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.

Overview Java 9 and 11 added three tools that change how you write, test, and deploy Java code: Tool JEP Release Purpose JShell JEP 222 Java 9 Interactive REPL for Java code jlink JEP 282 Java 9 Build minimal custom JRE images Single-file programs JEP 330 Java 11 Run .java files directly JShell — Interactive Java REPL JShell is Java’s Read-Eval-Print Loop: a command-line tool for evaluating Java expressions, statements, methods, and classes interactively, without creating a project.