Unknown error in XPath - xpath

Question

One of our web application, which has been running in our production environment for a long time, most recently it is running into an weird error when there is a high volume of transactions. We couldn't figure out what is exactly the root cause of the problem, but we found some similar issues in the previous version, WebSphere 6, related to a bug in Xalan version used by the app server. Our application server actually is WebSphere 7, which is supposed to have it fixed, besides it's not using Xalan under the hood anymore. Our application doesn't have Xalan jar embedded too.
To have it fixed we just restart the application itself.
One important note is that the Document is being cached (docs.get(tableName)) and reused to execute the XPath evaluation. We did it to avoid the cost of parsing the Document every time.
The app code is
Document doc = null;
try {
doc = docs.get(tableName);
if (doc == null)
return null;
XPathFactory xFactory = XPathFactory.newInstance();
XPath xpath = xFactory.newXPath();
XPathExpression expr = xpath.compile(toUpper(xPathQuery));
Object result = expr.evaluate(doc, XPathConstants.NODESET);
return (NodeList) result;
} catch (XPathExpressionException e) {
logger.error("Error executing XPath", e);
}
And the error stack is here
javax.xml.transform.TransformerException: Unknown error in XPath.
at java.lang.Throwable.<init>(Throwable.java:67)
at javax.xml.transform.TransformerException.<init>(Unknown Source)
at org.apache.xpath.XPath.execute(Unknown Source)
at org.apache.xpath.jaxp.XPathExpressionImpl.evaluate(Unknown Source)
Caused by: java.lang.NullPointerException
at org.apache.xerces.dom.ElementNSImpl.getPrefix(Unknown Source)
at org.apache.xml.dtm.ref.dom2dtm.DOM2DTM.processNamespacesAndAttributes(Unknown Source)
at org.apache.xml.dtm.ref.dom2dtm.DOM2DTM.nextNode(Unknown Source)
at org.apache.xml.dtm.ref.DTMDefaultBase._nextsib(Unknown Source)
at org.apache.xml.dtm.ref.DTMDefaultBase.getNextSibling(Unknown Source)
at org.apache.xml.dtm.ref.DTMDefaultBaseTraversers$ChildTraverser.next(Unknown Source)
at org.apache.xpath.axes.AxesWalker.getNextNode(Unknown Source)
at org.apache.xpath.axes.AxesWalker.nextNode(Unknown Source)
at org.apache.xpath.axes.WalkingIterator.nextNode(Unknown Source)
at org.apache.xpath.axes.NodeSequence.nextNode(Unknown Source)
at org.apache.xpath.axes.NodeSequence.runTo(Unknown Source)
at org.apache.xpath.axes.NodeSequence.setRoot(Unknown Source)
at org.apache.xpath.axes.LocPathIterator.execute(Unknown Source)
... 16 more
This is the similar issue what I mentioned.
http://www-01.ibm.com/support/docview.wss?uid=swg1PK42574
Thakns.

Answer 1

Many people don't realise that the DOM is not thread-safe (even if you are only doing reads). If you are caching a DOM object, make sure you synchronize all access to it.
Frankly, this makes DOM unsuited to this kind of application. I know I'm biased, but I would suggest switching to Saxon, whose native tree implementation is not only far faster than DOM, but also thread-safe. One user recently tweeted about getting a 100x performance improvement when they made this switch.