Java Module System: Modularize Your Applications with Java 9+

Explore the Java Module System (JPMS), introduced in Java 9 to enhance modularity in Java applications. Understand the concept of modules as self-contained collections of code and resources, allowing developers to encapsulate functionality, restrict access, and streamline large applications. Learn how Java's modular platform structure improves code organization and security.

Java Module System

In Java 9, the module system was introduced to improve modularity in Java applications. The module system is also known as JPMS, Java Platform Module System, and is commonly referred to as "Modules".

What is a Module?

A module is a self-contained collection of code and data that is identified by a unique name. It contains packages, resources, and configurations related to a specific functionality. Modules allow developers to restrict access to the packages they contain. By default, code in a module's package is not visible to the outside world, not even via reflection.

From Java 9 onwards, the Java platform itself is modularized. To list the modules supported by Java, you can use the --list-modules command:

C:\Users\JohnDoe>java --list-modules
java.base@20.0.2
java.compiler@20.0.2
java.sql@20.0.2
jdk.javadoc@20.0.2
... (list continues)

java.base@20.0.2
java.compiler@20.0.2
java.sql@20.0.2
jdk.javadoc@20.0.2
... (list continues)

Modules like jdk are for JDK-specific packages, while java modules are for standard library packages.

Features of Java Module System

The module system in Java 9 introduced several enhancements, such as:

• A new phase called "link time," which is optional and occurs between compile time and runtime. During this phase, modules can be assembled and optimized into a custom runtime image using the jlink tool.
• Additional options in javac, jlink, and java to specify module paths, allowing modules to be located more efficiently.
• Modular JARs, which include a module-info.class file in the root directory.
• The introduction of the JMOD format, which is similar to JAR but can include native code and configuration files.

Declaring a Module

To declare a module, you need to create a module-info.java file in the root folder of your application. This file contains the metadata for the module.

module com.example.greetings {
}

Adding Dependent Modules

You can declare dependencies of other modules in the module descriptor. For example, if you want to use the com.example.util module, you can add it as follows:

module com.example.greetings {
requires com.example.util;
}

Adding Optional Modules

To declare optional dependencies, use the static keyword:

module com.example.greetings {
requires com.example.util;
requires static com.example.logging;
}

Adding Transitive Modules

Transitive dependencies allow you to expose dependencies to modules that require your module. Use the transitive keyword:

module com.example.greetings {
requires com.example.util;
requires transitive com.example.base;
}

Exporting Public Classes

By default, no public classes in a module's package are accessible to the outside. You must export the package to make the public classes available:

module com.example.greetings {
exports com.example.greetings.HelloWorld;
}

Allowing Reflection

To allow reflection for accessing private members of a module, use the opens keyword:

module com.example.greetings {
opens com.example.greetings.HelloWorld;
}

Creating and Using a Java Module

Step 1: Create a Module Directory

Create a directory for the module: C:/JAVA/src/com/example/greetings.

Step 2: Create module-info.java

In the module folder, create a module-info.java file with the following content:

module com.example.greetings {
}

Step 3: Add Source Code

Create a Java9Tester.java file inside the module folder:

package com.example.greetings;

public class Java9Tester {
public static void main(String[] args) {
    System.out.println("Hello World!");
}
}

Step 4: Compile the Module

Compile the module using the following command:

C:/JAVA> javac -d mods/com.example.greetings src/com.example.greetings/module-info.java src/com.example.greetings/com/example/greetings/Java9Tester.java

Step 5: Run the Module

Run the compiled module using the command:

C:/JAVA> java --module-path mods -m com.example.greetings/com.example.greetings.Java9Tester

Hello World!