How to start and shut down gracefully

[CyJaySong]

How to start and shut down gracefully. To ensure that producers and consumers complete their work during the period

[geniusjoe]

I think we only need to do is to make sure we have called producer.close() consumer.close() and client.close() and wait them return before exiting the whole program.
These methods are all sync methods and they will block caller thread until they have done with all pending messages.