🧠 Java Modules (JPMS) Cheat Sheet

🔹 What Is a Module?

A Java module is a self-contained unit of code that groups related packages, resources, and configuration.
Introduced in Java 9, modules improve encapsulation, performance, and maintainability.

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🏗️ Basic Syntax: module-info.java

module com.example.myapp {
    requires com.example.utils;
    exports com.example.myapp.api;
}

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📦 Common JPMS Keywords

Keyword	Description
module	Declares a module
requires	Declares a dependency on another module
exports	Makes a package accessible to other modules
opens	Allows runtime reflection access (e.g., by frameworks like Jackson)
uses	Declares a service interface this module will consume
provides ... with	Declares an implementation of a service interface
requires transitive	Re-exports a dependency to downstream modules
requires static	Marks a compile-time-only dependency (not needed at runtime)
exports ... to	Makes a package accessible to specific modules only

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📁 Typical Module Structure

my-module/
├── src/
│   └── main/
│       └── java/
│           ├── module-info.java
│           └── com/mycompany/mypackage/
│               └── MyClass.java

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✅ exports vs opens

Feature	exports	opens
Compile-time use	✔ Required	❌ Not accessible
Reflection support	❌ Not allowed	✔ Accessible via reflection
Use case	Public APIs	JSON mapping, ORM, frameworks, etc.

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🔁 Services (SPI)

Step 1: Define a service interface

// module: payment.api
package com.mycompany.payment;

public interface PaymentGateway {
    void pay(String customer, double amount);
}

Step 2: Provide implementation

// module: payment.impl
module payment.impl {
    requires payment.api;
    provides com.mycompany.payment.PaymentGateway
        with com.mycompany.payment.impl.StripePaymentGateway;
}

Step 3: Load with ServiceLoader

// module: checkout
ServiceLoader<PaymentGateway> loader = ServiceLoader.load(PaymentGateway.class);
loader.findFirst().ifPresent(gw -> gw.pay("Zied", 100));

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📋 Best Practices

• ✅ Start modularizing early in new projects
• ✅ Keep one responsibility per module
• ✅ Export only what you intend to expose
• ✅ Use opens only for framework-based reflection
• ✅ Use provides/uses for pluggable architecture
• ✅ Test each module independently

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⚠️ Common Pitfalls

• ❌ Forgetting to export a package needed by other modules
• ❌ Attempting reflection on closed packages (without opens)
• ❌ Cyclic dependencies between modules
• ❌ Mixing modules with legacy classpath without control

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🧪 Compile & Run

Compile:

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

Run:

java --module-path out -m my.module/com.mycompany.Main

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📚 Resources

• JPMS Official Quick Start
• Java 9 Module System (Oracle Docs)
• Maven & JPMS Guide

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