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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0px; border-collapse: collapse; padding: 0 0 10px 0"> <p style="margin: 10px 0 0 0; margin-top: 0">That might be true in a general case, but here I don't think it's an issue. It's because the critical section is just the classic "check map and create key/value if it doesn't exist". So it (logically) should actually only happen once for a given key, the first time something requests that QName index. It should be ok now, since that's guarded and so (physically) it will absolutely only happen once - only 1 thread can now eval "!objectIndex.containsKey(index)" as true.</p> <p style="margin: 10px 0 0 0">And then the code below <tt>checkAndCreateIndex</tt> doesn't mutate anything, just reading the map, which is now guaranteed to be in a consistent state.</p> <p style="margin: 10px 0 0 0">My read of the race is: The actual error was an NPE in <tt>ListView#isEmpty()</tt>. That must be because the view's <tt>indexList</tt> member is null. That means that in the view's ctor the "<tt>indexList = backingList.get(index)"</tt> must have assigned null. And I think that could happen if one thread was partially through <tt>checkAndCreateIndex</tt> such that a second thread evaled the "!objectIndex.containsKey(index)" as false, fell through and later (before the first thread finished the critical section) called the map get() before the value was actually available from the map.</p> <p style="margin: 10px 0 0 0">I'm pretty such that can happen since - and I always forget this - the plain Java collection types (at least the hashtable-based ones) are NOT thread-safe for mutation. That is in fact the reason for the existence for the <tt>ConcurrentHashMap, -HashSet</tt> impls - to at least allow the collection to be structurally correct and not corrupted, etc in the fact of concurrent mods. Esp important since those will internally re-hash and re-structure themselves based on the capacity algorithm, when you add new items. (They don't of course help in this kind of logical concurrency issue of "check and create if doesn't exist, because that involves 2+ calls to the collection).</p> </td>
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