In this example, the RWThreadIOUFunction class closes the IOU when the sync_service() function returns a result back to the runnable.

Threads attempting to redeem an IOU that has not been closed block until another thread writes a result or exception to that IOU object. If an IOU is closed with an exception, any attempt to redeem the IOU produces that exception.

For more information, see Closing an IOU and Redeeming an IOU.