How Java 9 And Project Jigsaw May Break Your Code

Java 9's Project Jigsaw introduces a module system that will break existing code relying on internal APIs, specific JDK folder structures, or internal JARs. Key breaking changes include: inaccessibility of internal APIs (sun.* and some com.sun.* packages), merging of JDK and JRE directory structures, removal of lib/rt.jar and lib/tools.jar, a new jrt:// URL scheme replacing jar: URLs, and removal of the endorsed standards override and extension mechanisms. The jdeps tool (available since Java 8) can help identify dependencies on internal APIs before migration. Projects should run against Java 9 early access builds and fix identified issues proactively.