Obtains the object to be pointed by the IDREF value.
While a document is being unmarshalled, every time
an IDREF value is found, this method is invoked immediately to
obtain the object that the IDREF is pointing to.
This method returns a
Callable to support forward-references.
When this method returns with a non-null return value,
the JAXB RI unmarshaller invokes the
Callable#call() method immediately.
If the implementation can find the target object (in which case
it was a backward reference), then a non-null object shall be returned,
and it is used as the target object.
When a forward-reference happens, the
call method
should return null. In this case the JAXB RI unmarshaller invokes
the
call method again after all the documents are fully unmarshalled.
If the
call method still returns null, then the JAXB RI unmarshaller
treats it as an error.
A
Callable object returned from this method may not throw
any exception other than a
SAXException (which means a fatal error.)