Introduction to Java 9 Module

With Java 9 we can use module system called Jigsaw. Without talking about module a lot let’s dive into modular world of Java.

Before creating any module we have to keep few things in mind

1. Module name and directory name must be same
2. Module name are kept same as package name (reverse domain name) but not mandatory

Now let’s create a module #

For this example let’s create a module com.mymodule. To do so we need to create directory called com.mymodule. We will create all of our source files under src so our directory we create will be src/com.mymodule.

Inside the directory create module-info.java file with content

module com.mymodule {
    
}

We create module by using module keyword. But module is not a reserved keyword in Java 9. module keyword is only special on this module-info.java file. With this simple code we have created a module. But we still do not have any code to run so let’s write some.

Write some code to run #

I will create a MyModule.java file that’s prints some string inside com.mymodule package.

package com.mymodule;

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

Here’s a directory structure

└── src
    └── com.mymodule
        ├── com
        │   └── mymodule
        │       └── MyModule.java
        └── module-info.java

Compile and run the program #

To compile we can run

javac -d mods --module-source-path src $(find src -name "*.java")

So above command compiles all the modules that are located on src directory and stores the bytecode at mods directory.

To run it we do

java --module-path mods -m com.mymodule/com.mymodule.MyModule

Output #

That’s it and you will get the output

Hello Module World

I wrote a bash script which can do compile and run the java module at one. You can find script here

With this script you can do ./run.sh main.module.name MainClassToRun