Any recommendations for an open-source Java codebase to read that makes good use of the Java 8 concurrency features? Currently in a new job and struggling with

Any recommendations for an open-source Java codebase to read that makes good use of the Java 8 concurrency features? Currently in a new job and struggling with an undocumented, uncommented codebase containing streams, rxjava, executors and futures. Could do with a different, hopefully documented, take on Java concurrency.