xindice btree on disk

Hello,

I have been looking for a solid implementation of a Java BTree on disk for
the longest time. Could it be possible to use xindices btree on disk
seperately.
If so, where would be a good place to start. I have looked at the source
code, instantiated a new BTree and added values to it, but I got a null
pointer exception saying that the file was not opened.

Thanks to all,
-C.B.