When to use Java over Drools

Some caveats to remember when using Drools are that it scales best when a large number of rules are executed against a small number of facts. It also makes more sense to use Drools when the rules are loosely coupled; if many rules depend on each other in a chain, it may make more sense to write an if statement decision tree in Java code instead of presenting the logic as a series of individual rules spread throughout one or more DRL files.

https://www.credera.com/blog/technology-insights/open-source-technology-insights/using-drools-expert-apache-camel-part-3-embark-sample-route/

Volatile accesses are synchronizing

...volatile accesses are synchronizing actions (docs.oracle.com/javase/specs/jls/se7/html/…), as such, they define an order for concurrent events which makes them equivalent to locking on the same monitor. – Cephalopod 

http://stackoverflow.com/questions/3786825/volatile-boolean-vs-atomicboolean