Mark Reinhold, chief architect on the Java platform, explains how to create a module in this live coding session, Modules in One Lesson. He shows how to make one module depend on another and demonstrates Java 9 features that can be used in both the classpath and module systems.
Most likely you won’t migrate your applications and the libraries that you depend on at the same time. And, some of those libraries are managed by third parties. You won’t know when or if they will be migrated to modules.
In his presentation Migrating to Modules, Mark Reinhold shows how to migrate an application to modules when the libraries are not modularized and vice versa how to migrate libraries or use modularized libraries when your application is not. He explains what happens to the application when it is sitting on a classpath and it will consume modularized libraries.
More information about module system and JDK 9 can be found online:
JEP 261: Module System
JEP 200: The Modular JDK
JDK 9 release
Java 9 expert insights
JDK 9 documentation
Java SE & JDK 9 API Specification
Oracle JDK 9 and JRE 9 Certified System Configurations
Related blog: Modular Development with JDK 9