http://www.cnblogs.com/cxd4321/archive/2009/01/12/1374140.html
http://wenku.baidu.com/link?url=srA54aAcot14eb4rF72LnL_HmfVIO6A8wuhKkgmz1PDqaJSncaKbjPdWfbCcRwFit6OIOrxdkpkRCkQFANk2s7iVZSdmKuzvjF70c0caeHW