Chapter 133: Java Modules in Java

JPMS (Java Platform Module System) groups packages into module units with an explicit module-info.java: dependencies (requires), API surface (exports), and reflective access (opens). JDK 9+ libraries and apps can ship as modules; classpath-only jars still load in the unnamed module.

1. Topic title

Modules: compile-time and runtime dependency graph with enforced encapsulation

2. What it means

module com.example.app { requires java.net.http; exports com.example.api; }

requires transitive propagates readability to consumers of your module—use for API types that leak into signatures.

opens com.example.internal to spring.core` grants deep reflection to named friends without exporting internals to everyone.

3. Why it is used

Shrink runtime images with jlink—custom distros without unused JDK modules.

Security—default deny for reflective poking at jdk.internal.*.

4. Mental sketch

Classpath is a pile of jars in a shared attic—anything might touch anything. Modules are labeled boxes with signed permission slips for who may open which lid.

5. Java code example

/* file: src/main/java/module-info.java */
module demo.modules {
    requires java.base;
    exports com.example.demo.api;
}

Real apps add requires lines for logging, SQL drivers, HTTP clients—start minimal, let compiler errors teach missing requires.

6. Explanation of code

java.base is implicitly required but listing it is fine for clarity in learning snippets.

7. Common mistakes

Split-package across two modules—illegal; merge packages or stop exporting half.

Assuming Spring Boot apps are modular—many still run classpath-first; module migration is incremental.

8. Best practices

jdeps on fat jars before modularizing—see accidental coupling.

Keep exports lists small—prefer opens … to for framework introspection only.

9. Small practice task

Create a two-module Gradle or Maven build where api exports one package and impl requires api—run java --module-path … -m impl/com.example.Main.

Beginner tip

If reflection fails with InaccessibleObjectException, you hit JPMS walls—open the package, move code to an exported API, or rethink the hack.