Examples of java.util.concurrent.ArrayBlockingQueue$Itrs

• java.util.concurrent.ArrayBlockingQueue
  Shared data between iterators and their queue, allowing queue modifications to update iterators when elements are removed. This adds a lot of complexity for the sake of correctly handling some uncommon operations, but the combination of circular-arrays and supporting interior removes (i.e., those not at head) would cause iterators to sometimes lose their places and/or (re)report elements they shouldn't. To avoid this, when a queue has one or more iterators, it keeps iterator state consistent by: (1) keeping track of the number of "cycles", that is, the number of times takeIndex has wrapped around to 0. (2) notifying all iterators via the callback removedAt whenever an interior element is removed (and thus other elements may be shifted). These suffice to eliminate iterator inconsistencies, but unfortunately add the secondary responsibility of maintaining the list of iterators. We track all active iterators in a simple linked list (accessed only when the queue's lock is held) of weak references to Itr. The list is cleaned up using 3 different mechanisms: (1) Whenever a new iterator is created, do some O(1) checking for stale list elements. (2) Whenever takeIndex wraps around to 0, check for iterators that have been unused for more than one wrap-around cycle. (3) Whenever the queue becomes empty, all iterators are notified and this entire data structure is discarded. So in addition to the removedAt callback that is necessary for correctness, iterators have the shutdown and takeIndexWrapped callbacks that help remove stale iterators from the list. Whenever a list element is examined, it is expunged if either the GC has determined that the iterator is discarded, or if the iterator reports that it is "detached" (does not need any further state updates). Overhead is maximal when takeIndex never advances, iterators are discarded before they are exhausted, and all removals are interior removes, in which case all stale iterators are discovered by the GC. But even in this case we don't increase the amortized complexity. Care must be taken to keep list sweeping methods from reentrantly invoking another such method, causing subtle corruption bugs.

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