Monday, June 10, 2013

never edit table model on non-EDT?

A Swing interviewer asked me

Q: Can a non-EDT thread ever writes to the underlying data-structure of a (table/tree/list) model? 
A: If it does, we are at risk. EDT could be reading it -- uncoordinated and unexpected. EDT might see corrupted data. Classic reader/writer race scenario. Murphy's Law -- if it could happen it will.

(In theory, EDT can also update the data due to a message listener or a DB operation --writer/writer race, but) This one scenario alone is reason enough. All underlying data queried by EDT should Never be modified outside EDT.

In one real world live trading app though, some MOM listener threads do indeed update underlying data structure. When that happens, users are not allowed to trigger a refresh -- we can disable some buttons. After the update, we send invokeLater(fireXXX()) to refresh the view.

You may want to lock-protect the underlying data, but then EDT might be blocked. Yet, this is widespread. Look at DefaultTableModel in which EDT does access the underlying Vector, which is lock-protected. If the Vector is large, growing it (perhaps 2 folds) can take log(N) time, and EDT would be blocked for that duration. However, in practice this latency is probably below 50ms and unnoticeable.

As a minimum requirement, ALL models need to be thread-safe. That often entails locking or CAS or copy-on-write.

Swing models normally do not use large data structures (since a screen can't realistically show even 1000 rows). If it does, update should ideally be designed with a worker thread, to populate a replacement data structure, and then ask EDT to swap it in. Many non-ideal situations exist in practice, which calls for specific techniques and tactical solutions.

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http://www.linkedin.com/in/tanbin