Service Base. on Close Async(CancellationToken cancellationToken) 方法
This method is called as the final step of closing the service. Override this method to be notified that Close has completed for this instance's internal components.
protected CompletableFuture<?> onCloseAsync(CancellationToken cancellationToken)
CancellationToken object to indicate the cancellation status of the operation.
A CompletableFuture that represents outstanding operation.