运行出现错误

严重: Servlet.service() for servlet [WeixinServlet] in context with path [/Weixin] threw exception [Servlet execution threw an exception] with root cause

java.lang.ClassNotFoundException: org.xmlpull.v1.XmlPullParserException

at com.thoughtworks.xstream.XStream.<init>(XStream.java:350)

at com.shqc.utl.MessageUtil.textMessageToXml(MessageUtil.java:50)

at com.shqc.servlet.WeixinServlet.doPost(WeixinServlet.java:53)

这是什么情况，维信给公众号发一次消息就出来一次