The CommandAwareRpcDispatcher uses the timeout executor to trigger the cancellation of an async rpc after some timeout. However, in the event that the command executes prior to the timeout (i.e. most of the time), the task remains in the timeout executor's queue. Consequently, the reference to the RspListFuture/SingleResponseFuture will remain in the executor's queue unnecessarily for the duration of the timeout.
Using ScheduledThreadPoolExecutor.setRemoveOnCancelPolicy(true) should fix the problem.