Class Loading Issues

It is possible that the Java libraries you have specified in the plugin libraries list conflict with the ones already loaded by Oxygen XML Developer. In order to instruct the plugin to prefer its libraries over the ones used by Oxygen XML Developer, you can add the following attribute on the <plugin> root element:classLoaderType="preferReferencedResources" from the plugin.xml descriptor.

A Late Delegation Class Loader (the main class loader in Oxygen XML Developer) is a java.net.URLClassLoader extension which prefers to search classes in its own libraries list and only if a class is not found there to delegate to the parent class loader.

The main Oxygen XML Developer Class Loader uses as libraries all jars specified in the OXYGEN_INSTALL_DIR\lib directory. Its parent class loader is the default JVM Class loader. For each instantiated plugin a separate class loader is created having as parent the Oxygen XML Developer Class Loader.

The plugin class loader can be either a standard java.net.URLClassLoader or a LateDelegationClassLoader (depending on the attribute classLoaderType in the plugin.xml). Its parent class loader is always the Oxygen XML Developer LateDelegationClassLoader.

If you experience additional problems like the following:
java.lang.LinkageError: ClassCastException: attempting to cast jar:file:/C:/jdk1.6.0_06/jre/lib/rt.jar!/javax/xml/ws/spi/Provider.classtojar:file:/D:/Program
    Files/Oxygen XML Editor
    12/plugins/wspcaccess/../../xdocs/lib/jaxws/jaxws-api.jar!/javax/xml/ws/spi/Provider.class
    at javax.xml.ws.spi.Provider.provider(Provider.java:94) at
    javax.xml.ws.Service.<init>(Service.java:56)
......................................................................
The cause could be the fact that some classes are instantiated using the context class loader of the current thread. The most straightforward fix is to write your code in a try/finally statement:
  ClassLoader oldClassLoader = Thread.currentThread().getContextClassLoader();
  try {
    //This is the implementation of the WorkspaceAccessPluginExtension plugin interface.
    Thread.currentThread().setContextClassLoader(
        CustomWorkspaceAccessPluginExtension.this.getClass().getClassLoader());
    //WRITE YOUR CODE HERE
  } finally {
    Thread.currentThread().setContextClassLoader(oldClassLoader);
  }