There were only two threads stuck, so monitoring the threads wouldn't have set off any alarms. One was a Clojure agent thread, which basically allowed all the clients marked for retirement to queue up.

I used both Yourkit and Java Mission Control for monitoring the JVM. They both have some functionality of deadlock detection, but it did not flag these two threads. Yourkit identified these threads as waiting, not blocked, but I'm not sure how it makes that distinction.