Java Platform Module System — Java Cheat Sheets

1 WHY MODULES?

The problem before modules

Every public class/package was accessible to any other code ("the classpath")
No clear declared dependencies between components
Whole JRE shipped even if an app used a tiny fraction of it

What the module system adds

Explicit, declared dependencies between modules
Strong encapsulation — only exported packages are visible outside a module
Smaller, custom runtime images (with jlink)

2 MODULE DECLARATIONS

// file: module-info.java module com.example.welcome { requires java.base; // implicit, shown for clarity requires javafx.controls; exports com.example.welcome.api; }

Key directives

Directive	Meaning
`requires`	this module depends on another module
`exports`	makes a package visible to other modules
`requires transitive`	dependency is passed along to modules that require this one
`opens`	allows reflective access (e.g. for frameworks) to a package

3 MODULARIZING A WELCOME APP

Turning a plain app into a module is mostly about project layout: put a module-info.java at the source root, keep classes in a package, and declare what the module needs (requires) and shares (exports).

src/ └─ com.example.welcome/ ├─ module-info.java └─ com/example/welcome/Welcome.java

4 MODULE-DEPENDENCY GRAPHS

com.example.app

→ requires →

com.example.utils

→ requires →

java.base

The JVM builds a graph of all module dependencies at startup and verifies it's consistent (no missing or conflicting modules) before your program even starts running — catching configuration errors early.

5 MIGRATING CODE & RESOURCES IN MODULES

Migration strategies

Bottom-up — modularize dependencies first, then the app
Top-down — modularize the app first, treat old code as an "unnamed module"

Resources

Non-code files (images, config, properties) can live alongside module code and be loaded via Class.getResourceAsStream(), respecting the module's encapsulation rules.

6 CUSTOM RUNTIMES WITH jlink

jlink --module-path $JAVA_HOME/jmods:mods \ --add-modules com.example.welcome \ --output myruntime

Why it matters

jlink creates a minimal, self-contained Java runtime image containing only the modules your app actually needs — much smaller than shipping the full JDK, great for containers and embedded devices.

7 SERVICES & ServiceLoader

// module-info.java (provider module) provides com.example.PaymentService with com.example.impl.CardPaymentService; // consumer code ServiceLoader<PaymentService> loader = ServiceLoader.load(PaymentService.class);

Concept

Modules can declare that they provide an implementation of a service interface, and other modules can discover implementations at runtime via ServiceLoader — a plug-in style architecture built into the platform.