final void removeStale() {
            KeyReference ref;
            while ((ref = (KeyReference) refQueue.poll()) != null) {
                remove(ref.keyRef(), ref.keyHash(), null, true);
            }
        }
Remove; match on key only if value null, else match both.
commons-profiler/src/main/java/com/navercorp/pinpoint/common/profiler/concurrent/jsr166/ConcurrentReferenceHashMap.java