Parts of it are indeed written in Java. You probably want to reach out to the developers list to talk about changing Spark.

On Sat, Nov 3, 2018, 11:42 AM Soheil Pourbafrani <soheil.ir08@gmail.com wrote:
Hi, I want to customize some part of Spark. I was wondering if there any Spark source is written in Java language, or all the sources are in Scala language?