A key to differentiate objects generated by the same factory for a
SharedResourcesBroker. The
SharedResourcesBroker is guaranteed to return the same object for the same factory,
ScopeInstance,
and
SharedResourceKey, but different objects if any of this differ. The key can contain information relevant
to the factory.
Note: keys are compared using
#equals(Object). It is important for implementations to override this method.
Example: for a file handle factory, the key could specify the path of the file for which the handle is needed.