catch (Exception e)
		{
			isTrue = false;
		}
		finally
		{
			if (!isTrue)
			{
				System.out.println(URLName + " is not exist");
			}
			if (con != null)
			{
				con.disconnect();
			}
			return isTrue;
		}
	}