This JEP might affect source code that uses underscore ("_") as an identifier. Its use generated a warning as of Java SE 8, and has been turned into an error in JDK 9.
New JDK 9 features
REPL (aka Project Kulla)
Project Kulla introduces a new REPL for Java 9. You can test this by following the instructions from the latest Adoption Group hackday.
Other potential changes to consider