When the POA has the NON_RETAIN policy it uses servant
managers that are ServantLocators. Because the POA
knows that the servant returned by this servant
manager will be used only for a single request,
it can supply extra information to the servant
manager's operations and the servant manager's pair
of operations may be able to cooperate to do
something different than a ServantActivator.
When the POA uses the ServantLocator interface,
immediately after performing the operation invocation
on the servant returned by preinvoke, the POA will
invoke postinvoke on the servant manager, passing the
ObjectId value and the Servant value as parameters
(among others). This feature may be used to force
every request for objects associated with a POA to
be mediated by the servant manager.