Java Pocket Guide, 4th Edition

Chapter 21. Java Module System

Java 9 introduces the Project Jigsaw, which both adds modularization to the platform and modularizes the JDK itself. The goal of Jigsaw is twofold: to enable reliable configuration and add strong encapsulation to Java. With modularization, it is now possible to restrict which packages are public and also ensure that runtime dependencies are present when an application is launched.

The Java Platform Module System (JPMS) is implemented as a separate layer within the JVM. This distinguishes it from other module systems, such as OSGi, which are implemented using Classloaders. JPMS enables modularization of the JDK itself.

Project Jigsaw

Project Jigsaw is made up of a JSR (Java Specification Request) and multiple JEPs (JDK Enhancement Proposals). The specifications that make up Jigsaw are as follows:

• JSR 376 Java Platform Module System

• JEP 200: Modular JDK

• JEP 201: Modular Source Code

• JEP 220: Modular Runtime Images

• JEP 260: Encapsulate Most Internal APIs

• JEP 261: Module System

• JEP 282: jlink: Java Linker

The main project page for Jigsaw has links to each of these specifications.

Java Modules

Java modules are a JAR file containing a module-info.java file in the default package. Since “module-info” is an invalid Java class name, it is ignored by Java 8 and earlier. It does get compiled to bytecode and is available via reflection. The module file declares the name of the module, dependencies of a module, and which packages are exported ...