• Java Module System (introduced in Java 9) is used to organize large applications into modules.
• A module is like a group of packages with a clear boundary: what it exports and what it requires.
• Main file: module-info.java (module descriptor).

1) Introduction to Java Module System

• Before modules, Java apps used classpath which could cause:
  • Dependency conflicts (same library multiple versions)
  • Hard to control access to internal packages
  • Large applications become difficult to manage
• Modules solve this using explicit dependencies and strong encapsulation.

2) module-info.java (Module Descriptor)

• Defines module name and rules.
• Common keywords:
  • module : declares module
  • requires : dependency module
  • exports : packages visible to other modules
  • opens : allows reflection (used with frameworks)
  • provides ... with ... : service provider
  • uses : service usage

// module-info.java (basic)
module com.webtechio.app {
    requires java.base; // optional (java.base is default)
}

3) Creating and Using Modules (Example: math + app)

• Create two modules:
  • com.webtechio.math (exports math package)
  • com.webtechio.app (requires math module and uses it)

// Folder Structure (example)
// src
//   com.webtechio.math
//     module-info.java
//     com/webtechio/math/Calc.java
//   com.webtechio.app
//     module-info.java
//     com/webtechio/app/Main.java

// src/com.webtechio.math/module-info.java
module com.webtechio.math {
    exports com.webtechio.math;
}

// src/com.webtechio.math/com/webtechio/math/Calc.java
package com.webtechio.math;

public class Calc {
    public int add(int a, int b) {
        return a + b;
    }
}

// src/com.webtechio.app/module-info.java
module com.webtechio.app {
    requires com.webtechio.math;
}

// src/com.webtechio.app/com/webtechio/app/Main.java
package com.webtechio.app;

import com.webtechio.math.Calc;

public class Main {
    public static void main(String[] args) {
        Calc c = new Calc();
        System.out.println("Sum: " + c.add(10, 20));
    }
}

4) Compiling and Running Modules (Command Line)

• Compile modules using --module-source-path.
• Run using --module-path and -m (module/class).

// Compile (creates output in out folder)
javac -d out --module-source-path src $(find src -name "*.java")

// Run module
java --module-path out -m com.webtechio.app/com.webtechio.app.Main

5) Advantages of Modular Programming

• Strong Encapsulation: only exported packages are accessible.
• Reliable Configuration: dependencies are declared using requires.
• Better Maintainability: modules are easier to manage and update.
• Improved Security: internal APIs can be hidden.
• Faster Startup for large apps (only required modules loaded).
• Easy Reuse: modules can be reused across projects.

6) Quick Notes

• java.base is automatically available to all modules.
• Use exports to make a package public to other modules.
• Use requires to depend on another module.
• Module name should look like a reversed domain style: com.company.module.