# Java Platform Module System

The Java Platform Module System specifies a distribution format for collections of Java code and associated resources. It also specifies a repository for storing these collections, or modules, and identifies how they can be discovered, loaded and checked for integrity. It includes features such as namespaces with the aim of fixing some of the shortcomings in the existing JAR format, especially the JAR Hell, which can lead to issues such as classpath and class loading problems.
The Java Module System was initially being developed under the Java Community Process as JSR 277 and was scheduled to be released with Java 7.
JSR 277 later was put on hold and Project Jigsaw was created to modularize the JDK. This JSR was superseded by JSR 376 (Java Platform Module System).
Project Jigsaw was originally intended for Java 7 (2011) but was deferred to Java 8 (2014) as part of Plan B, and again deferred to a Java 9 release in 2017. Java 9 including the Java Module System was released on September 21, 2017.

## Architecture

The Java Module System implemented in Java 9 includes the following JEPs and JSR (Java Specification Request):
* 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
* JSR 376: Java Platform Module System
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.
* 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.
* 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.
* 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.

## Properties of modules

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. Explicit dependency declarations improve the integrity of the code, by making it easier to reason about large applications and the dependencies between software components.
The module declaration is placed in a file named module-info.java at the root of the module’s source-file hierarchy. The JDK will verify dependencies and interactions between modules both at compile-time and runtime.
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:
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. Note that in Java versions 9 through 16, whether such 'illegal access' is de facto permitted depends on a command line setting.
The JDK itself has been modularized in Java 9. For example, the majority of the Java standard library is exported by the module java.base.

## Core modules

The following modules belong to the Java Standard Edition.
| java.base | Defines the foundational APIs of the Java SE Platform. |
| java.compiler | Defines the Language Model, Annotation Processing, and Java Compiler APIs. |
| java.datatransfer | Defines the API for transferring data between and within applications. |
| java.desktop | Defines the AWT and Swing user interface toolkits, plus APIs for accessibility, audio, imaging, printing, and JavaBeans. |
| java.instrument | Defines services that allow agents to instrument programs running on the JVM. |
| java.logging | Defines the Java Logging API. |
| java.management | Defines the Java Management Extensions (JMX) API. |
| java.management.rmi | Defines the RMI connector for the Java Management Extensions (JMX) Remote API. |
| java.naming | Defines the Java Naming and Directory Interface (JNDI) API. |
| java.net.http | Defines the HTTP Client and WebSocket APIs. |
| java.prefs | Defines the Preferences API. |
| java.rmi | Defines the Remote Method Invocation (RMI) API. |
| java.scripting | Defines the Scripting API. |
| java.se | Defines the API of the Java SE Platform. |
| java.security.jgss | Defines the Java binding of the IETF Generic Security Services API (GSS-API). |
| java.security.sasl | Defines Java support for the IETF Simple Authentication and Security Layer (SASL). |
| java.smartcardio | Defines the Java Smart Card I/O API. |
| java.sql | Defines the JDBC API. |
| java.sql.rowset | Defines the JDBC RowSet API. |
| java.transaction.xa | Defines an API for supporting distributed transactions in JDBC. |
| java.xml | Defines the Java APIs for XML Processing (JAXP). |
| java.xml.crypto | Defines the API for XML cryptography. |

## Links with OSGi

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. A couple of articles exploring how the Java Module System and OSGi could interoperate were published in 2016. These can be found on InfoQ and also the OSGi Alliance Blog.