Deadlock Potential with ORM XML Processing

There exists the potential for deadlock with the ORM XML processing function. In an environment
with multiple threads (such as an application server), operations (such as creating a new
EntityManagerFactory or transformation Classloader activity) can lead to a point where
a Xerces SAX Parser (acquired by XMLFactory) is constituted and executed. Xerces calls
Thread.currentThread().getContextClassloader() during its execution to construct the
SAXParser (via ObjectFactory.createObject()). This means that within the call to Xerces,
a ClassLoader lock will be attempted on the Thread's ContextClassLoader. If there is already
a lock with a ClassLoader higher in the heirarchy, and another thread with a lock on the
same ContextClassLoader that is waiting to acquire a lock on a higher level ClassLoader, then
a deadlock will occur.

A solution to this problem is to temporally set the Thread's ContextClassLoader at the
point where XML APIs are invoked (such as in XMLFactory, XMLMetaDataParser) to the
ClassLoader that loaded the OpenJPA implementation classes. This will prevent Xerces from
starting with a lower level classloader and avoid this deadlock potential.