XMLConfiguraction:Content is not allowed in prolog

这读的问题已经解决,
但现在又出现了写的问题:
org.apache.commons.configuration.ConfigurationException: java.lang.NullPointerException
at org.apache.commons.configuration.AbstractFileConfiguration.save(AbstractFileConfiguration.java:344)
at org.apache.commons.configuration.AbstractFileConfiguration.save(AbstractFileConfiguration.java:315)
at org.apache.commons.configuration.XMLConfiguration.save(XMLConfiguration.java:405)
at com.rareliu.xml.util.XMLConfigurationUtil.setNodeValue(XMLConfigurationUtil.java:100)
at com.rareliu.xml.action.EmailConfigXMLAction.updateObject(EmailConfigXMLAction.java:93)
at com.rareliu.xml.action.EmailConfigXMLAction.actionProcess(EmailConfigXMLAction.java:44)
at com.eroadsoftware.infogate.web.action.BaseAction.execute(BaseAction.java:212)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)......
出了这个错,不明白为什么会有空指针异常:
load文档没有出错,load后可以读,xmlConfigU.getString("hostConfigs.hostConfig[@name]");
写:xmlConfigU.setProperty("hostConfigs.hostConfig[@name]", hostConfigName);时就报上面的错了!
求大家帮忙解决问题!