I need help for the following problem: I use Websphere Liberty 19.0.0.9 with Oracle and IBM Java 1.8 an run an older application (EAR) containing an EJB, which serializes XML with JAXB. The application needs to control XML namespace definitions and prefixes and does this by providing an implementation of com.sun.xml.bind.namespacePrefixMapper to javax.xml.bind.Marshaller.setProperty with property "com.sun.xml.bind.namespacePrefixMapper". At runtime the error java.lang.NoClassDefFoundError: com/sun/xml/bind/marshaller/NamespacePrefixMapper occurs when loading the implementation class. The server.xml contains feature javaee-8.0, and the liberties’ JAXB implementation wlp-19.0.0.9\lib\com.ibm.ws.jaxb.tools.2.2.10_1.0.32.jar contains the class com.sun..xml.bind.marshaller.NamespacePrefixMapper.
I tried to solve it by putting the jaxb-impl-2.2.4.jar to the EAR/lib (which is the wrong way because JAXB is provided by JEE) but then an error occurred in the com.sun.xml.bind.v2.runtime.MarshallerImpl.setProperty(MarshallerImpl.java:511) because the check if(!(value instanceof NamespacePrefixMapper)) failed, because the Classloader (AppClassLoader) of the implementation provided another class object for class NamespacePrefixMapper than the MarshallerImpls’ classloader (org.eclipse.osgi.internal.loader.EquinoxClassLoader). But this showed that the liberty can access the the NamespacePrefixMapper.
I made several attempts to use the same classloader for the implementation and the MarschallerImpl when loading them, and I tried to solve it by classloder settings in the server.xml. No success. I know that it is not recommended to use such JAXB implementation specific classes, but the application was developed this way and cannot be changed easily.
Any help is appreciated which tells me how to convince liberty to either provide the NamespacePrefixMapper class to the application classloader, or to use the application classloaders NamespacePrefixMapper also in the MarschallerImpl. Thank you.
//The implementation class looks for example like this:
public class MyNamespacePrefixMapperImpl extends com.sun.xml.bind.marshaller.NamespacePrefixMapper {...}
JAXBContext c = JAXBContext.newInstance(some mapped class);
Marshaller m = c.createMarshaller();
com.sun.xml.bind.marshaller.NamespacePrefixMapper mapper = new MyNamespacePrefixMapperImpl();// Here the NoClassDefFoundError occurs.
m.setProperty("com.sun.xml.bind.namespacePrefixMapper", mapper); // Here the instanceof check fails if jaxb-impl.jar is in EAR/lib.
this is a precarious situation without an easy solution. Liberty attempts to "hide" internal packages to avoid scenarios where users want a slightly different version of the implementation than what the framework provides - the most glaring example of this problem was in traditional WAS where users wanted to use a different version of Jakarta Commons Logging than what was shipped with WAS - this required users to provide their own, either in an isolated shared library, or use other parent-last classloading hacks to make it work. Liberty avoids those issues by isolating the internal implementations from the user applications.
So that works great when a user wants to use a different version of a third party library than what Liberty provides, but as you have discovered, that doesn't work so great when your legacy application depends on those hidden/isolated third party libraries.
The most ideal solution would be to refactor the application code so as to not depend on internal JAXB classes - somebody with more JAXB expertise may be able to help with this... But it sounds like that may not be feasible, so another alternative would be to create a user feature. A user feature is essentially an extension to Liberty's runtime - so it has access to packages that user applications do not. It also allows you to add packages as APIs for the user applications - so you could use a user feature to add the com.sun.xml.bind.marshaller as a public API - then your user application could extend it freely. You could also include your MyNamespacePrefixMapperImpl class in your user feature and register it there so that it would automatically apply to all applications in your server.
You can find more information on user features here: https://www.ibm.com/support/knowledgecenter/en/SSEQTP_liberty/com.ibm.websphere.wlp.doc/ae/twlp_feat_example.html
Hope this helps, Andy