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Best way to control concurrent access to Java collections

Asked 2009-02-18T15:53:08.110
27

Should I use old synchronized Vector collection, ArrayList with synchronized access or Collections.synchronizedList or some other solution for concurrent access?

I don't see my question in Related Questions nor in my search (Make your collections thread-safe? isn't the same).

Recently, I had to make kind of unit tests on GUI parts of our application (basically using API to create frames, add objects, etc.). Because these operations are called much faster than by a user, it shown a number of issues with methods trying to access resources not yet created or already deleted.

A particular issue, happening in the EDT, came from walking a linked list of views while altering it in another thread (getting a ConcurrentModificationException among other problems). Don't ask me why it was a linked list instead of a simple array list (even less as we have in general 0 or 1 view inside...), so I took the more common ArrayList in my question (as it has an older cousin).

Anyway, not super familiar with concurrency issues, I looked up a bit of info, and wondered what to choose between the old (and probably obsolete) Vector (which has synchronized operations by design), ArrayList with a synchronized (myList) { } around critical sections (add/remove/walk operations) or using a list returned by Collections.synchronizedList (not even sure how to use the latter).

I finally chose the second option, because another design mistake was to expose the object (getViewList() method...) instead of providing mechanisms to use it.

But what are the pros and cons of the other approaches?

[EDIT] Lot of good advices here, hard to select one. I will choose the more detailed and providing links/food for thoughts... :-) I like Darron's one too.

To summarize:

  • As I suspected, V
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1 Answer

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Vector and the List returned by Collections.synchronizedList() are morally the same thing. I would consider Vector to be effectively (but not actually) deprecated and always prefer a synchronized List instead. The one exception would be old APIs (particularly ones in the JDK) that require a Vector.

Using a naked ArrayList and synchronizing independently gives you the opportunity to more precisely tune your synchronization (either by including additional actions in the mutually exclusive block or by putting together multiple calls to the List in one atomic action). The down side is that it is possible to write code that accesses the naked ArrayList outside synchronization, which is broken.

Another option you might want to consider is a CopyOnWriteArrayList, which will give you thread safety as in Vector and synchronized ArrayList but also iterators that will not throw ConcurrentModificationException as they are working off of a non-live snapshot of the data.

You might find some of these recent blogs on these topics interesting:

answered 2009-02-18T21:31:21.027

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