How to get Drupal to call hook_stream_wrappers() implementation?

Hi

I am creating a set of stream wrappers. One of which is intended to override "public", in other words, a class that extends DrupalPublicStreamWrapper.

I have created an implementation of hook_stream_wrappers(), but it is not getting called.

Thereby, when file_stream_wrapper_get_instance_by_scheme() is called with scheme as "public" is using DrupalPublicStreamWrapper and not my extension.

What do I need to do to get Drupal to call hook_stream_wrappers()?

Thanks