Java SDK: RestClient Thread Safe?

I'm currently using a Singleton of the RestClient for all RingCentral requests. I did this after having issues instantiating and authentication a RestClient for each request:

https://forums.developers.ringcentral.com/questions/7503/java-sdk-subscription-refresh-causing-401-error.html

Is this an acceptable approach? If not, is there a recommended pattern to follow for server applications?