Java 9 will introduce modularity, which will be a major change to the Java platform. To help you understand those changes Alan Bateman and Alex Buckley presented a series of sessions about Java 9 at JavaOne. In the 'Prepare for JDK 9' presentation, Alan describes JDK 9 changes and their implications in existing code and future development.
In the 'Introduction to Modular Development' presentation, Alan presents the structure of a module and he compiles basic modules.
In the Project Jigsaw: Under the Hood presentation, Alex explains the difference between readability, observability, visibility, and accessibility in the Java Platform Module System. He also discusses the unnamed module, the platform's built-in class loaders and how you can load two different versions of a module at the same time