Returns a Java reference to the servant which should be used for this
request. servant_preinvoke() is invoked by a local stub.
If a ServantObject object is returned, then its servant field
has been set to an object of the expected type (Note: the object may
or may not be the actual servant instance). The local stub may cast
the servant field to the expected type, and then invoke the operation
self - The object reference which delegated to this delegate.
operation - a string containing the operation name.
The operation name corresponds to the operation name as it would be
encoded in a GIOP request.
expectedType - a Class object representing the expected type of the servant.
The expected type is the Class object associated with the operations
class of the stub's interface (e.g. A stub for an interface Foo,
would pass the Class object for the FooOperations interface).
a ServantObject object.
The method may return a null value if it does not wish to support
this optimization (e.g. due to security, transactions, etc).
The method must return null if the servant is not of the expected type.