Create a new {@link ExecutorSubscribableChannel} instance where messages will be sent in the callers thread.
Create a new {@link ExecutorSubscribableChannel} instance where messages will be sent via the specified executor. @param executor the executor used to send the message, or {@code null} to execute in the callers thread.
A {@link SubscribableChannel} that sends messages to each of its subscribers.
@author Phillip Webb @author Rossen Stoyanchev @since 4.0