android xml dom解释

private
NodeList root(
final
String url , 
final
String str){
      
NodeList root =
null
;
      
try
{
      
InputSource is=
new
InputSource(
new
InputStreamReader(
new
URL(url).openStream()));
      
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance
();
          
DocumentBuilder dom = factory.newDocumentBuilder();
          
Document doc = dom.parse(is);
          
root = doc.getElementsByTagName(str);
          
          
      
}
catch
(Exception e){
          
e.printStackTrace();
      
}
      
return
root;
   
}