akka-in-action | Accompanying source code for akka in action

What is going wrong here is that there is blocking on the dispatcher.

On the JVM, threads backed by operating system threads, which are expensive both in memory and process scheduler overhead. One of the advantages of Akka is that it allows more efficient use of threads, by allowing you to run many actors on a smaller number of threads.

This is great, but does mean you should never perform a blocking call inside an actor. The CompletableFuture::join call here is blocking, which likely is the cause of your problem.

By avoiding blocking calls and using async API's (such as CompletableFuture.allOf) your problem should go away.