Java 9 - Module System

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Java 9, a new kind of programming component called module has been introduced. A module is a self-describing collection of code and data and has a name to identify it.

Features

With the Modules component, following enhancements has been added in Java 9 −

  • A new optional phase,link time, is introduced. This phase is in-between compile time and run time. During this phase, a set of modules can be assembled and optimized, making a custom runtime image using jlink tool.

  • javac, jlink, and java have additional options to specify module paths, which further locate definitions of modules.

  • JAR format updated as modular JAR, which contains module-info.class file in its root directory.

  • JMOD format introduced, a packaging format (similar to JAR) which can include native code and configuration files.

Creating Module

Following the steps to create a module say com.howcodex.greetings.

Step 1

Create a folder C:\>JAVA\src. Now create a folder com.howcodex.greetings which is same as the name of module we're creating.

Step 2

Create module-info.java in C:\>JAVA\src\com.howcodex.greetings folder with following code.

module-info.java

module com.howcodex.greetings { }

module-info.java is the file which is used to create module. In this step we've created a module named com.howcodex.greetings. By convention this file should reside in the folder whose name is same as module name.

Step 3

Add the source code in the module. Create Java9Tester.java in C:\>JAVA\src\com.howcodex.greetings\com\ howcodex\greetings folder with following code.

Java9Tester.java

package com.howcodex.greetings;

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

By convention, the source code of a module to lie in same directory which is the name of the module.

Step 4

Create a folder C:\>JAVA\mods. Now create a folder com.howcodex.greetings which is same as the name of module we've created. Now compile the module to mods directory.

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

Step 5

Let's run the module to see the result. Run the following command.

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

Here module-path provides the module location as mods and -m signifies the main module.

Output

It will print the following output on console.

Hello World!
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