Java - Module

Java Conceptuel Diagram


A Java module is a packaging format that create a modular JAR file.

It's a feature of JDK 9 from the Java Platform Module System (JPMS) that divide the monolithic rt.jar and tools.jar files into 75 distinct modules (at least in the OpenJDK, the Oracle JDK contained a few more).

Format and minimal example


The module consists of two type of source files:

  • the module declaration file:
  • and the classes

By convention, the source code for the module is in a directory that is the name of the module.


  • Minimal
module com.greetings { }
  • Exporting code to be used as a library (ie making it visible to other)
module com.greetings {
        exports com.greetings;
module com.socket {
        exports com.socket;
        exports com.socket.spi;
        uses com.socket.spi.NetworkSocketProvider;


With javac

Creation of the directory tree to get the output of the compilation

mkdir -p mods # the root module path
mkdir -p mods/com.greetings # the location of the module

The compilation

    -d mods/com.greetings \ # the target directory for the output of the compiled module
    --module-path mods \ # the module path permits to resolve external modules (ie code dependencies)
    src/com.greetings/ \

compile multiple modules with one javac command

mkdir mods
javac -d mods --module-source-path src $(find src -name "*.java")
package com.greetings;
public class Main {
        public static void main(String[] args) {

Package (Modular Jar)

package - The contents of the compiled modules are exploded on the file system. For transportation and deployment purposes then it is usually more convenient to package a module as a modular JAR.

A modular JAR is a regular JAR file that has a module-info.class in its top-level directory.

mkdir mlib
# create [email protected]
jar --create --file=mlib/[email protected] \
      --module-version=1.0 -C mods/org.astro .
# create com.greetings.jar 
jar --create --file=mlib/com.greetings.jar \
     --main-class=com.greetings.Main -C mods/com.greetings .


To run a modular jar

java -p mlib -m com.greetings


  • m specifies the main module

To run raw class file exploded on the file system

java \
   --module-path mods \
   -m com.greetings/com.greetings.Main


  • The m option specifies the main module, the value after the slash is the class name of the main class in the module.



Modular JAR files can also be Multi-Release JAR files - ie extend the JAR file format to allow multiple, Java-release-specific versions of class files to coexist in a single archive. (per JEP 238)


Describe the module

print the module declaration for a module packaged as a modular JAR.

jar --describe-module --file=mlib/[email protected]



jlink is the linker tool and can be used to link a set of modules, along with their transitive dependences, to create a custom modular run-time image.


Documentation / Reference

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