message: Use ConcurrentHashMap.putIfAbsent() for synchronization safety. Credit to
diff: @@ -90,9 +90,8 @@ public class SimpleTokenFactory implements TokenFactory {
  // Accept if there is no non-replayable ID.
  final String key = masterToken.getIdentity() + ":" + masterToken.getSerialNumber();
  - final Long largestNonReplayableId = nonReplayableIds.get(key);
  + final Long largestNonReplayableId = nonReplayableIds.putIfAbsent(key, nonReplayableId);
  if (largestNonReplayableId == null) {
  - nonReplayableIds.put(key, nonReplayableId);
  return null;
  }
  