DOM java question

Hi all,

      I am saving my configuration file in XML. I retrieve and update the config file using DOM. I am able to retrieve the data but when i update the valuse changes in memory but not in file.

/* xml file */
<Configuration>
  // Some other stuffs here //
      <Config>
            <ip>192.168.3.120</ip>
            <port>1521</port>
            <database>hitech</database>
      </Config>
</Configuration>

method to update value is like this
       
        static final String NODENAME = "Config";

      public void setConfiguration(String configName, String data)
      {
            NodeList listOfTag = doc.getElementsByTagName(NODENAME);
            int totalTag = listOfTag.getLength();
            for(int i=0;i<totalTag;i++)
            {
                  Node fieldNode = listOfTag.item(i);
                  if(fieldNode.getNodeType()==Node.ELEMENT_NODE)
                  {
                        Element fieldElement = (Element)fieldNode;
                        NodeList tagList = fieldElement.getElementsByTagName(configName);
                        Element tagElement = (Element)tagList.item(0);
                        NodeList textTList = tagElement.getChildNodes();
                        ((Node)textTList.item(0)).setNodeValue(data);
                        System.out.println("Written " + data + " reread " + ((Node)textTList.item(0)).getNodeValue().trim());
                        break;
                  }
            }
      }

The change is only seen in memory but not in file.

Thanks in advance
Manish Regmi