Another way to use util.concurrent 2-way async without callbacks would be to have an executor run a “request service” and also run a second queue back to a “response handling service.” That way you get the benefit of queueing, and you can free up the “request service” threads a little sooner. But callbacks will work well in many situations where response handling is fast (setting values or using in-memory data structures). The javadocs for javax.xml.ws.Service hint that Service may use this strategy.

One suggestion: change the formatting a little – there are some long lines with “2 lines of code” on them. I was initially confused because I was reading with a large font size, and the 2nd line (very important code) was invisible off to the right of the text box.

This is the line that clued me in to the problem, since I originally did not see “cs” defined:

ExecutorService es = Executors.newFixedThreadPool(3); CompletionService cs = new ExecutorCompletionService(es);