Java How to Program, Early Objects, 11th Edition

36.6 Migrating Code to Java 9

Many pre-Java-9 apps will run unaltered on Java 9. In fact, as we prepared this book, we tested every app using JDK 9 and they all compiled and ran without issue. In Java 9, all programs are compiled and executed using the module system. Java 9 strongly encapsulates types that are not exported by modules, so it’s possible that some apps will fail to compile because types that were accessible to them prior to Java 9 no longer are. For example, there are many pre-Java-9 internal APIs that were not meant for use outside the JDK, but were in fact used outside the JDK—many of these are not exported in Java 9 and thus are inaccessible.¹⁹ If your code uses such internal APIs directly or indirectly, it will fail to compile. ...