This wiki page, http://wiki.apache.org/couchdb/Getting_started_with_Java, lists several CouchDB Java clients. I wonder if any of the clients mentioned is significantly more dominant/popular/better/supported than the others.
I think Ektorp is pretty much the clear leader at the mo. I've certainly been very happy with it, and I've never seen anybody using anything else.
In addition, as a quick metric, Ektorp is clearly far more actively maintained than any of the others: currently the last commit on Ektorp was 5 hours ago, vs 4 years for CouchDB4j, a little over 1 year for jcouchdb and 2 years for jRelax.
Lightcouch seems to be most active nowadays. Ektorp is also still active, however.
Each has its own strengths, however, so I'd pick based on what level you want to be abstracted from CouchDB's HTTP API. Myself, I'd use Lightcouch.
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