24.50.56.74: /* Core modules */

2025-06-02T21:27:16Z

Core modules

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{{short description|Distribution format for Java code and resources}}
The '''Java Platform Module System'''<ref>{{cite web
| url=http://openjdk.java.net/projects/jigsaw/spec/
| title=Java Platform Module System (JSR 376)
| publisher=[[Oracle Corporation]]
| accessdate=2018-07-02}}</ref> specifies a distribution format for collections of [[Java (programming language)|Java]] code and associated resources. It also specifies a repository for storing these collections, or ''[[modular programming|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 (file format)|JAR]] format, especially the [[Java class loader#JAR hell|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<ref name="jigsaw">{{cite web
| url=http://openjdk.java.net/projects/jigsaw/
| title=Project Jigsaw
| publisher=[[Oracle Corporation]]
| accessdate=2015-11-29}}</ref> 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 This JSR was superseded by JSR 376 (Java Platform Module System).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,<ref>{{cite web
| url=http://mreinhold.org/blog/plan-b
| title=It's time for … Plan B
| publisher=[[Oracle Corporation]]
| author=Mark Reinhold
| date=2009-09-20
| accessdate=2017-06-21}}</ref> and again deferred to a [[Java 9]] release in 2017.<ref>{{cite web
| url=http://openjdk.java.net/projects/jdk9/
| title=JDK 9
| publisher=[[Oracle Corporation]]
| accessdate=2016-02-24}}</ref> Java 9 including the Java Module System was released on September 21, 2017.<ref>{{cite web|url=https://www.techworld.com/news/developers/java-9-release-date-new-features-3660988/|title=Java 9: Release date and new features|publisher=techworld.com|date=2017-07-21|accessdate=2017-11-18}}</ref>

== Architecture ==
The Java Module System implemented in Java 9 includes the following [[JDK Enhancement Proposal|JEP]]s and [[Java Community Process|JSR (Java Specification Request)]]:<ref name="jigsaw"/>
* 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<ref>{{cite web
| url=http://openjdk.java.net/jeps/282
| title=jlink: The Java Linker (JSR 282)
| publisher=[[Oracle Corporation]]
| accessdate=2016-03-12}}</ref>
* JSR 376: Java Platform Module System<ref>{{cite web
| url=http://openjdk.java.net/projects/jigsaw/spec/
| title=Java Platform Module System (JSR 376)
| publisher=[[Oracle Corporation]]
| accessdate=2015-11-29}}</ref>

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.<ref>{{cite web
| url=http://openjdk.java.net/jeps/238
| title=JEP 238: Multi-Release JAR Files
| publisher=[[Oracle Corporation]]
| accessdate=2017-07-31}}</ref>
* 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.<ref>{{cite web
| url=http://openjdk.java.net/jeps/253
| title=JEP 253: Prepare JavaFX UI Controls & CSS APIs for Modularization
| publisher=[[Oracle Corporation]]
| accessdate=2017-07-31}}</ref>
* 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.<ref>{{cite web
| url=http://openjdk.java.net/jeps/260
| title=JEP 260: Encapsulate Most Internal APIs
| publisher=[[Oracle Corporation]]
| accessdate=2017-07-31}}</ref>
* 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.<ref>{{cite web
| url=http://openjdk.java.net/jeps/275
| title=JEP 275: Modular Java Application Packaging
| publisher=[[Oracle Corporation]]
| accessdate=2017-07-31}}</ref>

== Properties of modules ==

Modules are a new way of grouping code. Contrary to [[JAR (file format)|Jar files]], modules explicitly declare which modules they depend on, and what packages they export.<ref>{{cite web
| url=http://openjdk.java.net/projects/jigsaw/spec/sotms/
| title=The State of the Module System
| publisher=[[Oracle Corporation]]
| author=Mark Reinhold
| date=2016-03-08
| accessdate=2017-02-18}}</ref> 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 {{mono|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 {{mono|com.foo.bar}} depends on another {{mono|com.foo.baz}} module, and exports the following packages: {{mono|com.foo.bar.alpha}} and {{mono|com.foo.bar.beta}}:
<syntaxhighlight lang="cpp">
module com.foo.bar {
requires com.foo.baz;

exports com.foo.bar.alpha;
exports com.foo.bar.beta;
}
</syntaxhighlight>
The public members of {{mono|com.foo.bar.alpha}} and {{mono|com.foo.bar.beta}} packages will be accessible by dependent modules. Private members are inaccessible even through a means such as [[reflective programming|reflection]]. Note that in [[Java version history|Java versions]] 9 through 16, whether such 'illegal access' is ''de facto'' permitted depends on a command line setting.<ref name="JEP 396: Strongly Encapsulate JDK Internals by Default">{{cite web | url=https://openjdk.java.net/jeps/396 | title=JEP 396: Strongly Encapsulate JDK Internals by Default | access-date=2021-02-06}}</ref>

The JDK itself has been modularized in [[Java version history#Java SE 9|Java 9]].<ref>{{cite web
| url=http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html
| title=JDK Module Summary
| publisher=[[Oracle Corporation]]
| date=2016-06-24
| accessdate=2017-02-18
| archive-date=2015-12-08
| archive-url=https://web.archive.org/web/20151208074800/http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html
| url-status=dead
}}</ref> For example, the majority of the Java standard library is exported by the module <code>java.base</code>.

Modules can themselves be imported, automatically importing all exported packages.<ref>{{Cite web|url=https://openjdk.org/jeps/494|title=JEP 494: Module Import Declarations (Second Preview)|website=openjdk.org}}</ref> This is done using <code>import module</code>. For example, <syntaxhighlight lang="Java" inline>import module java.sql;</syntaxhighlight> is equivalent to
<syntaxhighlight lang="Java">
import java.sql.*;
import javax.sql.*;
// Remaining indirect exports from java.logging, java.transaction.xa, and java.xml
</syntaxhighlight>
Similarly, <syntaxhighlight lang="Java" inline>import module java.base;</syntaxhighlight>, similarly, imports all 54 packages belonging to <code>java.base</code>.

Modules use the following [[List of Java keywords|keywords]]:
* <code>exports</code>: used in a module declaration to specify which packages are available to other modules
* <code>module</code>: declares a module
* <code>open</code>: indicates that all classes in a package are accessible via reflection by other modules
* <code>opens</code>: used to open a specific package for reflection to other modules
* <code>provides</code>: used to declare that a module provides an implementation of a service interface
* <code>requires</code>: used in a module declaration to specify that the module depends on another module
* <code>to</code>: used with the <code>opens</code> directive to specify which module is allowed to reflectively access the package
* <code>transitive</code>: used with the <code>requires</code> directive to indicate that a module not only requires another module but also makes that module's dependencies available to modules that depend on it
* <code>uses</code>: used in a module to declare that the module is using a service (i.e. it will consume a service provided by other modules)
* <code>with</code>: used with the <code>provides</code> directive to specify which implementation of a service is provided by the module

== Core modules ==
The modules under namespace <code>java.*</code> belong to the [[Java Platform, Standard Edition]], and modules under namespace <code>jdk.*</code> belong to the [[Java Development Kit]].<ref>{{Cite web|url=https://docs.oracle.com/en/java/javase/24/docs/api/index.html|title=Java® Platform, Standard Edition & Java Development Kit Version 24 API Specification|website=docs.oracle.com}}</ref>
{| class="wikitable plainrowheaders"
! scope="row" | {{code|java.base}}
| Defines the core APIs that form the foundation of the Java SE Platform.
Implicitly required by all modules and does not need to be declared with <code>requires</code> inside a module declaration.
|-
! scope="row" | {{code|java.compiler}}
| Defines APIs related to the language model, [[Java annotation]] processing, and the [[Java compiler]].
|-
! scope="row" | {{code|java.datatransfer}}
| Defines the APIs that facilitate data transfer between applications or within an application.
|-
! scope="row" | {{code|java.desktop}}
| Defines the [[Abstract Window Toolkit]] (AWT) and [[Swing (Java)|Swing]] libraries for user interfaces, along with APIs for accessibility, audio, imaging, printing, and [[JavaBeans]].
|-
! scope="row" | {{code|java.instrument}}
| Defines services that allow for the instrumentation of programs running on the [[Java Virtual Machine]] (JVM).
|-
! scope="row" | {{code|java.logging}}
| Defines the API for [[Logging (computing)|logging]] events in Java applications (Java Logging API).
|-
! scope="row" | {{code|java.management}}
| Defines the APIs for the [[Java Management Extensions]] (JMX) framework.
|-
! scope="row" | {{code|java.management.rmi}}
| Defines the [[Remote Method Invocation]] (RMI) connector for the Java Management Extensions (JMX) Remote API.
|-
! scope="row" | {{code|java.naming}}
| Defines the API for [[Java Naming and Directory Interface]] (JNDI) services.
|-
! scope="row" | {{code|java.net.http}}
| Defines the API for [[HTTP client]] and [[WebSocket]] functionality.
|-
! scope="row" | {{code|java.prefs}}
| Defines the API for managing and storing user preferences.
|-
! scope="row" | {{code|java.rmi}}
| Defines the Remote Method Invocation (RMI) API for object communication across Java virtual machines.
|-
! scope="row" | {{code|java.scripting}}
| Defines the API for integrating [[scripting engine]]s into Java applications (Scripting API).
|-
! scope="row" | {{code|java.se}}
| Defines the standard API for the Java SE Platform.
|-
! scope="row" | {{code|java.security.jgss}}
| Defines the Java binding for the [[Internet Engineering Task Force|IETF]] [[Generic Security Services Application Programming Interface|Generic Security Services API]] (GSS-API) for security contexts.
|-
! scope="row" | {{code|java.security.sasl}}
| Defines Java's support for the [[Simple Authentication and Security Layer]] (SASL) protocol.
|-
! scope="row" | {{code|java.smartcardio}}
| Defines the API for interacting with [[smart card]] devices (Java Smart Card I/O API).
|-
! scope="row" | {{code|java.sql}}
| Defines the [[Java Database Connectivity]] (JDBC) API for database interaction.
|-
! scope="row" | {{code|java.sql.rowset}}
| Defines the JDBC RowSet API for managing and interacting with tabular data in Java.
|-
! scope="row" | {{code|java.transaction.xa}}
| Defines an API for managing distributed transactions within JDBC.
|-
! scope="row" | {{code|java.xml}}
| Defines the [[Java API for XML Processing]] (JAXP), including tools for parsing and transforming XML data.
|-
! scope="row" | {{code|java.xml.crypto}}
| Defines the API for [[XML Encryption|cryptographic operations]] with XML data.
|-
! scope="row" | {{code|jdk.accessibility}}
| Defines utility classes for implementing [[Assistive technology|Assistive Technologies]] in the JDK.
|-
! scope="row" | {{code|jdk.attach}}
| Defines the attach API for connecting to and interacting with a running JVM instance (attach API).
|-
! scope="row" | {{code|jdk.charsets}}
| Provides additional [[character encoding|character sets]] ({{mono|charset}}s), including double-byte and IBM-specific charsets, not included in <code>java.base</code>.
|-
! scope="row" | {{code|jdk.compiler}}
| Defines the internal implementation of the Java compiler and the command-line tool ''[[javac]]''.
|-
! scope="row" | {{code|jdk.crypto.cryptoki}}
| Defines the implementation of the SunPKCS11 security provider for cryptographic operations.
|-
! scope="row" | {{code|jdk.dynalink}}
| Defines the API for dynamically linking high-level operations on objects.
|-
! scope="row" | {{code|jdk.editpad}}
| Implements the edit pad service used by <code>jdk.jshell</code> for evaluating code.
|-
! scope="row" | {{code|jdk.hotspot.agent}}
| Provides the implementation for the [[HotSpot (virtual machine)|HotSpot]] Serviceability Agent.
|-
! scope="row" | {{code|jdk.httpserver}}
| Defines the JDK-specific HTTP server API and the ''jwebserver'' tool for running a simple HTTP server.
|-
! scope="row" | {{code|jdk.incubator.vector}}
| Defines the API for expressing computations that can be compiled into SIMD instructions at runtime, such as AVX or NEON instructions.
|-
! scope="row" | {{code|jdk.jartool}}
| Provides tools for manipulating [[JAR (file format)|Java Archive]] (JAR) files, such as the ''jar'' and ''jarsigner'' commands.
|-
! scope="row" | {{code|jdk.javadoc}}
| Defines the system documentation tool implementation and its command-line version, ''[[Javadoc|javadoc]]''.
|-
! scope="row" | {{code|jdk.jcmd}}
| Defines tools for diagnosing and troubleshooting JVM processes, including ''jcmd'', ''jps'', and ''jstat''.
|-
! scope="row" | {{code|jdk.jconsole}}
| Defines the JMX-based graphical tool, ''[[JConsole|jconsole]]'', for monitoring and managing Java applications.
|-
! scope="row" | {{code|jdk.jdeps}}
| Defines tools for analysing dependencies in Java programs and libraries, such as ''jdeps'', ''javap'', ''jdeprscan'', and ''jnativescan''.
|-
! scope="row" | {{code|jdk.jdi}}
| Defines the [[Java Platform Debugger Architecture#Java Debugger Interface (JDI)|Java Debug Interface]] (JDI) for debugging Java applications.
|-
! scope="row" | {{code|jdk.jdwp.agent}}
| Provides the implementation of the [[Java Debug Wire Protocol]] (JDWP) agent for remote debugging.
|-
! scope="row" | {{code|jdk.jfr}}
| Defines the API for [[JDK Flight Recorder]] (JFR) for collecting performance and diagnostic data.
|-
! scope="row" | {{code|jdk.jlink}}
| Defines the ''jlink'' tool for creating custom runtime images, and ''jmod'' and ''jimage'' tools for managing JMOD and JDK container files.
|-
! scope="row" | {{code|jdk.jpackage}}
| Defines the [[Java packager]] tool (''jpackage'') for creating platform-specific application packages.
|-
! scope="row" | {{code|jdk.jshell}}
| Provides the ''jshell'' tool for interactively evaluating Java code snippets.
|-
! scope="row" | {{code|jdk.jsobject}}
| Defines the API for integrating [[JavaScript]] objects in Java applications.
|-
! scope="row" | {{code|jdk.jstatd}}
| Defines the ''jstatd'' tool for remotely monitoring JVM statistics.
|-
! scope="row" | {{code|jdk.localedata}}
| Provides locale-specific data for non-US locales.
|-
! scope="row" | {{code|jdk.management}}
| Defines JDK-specific management interfaces for JVM monitoring and control.
|-
! scope="row" | {{code|jdk.management.agent}}
| Defines the JMX-based management agent for the JVM.
|-
! scope="row" | {{code|jdk.management.jfr}}
| Defines the management interface for JDK Flight Recorder (JFR).
|-
! scope="row" | {{code|jdk.naming.dns}}
| Implements the [[Domain Name System|DNS]] Java Naming provider for resolving domain names.
|-
! scope="row" | {{code|jdk.naming.rmi}}
| Implements the RMI Java Naming provider for remote method invocation.
|-
! scope="row" | {{code|jdk.net}}
| Defines the JDK-specific API for network programming.
|-
! scope="row" | {{code|jdk.nio.mapmode}}
| Defines specific file mapping modes available in the JDK.
|-
! scope="row" | {{code|jdk.sctp}}
| Provides the API for [[Stream Control Transmission Protocol]] (SCTP) in the JDK.
|-
! scope="row" | {{code|jdk.security.auth}}
| Implements authentication modules and security-related interfaces within the <code>javax.security.auth</code> package (interfaces in <code>javax.security.auth.*</code>).
|-
! scope="row" | {{code|jdk.security.jgss}}
| Defines extensions to the GSS-API and the implementation of the SASL GSSAPI mechanism in the JDK.
|-
! scope="row" | {{code|jdk.xml.dom}}
| Defines the JDK's subset of the [[World Wide Web Consortium]] (W3C) [[Document Object Model]] (DOM) API not covered by Java SE.
|-
! scope="row" | {{code|jdk.zipfs}}
| Provides the implementation for the ZIP file system provider, enabling access to [[ZIP (file format)|ZIP files]] as file systems.
|}

== 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.<ref>{{cite web
| url=http://mreinhold.org/blog/late-for-the-train-qa
| title=Project Jigsaw: Late for the train: The Q&A
| publisher=[[Oracle Corporation]]
| author=Mark Reinhold
| date=2012-08-24
| accessdate=2015-11-29}}</ref> A couple of articles exploring how the Java Module System and OSGi could interoperate were published in 2016. These can be found on InfoQ<ref>{{cite web
| url=https://www.infoq.com/articles/java9-osgi-future-modularity/
| title=Java 9, OSGi and the Future of Modularity
| publisher=InfoQ
| accessdate=2016-09-26}}</ref> and also the OSGi Alliance Blog.<ref>{{cite web
| url=http://blog.osgi.org/2016/08/java-module-layers-and-osgi-bundles.html
| title=Java Module Layers and OSGi Bundles
| publisher=OSGi Alliance
| accessdate=2016-08-01}}</ref>

== See also ==
* [[Java package]]
* [[Classpath]]
* [[Java class loader]]
* [[Precompiled header#Modules]], C++ modules

== References ==
{{reflist}}

== External links ==
* [http://www.jcp.org/en/jsr/detail?id=277 JSR 277]
* [http://www.jcp.org/en/jsr/detail?id=294 JSR 294]
* [https://www.jcp.org/en/jsr/detail?id=376 JSR 376]
* [http://openjdk.java.net/projects/jigsaw/ Project Jigsaw]
* [http://openjdk.java.net/projects/jigsaw/spec/sotms/ The State of the Module System]
* [http://openjdk.java.net/projects/jigsaw/spec/issues/ Java Platform Module System: Issue Summary]

{{Java (software platform)}}

[[Category:Java specification requests]]

Java Platform Module System - Revision history

24.50.56.74: /* Core modules */