XPath class resolution in JBoss5

Question

I'm having a hard time figuring out where the problem is coming from, so I'm posting this in the hopes that others might have found something similar to this elsewhere and are kind enough to share their insight.

I'm using a JBoss 5.0.1.GA application server running on top of a Sun Java 1.6.0-13 JDK. For the WAR file in the generated Web Service, I use a Axis2 1.4 WS engine whose JAR files are inserted by Eclipse Galileo into the project's WEB-INF/lib directory when creating the Webservice from the given "worker" class in the Dynamic Web Project. The relevant code snippet follows:

String sUrl = "http://example.com/datafile.xml";
String sPath = "/some/xpath/string";
InputStream input = new URL(sUrl).openStream();
InputSource source = new InputSource(input);
DocumentBuilderFactory docFact = DocumentBuilderFactory.newInstance();
docFact.setNamespaceAware(false);
DocumentBuilder parser = docFact.newDocumentBuilder();
Document doc = parser.parse(source);
XPath xpath = XPathFactory.newInstance().newXPath();
// error occurs here:
String result = (String) xpath.evaluate(path,doc,XPathConstants.STRING);
input.close();

This is the error I'm getting from the JBoss log:

java.lang.LinkageError: loader constraint violation: when resolving field "STRING" the class loader (instance of org/jboss/classloader/spi/base/BaseClassLoader) of the referring class, javax/xml/xpath/XPathConstants, and the class loader (instance of <bootloader>) for the field's resolved type, javax/xml/namespace/QName, have different Class objects for that type

I could use the XPath.evaluate(String,Document) — however there are occasions where I need to get (for example) a XPathConstants.NODESET instead, so it's a no-go. I have also tried to fumble a little by littering some jboss-web.xml files here and there in the WAR file, but with no effect.

What I'm trying to understand is:

• Where could the error be coming from? The JBoss class loader? Some weird interaction between JBoss and the Sun JDK? Some weirdness introduced by Eclipse when creating the Web Service? Maybe some confusion introduced by the Axis2 libraries deployed within the WAR?
• I've found instances of compiled class files in what looks like a triple-whammie:
  • Sun JDK (file rt.jar);
  • JBoss libraries ($JBOSS_HOME/lib/endorsed/stax-api.jar); and
  • Axis2-deployed libraries ($JBOSS_HOME/server/deploy/MyProject.ear/MyProject.war/WEB-INF/lib/axis2-saaj-api-1.4.jar and woden-impl-dom-1.0M8.jar).
• How exactly am I supposed to configure JBoss to tell it which classes it's OK to load from "other" libraries from? Specifically, the jaxax.xml.namespace.QName is is causing the grief.

Thank you in advance.

Answer 1

JBoss will throw a LinkageError when the application's classpath contains classes which JBoss considers "protected", i.e. it does not permit the application to contain its own copies of certain key APIs.

In this case, it looks like your appcontains its own copies of the javax.xml.xpath API, and possibly some others as well, as you mentioned.

You need to remove anything from your EAR/WAR's lib directories that clashes with JBoss's own libraries (e.g. axis2-saaj-api-1.4.jar).

Answer 2

It seems that the problem was solved by removing the javax.xml.namespace.* package and respective classes from the deployed Axis2 JAR files. Namely, using Axis2 1.4.1 (instead of 1.4), I repackaged these JAR files:

• axis2-saaj-api-1.4.1.jar, by removing javax.xml.namespace
• woden-impl-dom-1.0M8.jar, by removing javax

Also, Eclipse is extremely picky at the project configuration. So far, I've found that the Project Facet for the Dynamic Web Project has to be created with a Dynamic Web Module of version 2.4 (and not 2.5 as it suggests by default), but with a Java version 6 (same as the branch of the used JDK). I don't know why this happens, I suppose the Dynamic Web Module version 2.4 tying up by default with Java 1.4 in Eclipse is where all the confusion comes from. Some googling led me to believe that package javax.xml didn't become incorporated into the JDK until Java 5 or Java 6 -- hence the possible mixup! However, I'm not knowledgeable enough to investigate if the problem comes from how Eclipse packages the archive files for deployment so this is just a suspicion I have so far.