Github user mars commented on the issue:
https://github.com/apache/incubator-predictionio/pull/421
Based on these [Scala Concurrency/Thread Safety
docs](https://twitter.github.io/scala_school/concurrency.html#danger), I
believe simply annotating `@volatile` will cause the synchronization needed for
thread-safety in this case.
So, I updated this PR with that change.
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