I've been able to obtain the index reader, but for the use of the FacetsCollector of the lucene 4.8 seems that I need an further index (taxonomy index). I would like to resolve with the "SortedSetDocValues strategy", using BitSets to collect data.
Do you have any suggestion?

I'm searching for rapid solution. I hope to resolve in this week!
Do you think that a valid solution could be to use Solr for faceting? Is there a way to read only the facet query from Solr?