join()
join()
A callback function which will be invoked periodically by a method of another object that runs a long time.
Marker interface used to say that a class has supports a join() callback method. IndexArchiveBundle has methods which take objects that implement Join. For activities which may take a long time such as index saving index tier merging IndexArchiveBundle will periodically call the Join objects join method so that it can continue processing rather than blocking entirely until the long running method completes