OCP 17 Exam - chapter 12 notes - modules

Note: Java Platform Module System was introduced in JDK 9, see: https://openjdk.org/projects/jigsaw/spec/ In java 25 modules would be available through imports like regular classes, see: https://www.baeldung.com/java-25-features#2-module-import-declarations-jep-511–preview.

Running and packaging simple module

Folder structure of app:

- app
  - src/pkg
    - App.java // prints hello world
  - module-info.java // only module app { }

Run below commands in parent directory https://github.com/RG9/rg-playground-ocp17/blob/main/java-modules-demo.

Compile to app/out:

$ javac --module-path mods -d app/out app/src/pkg/*.java app/module-info.java

Note: –module-path is not needed now as “app” module do not have any dependencies, but good to know that parameter

Package into jar:

$ jar -cvf mods/app.jar -C app/out  .
added manifest
added module-info: module-info.class
adding: pkg/(in = 0) (out= 0)(stored 0%)
adding: pkg/App.class(in = 414) (out= 289)(deflated 30%)

Run:

$ java -p mods -m app/pkg.App
Hello app!

-p is abbreviation of --module-path -m is abbreviation of --module

Be aware of format of --module param, which should be separated by slash: -m module/class

Side note: Java module system is different than one in Intellij! https://www.jetbrains.com/help/idea/creating-and-managing-modules.html#modules-idea-java Here you can read more about support for Java modules in Intellij: https://blog.jetbrains.com/idea/2017/03/support-for-java-9-modules-in-intellij-idea-2017-1/ Instead of creating a project, you should just “Open” parent directory.

Defining service

TBD

Playground code

https://github.com/RG9/rg-playground-ocp17/blob/main/java-modules-demo

---

Credit: OCP Oracle Certified Professional Java SE 17 Developer Study Guide