The Problem JPMS Solves Before Java 9, the JDK had no real notion of encapsulation at the library level. public meant accessible to everyone — including internal JDK classes like sun.misc.Unsafe and com.sun.internal.*. Large codebases suffered from: No reliable encapsulation: Any public class in any JAR was reachable from any other JAR. Classpath hell: Duplicate or conflicting classes from different JARs led to unpredictable behaviour. Monolithic JDK: The entire 60+ module JDK had to ship with every application.