[ http://issues.apache.org/jira/browse/XERCESJ-727?page=comments#action_60359 ] Michael Glavassevich commented on XERCESJ-727: ----------------------------------------------
I still believe this is a threading problem. A cloned node has the same owner document as the original node, so they're part of the same DOM even though the new node doesn't have a parent yet. In DOM, there are two ways of traversing a node's children. One is to walk to the child/sibling chain with getFirstChild()/getNextSibling(). The other is to iterate over the node list returned from getChildNodes(). In Xerces DOM implementation, siblings are stored as a linked list. So to get from node 1 to n, you have to walk through all the nodes in between. To improve the performance of sequential lookups as well as getting the length, Xerces uses a cache. These caches which keep track of the last position accessed are managed by the owner document and get redistributed to other nodes when needed. If you're multi-threading with node lists, the cache could get swiped away from one and given to another while the first one is still using it, hence the NullPointerException. > cloneNode(true) Element are still linked to original Element > ------------------------------------------------------------ > > Key: XERCESJ-727 > URL: http://issues.apache.org/jira/browse/XERCESJ-727 > Project: Xerces2-J > Type: Bug > Components: DOM > Versions: 2.4.0 > Environment: Operating System: Windows NT/2K > Platform: PC > Reporter: Francois Abot > Assignee: Xerces-J Developers Mailing List > > NB : tested only under Windows 2000/ jdk 1.4.1_02 > ------- > - Create a DOM Element. > - Create many threads, giving each of them a cloneNode(true) copy of the > Element. > - Start all the threads. > - Each thread just finds the Element's embedded sub-Elements. > The following Exceptions occur many times (but not always) : > java.lang.NullPointerException > at org.apache.xerces.dom.ParentNode.nodeListItem(Unknown Source) > at org.apache.xerces.dom.ParentNode.item(Unknown Source) > at Tst2Threads.run(Tst2Threads.java:40) > java.lang.NullPointerException > at org.apache.xerces.dom.ParentNode.nodeListItem(Unknown Source) > at org.apache.xerces.dom.ParentNode.item(Unknown Source) > at Tst2Threads.run(Tst2Threads.java:41) > where lines 40 and 41 are : > if ( lesNoeuds.item(i).getNodeType() != Element.ELEMENT_NODE ) > continue; > noeud = (Element)(lesNoeuds.item( i )); > in the following example. > NB : if I create an different Element from file for each thread creation > (different Element instances, not created from cloneNode) then no Exception > occurs > This seems a wrong behaviour, keeping Xerces from being used in > multi-threading > environment. > I understand that Xerces is not thread safe, but how can I use same data in > different threads if the cloning fails this way ? > How can I write correct thread safe code using DOM data ? > These symptoms seem close to bug #6885 but are NOT DUPLICATE at first sight : > there is no concurrent access on the same instance in my example. > Thanks to tell me if there is any error in my code. I am interested in any > workaround for that behaviour... > Francois. > *************************************** > Here follows a code example (sorry for poor comments ;-). > Usage : > - just run it from command line, > - enter a XML file name, Element used will be the first one read from file (I > suggest a consequent Element), > - enter a number of threads, > ==> with 200 threads the Exception occurs almost immediately. > *************************************** > import org.w3c.dom.*; > public class Tst2Threads extends Thread > { > static protected Document readDOM( String nomFichier ) throws Exception > { > java.net.URL url= new java.net.URL( nomFichier ); > java.io.InputStream inputXML = url.openStream(); > > javax.xml.parsers.DocumentBuilderFactory factory = > javax.xml.parsers.DocumentBuilderFactory.newInstance(); > factory.setValidating( false ); > factory.setNamespaceAware( false ); > factory.setIgnoringComments( true ); > > javax.xml.parsers.DocumentBuilder builder = factory.newDocumentBuilder(); > > Document document = builder.parse( inputXML ); > > return document; > } > /*******************************************************/ > Element element = null; > > public Tst2Threads(ThreadGroup gp, Element elt, String _nom) > { > super( gp, _nom); > element = elt; > } > > public void run() > { > NodeList lesNoeuds = element.getChildNodes(); > Element noeud; > while (true) > { > for (int i=0, max=lesNoeuds.getLength(); i<max ; i++) > { > if ( lesNoeuds.item(i).getNodeType() != Element.ELEMENT_NODE ) > continue; > noeud = (Element)(lesNoeuds.item( i )); > } > } > } > // get first Element > static Element readElmt(String nomFic) throws Exception > { > Document doc = readDOM( "file://"+nomFic ); > NodeList liste = doc.getChildNodes(); > boolean done = false; > Element elmt = null; > if (liste != null) > { > for(int i=0, max=liste.getLength(); (i<max) && (! done); i++) > { > if ( liste.item(i).getNodeType() != Element.ELEMENT_NODE ) continue; > elmt = (Element)(liste.item( i )); > done = true ; > } > } > return elmt; > } > > /*******************************************************/ > public static void main( java.lang.String[] args) > { > try > { > java.io.BufferedReader br=new java.io.BufferedReader( new > java.io.InputStreamReader( System.in)); > System.out.print("XML file name for Element description > [d:/tmp/menu.xml] :"); > String nomFic= br.readLine(); > if ("".equals(nomFic)) nomFic = "d:/tmp/menu.xml"; > Element elmt = readElmt(nomFic); > System.out.print("Nb of threads [200] :"); > String s= br.readLine(); > if ("".equals(s)) s="200"; > int nbThreads = Integer.valueOf(s).intValue() - 1; > System.out.println(" Starting for :\n file="+nomFic+"\n nb > threads="+nbThreads); > > ThreadGroup groupe = new ThreadGroup("TEST"); > Tst2Threads t; > for (int i=0; i<nbThreads; i++) > { > // Element cloned here (from main thread --v) > t = new Tst2Threads(groupe, (Element)(elmt.cloneNode(true)), "Th"+i); > t.start(); > } > } > catch (Throwable t) > { > t.printStackTrace(); > } > } > } -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - If you want more information on JIRA, or have a bug to report see: http://www.atlassian.com/software/jira --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
