Future and Promise are the two separate sides of an asynchronous operation.
promise is used by the "producer/writer" of the asynchronous operation.
future is used by the "consumer/reader" of the asynchronous operation.
1.
GenericFutureListener.operationComplete(Future) is directly called by an I/O thread. Therefore, performing a time consuming task or a
blocking operation in the handler method can cause an unexpected pause during I/O. If you need to perform a blocking operation on I/O completion,
try to execute the operation in a different thread using a thread pool.