Hibernate Spatial requires Spatial Dialect Providers to supply the database specific support for geographic data. Currently providers exist for Postgresql (PostGis spatial Extension), MySQL and for Oracle 10g (Standard Locator).
If you use only one database in you application, you just need to put the appropriate spatial dialect provider in your classpath. The Hibernate Spatial Extension will pick it up, and since it is the only provider in the classpath, use the dialect it provides as the spatial dialect