[XERCESJ-727] cloneNode(true) Element are still linked to original Element

• Type: Bug
• Status: Open
• Resolution: Unresolved
• Affects Version/s: 2.4.0
• Component/s: DOM (Level 3 Core)
• Environment: Operating System: Windows NT/2K; Platform: PC
• Bugzilla Id: 19982
• Assignee: Unassigned
• Reporter: Francois Abot
• Created: 16/May/03 16:35
• Updated: 16/Feb/11 23:56

Description

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.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(); }

}
}