Java Platform Module System

The Java Module System implemented in Java 9 includes the following JEPs and JSR (Java Specification Request):[2]

• JEP 200: The Modular JDK: Define a modular structure for the JDK
• JEP 201: Modular Source Code: Reorganize the JDK source code into modules, enhance the build system to compile modules, and enforce module boundaries at build time
• JEP 220: Modular Run-Time Images: Restructure the JDK and JRE run-time images to accommodate modules and to improve performance, security, and maintainability
• JEP 261: Module System: Implement the Java Platform Module System
• JEP 282: The Java Linker: Create a tool that can assemble and optimize a set of modules and their dependencies into a custom run-time image[6]
• JSR 376: Java Platform Module System[7]

Additionally, several other JDK 9 features have been added to ease transition to the module system:

• JEP 238: Multi-Release JAR Files: Extend the JAR file format to allow multiple, Java-release-specific versions of class files to coexist in a single archive.[8]
• JEP 253: Prepare JavaFX UI Controls & CSS APIs for Modularization: Define public APIs for the JavaFX functionalities that is presently only available via internal APIs and would become inaccessible due to modularization.[9]
• JEP 260: Encapsulate Most Internal APIs: Make most of the JDK's internal APIs inaccessible by default but leave a few critical, widely-used internal APIs accessible, until supported replacements exist for all or most of their functionality.[10]
• JEP 275: Modular Java Application Packaging: The Java packager will evolve for JDK 9, making it aware of modules, allowing for example to package a module and all the modules it depends on.[11]

Modules are a new way of grouping code. Contrary to Jar files, modules explicitly declare which modules they depend on, and what packages they export.[12] Explicit dependency declarations improve the integrity of the code, by making it easier to reason about large applications and the dependencies between software components.

For example, the following module declaration declares that the module com.foo.bar depends on another com.foo.baz module, and exports the following packages: com.foo.bar.alpha and com.foo.bar.beta:

module com.foo.bar {
    requires com.foo.baz;
    exports com.foo.bar.alpha;
    exports com.foo.bar.beta;
}

The public members of com.foo.bar.alpha and com.foo.bar.beta packages will be accessible by dependent modules. Private members are inaccessible even through a means such as reflection, though whether 'illegal access' is de facto permitted depends on a command line setting.[13]

Contrary to the Jar file format, the module will describe these dependencies in a module declaration which will be placed in a file named module-info.java at the root of the module’s source-file hierarchy. The JDK will be able to verify dependencies and interactions between modules both at compile-time and runtime. The JDK itself has been modularized in Java 9.[14]

The Java Module System does not intend to support all the functionalities that the OSGi platform currently supports (for example the Life-Cycle model and the Services Registry). However the Java Module System will support functions which are not supported by OSGi, such as modularity at compile-time, and built-in support for native libraries.[15] A couple of articles exploring how the Java Module System and OSGi could interoperate were published in 2016. These can be found on InfoQ[16] and also the OSGi Alliance Blog.[17]

• Java package
• Classpath (Java)
• Java Classloader

• JSR 277
• JSR 294
• JSR 376
• Project Jigsaw
• The State of the Module System
• Java Platform Module System: Issue Summary