Iterating Collection In Two Threads

This question relates both to C# and Java

If you have a collection which is not modified and that collection reference is shared between two threads what happens when you iterate on each thread??

ThreadA: Collection.iterator ThreadA: Collection.moveNext ThreadB: Collection.iterator T开发者_运维技巧hreadB: Collection.moveNext

Will threadB see the first element?

Is the iterator always reset when it is requested? What happens if this is interleaved so movenext and item is interleaved? Is there a danger that you dont process all elements??

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It works as expected because each time you request the iterator you get a new one.

If it didn't you wouldn't be able to do foreach followed by foreach on the same collection!

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By convention, an Iterator is implemented such that the traversal action never alters the collection's state. It only points to the current position in the collection, and manages the iteration logic. Therefore, if you scan the same collection by N different threads, everything should work fine.

However, note that Java's Iterator allows removal of items, and ListIterator even supports the set operation. If you want to use these actions by at least one of the threads, you will probably face concurrency problems (ConcurrentModificationException), unless the Iterator is specifically designed for such scenarios (such as with ConcurrentHashMap's iterators).

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In Java (and I am pretty sure also in C#), the standard API collections typically do not have single iterator. Each call to iterator() produces a new one, which has its own internal index or pointer, so that as long as both threads acquire their own iterator object, there should be no problem.

However, this is not guaranteed by the interface, nor is the ability of two iterators to work concurrently without problems. For custom implementations of collections, all bets are off.

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In c# - yes, java - seems to be but I'm not familiar. About c# see http://csharpindepth.com/Articles/Chapter6/IteratorBlockImplementation.aspx and http://csharpindepth.com/Articles/Chapter11/StreamingAndIterators.aspx

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At least in C#, all of the standard collections can be enumerated simultaneously on different threads. However, enumeration on any thread will blow up if you modify the underlying collection during enumeration (as it should.) I don't believe any sane developer writing a collection class would have their enumerators mutate the collection state in a way that interferes with enumeration, but it's possible. If you're using a standard collection, however, you can safely assume this and therefore use locking strategies like Single Writer / Multiple Reader when synchronizing collection access.