Problem: Application began spewing out the following error when clicking on our Ajax-ified radio buttons:

java.lang.LinkageError: loader constraint violation in interface itable initialization: when resolving method "org.ajax4jsf.org.w3c.tidy.DOMNodeImpl.setUserData(Ljava/lang/String;Ljava/lang/Object; Lorg/w3c/dom/UserDataHandler;)Ljava/lang/Object;"  the class loader (instance of com/ibm/ws/classloader/CompoundClassLoader) of the current class, org/ajax4jsf/org/w3c/tidy/DOMNodeImpl, and the class loader (instance of <bootloader>) for interface org/w3c/dom/Node have different Class objects for the type

Background: We were developing a Java EE application using RichFaces and A4J on the front end. The application was running fine the previous day, and then this happened. It was easy to enough to google “linkage error”, but it was difficult to locate the culprit. The error is often associated with the same JAR being loaded twice, but we checked and double-checked and the application was configured just fine.

So we tried locating where the org.w3c.dom classes were located in the JARs, and it turns out that the culprit was our upgrade of Apache POI.

Solution: For the purposes of enhancing another piece of functionality in our application, we needed to upgrade Apache POI to 3.6, including the JARs for the Open Office XML (OOXML) standards. Using Ant+Ivy, this resolved the dependency on Apache XMLBeans, which was packaged with 4 org.w3c.dom classes.

Naturally, this was what was causing the linkage error, since the WebSphere 7 libraries included an xml.jar, containing these classes. With no fix for the issue, we repackaged xmlbeans.jar by removing the org.w3c.dom classes and placing this repackaged version in our internal repository. After rebuilding the application, this made the errors go away.

This JIRA issue, https://issues.apache.org/jira/browse/XMLBEANS-376, confirms the addition of the org.w3c.dom classes (namely, DOMConfiguration, DOMStringList, TypeInfo, and UserDataHandler) to xmlbeans.jar, but the issue is considered minor. So far, no one from Apache XMLBeans team has confirmed whether they have added special logic in these classes.

Whether they did or they didn’t, it would have been a cleaner solution to not have packaged these classes with the JAR, in the first place.

References: Here are some of the articles / posts I used while researching this puzzler:

1. The java.lang.LinkageError: loader constraint violation demystified
2. Debug Your  App – java.lang.LinkageError

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