Enables Shiro end-users to configure and initialize one or more {@link Realm Realm} instances in any manner desired. <p/> This interface exists to support environments where end-users may not wish to use Shiro's default text-based configuration to create and configure realms, and instead wish to retrieve a realm configured in a proprietary manner. An implementation of this interface can access that proprietary mechanism to retrieve the already-created <tt>Realm</tt>s.