putIfAbsent - #825
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What this fixes: Under concurrent access, two threads could both miss the cache, both compute the value (expensive for _descriptorCache which runs BeanIntrospector), and then both call put — the second overwriting the first. With putIfAbsent, only the first computed value is stored; the second thread's result is discarded safely. The redundant computation still happens but the cache is never corrupted by a race.