As reported via users list. https://lists.apache.org/thread.html/7b6c57b1838f180019bdd2bf8bf673b3b2365fe74f0e035b3b20cf8c@%3Cusers.tomcat.apache.org%3E With multiple start/stop threads, concurrent calls are possible to the RMI leak cleaning which is not thread safe.
I added sync on the (static) objTable field from sun.rmi.transport.ObjectTable, and this should fix the issue, although I didn't attempt to reproduce the exception. It will be in 9M16, 8.5.10, 8.0.40 and 7.0.74.