In this chapter, we looked, at a high level, at some limitations of the traditional way of building reusable components in Java using JAR files. We saw how packaging libraries in JAR files doesn't allow developers to encapsulate inner APIs and types. There's also no way to reliably figure out whether a given application has all the necessary classes in the classpath. We learned how these problems that developers face in their code are not only present in the JDK code base itself, but are actually an issue on a much bigger scale. We understood the Java Platform Module System and the two primary goals that it set to achieve--strong encapsulation and reliable configuration. We learned about Project Jigsaw and the effort to modularize the core JDK using the same modular paradigm that's available to developers to use in their code.

At this time, you are probably wondering how the concept of modularity manifests in the Java language. What does a Java module look like?

In the next chapter, we'll answer these questions by creating our first Java 9 module, and get started on our sample application project which we'll be working on throughout this book.