OCP 17 Exam - chapter 12 notes - modules

Note: Java Platform Module System was introduced in JDK 9 in project Jigsaw. In Java 25 modules would be available through imports like regular classes.

Running and packaging simple module

Folder structure of app:

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

Package names must be unique: e.g. I cannot add class in package “java.util” as already exists in module “java.base”.

error: duplicate module on application module path module in products

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

Note: we don’t have to export any package to be able to run class with main method.

Side note: Java module system is different than one in Intellij! See 1 and 2. Instead of creating a project, you should just “Open” parent directory.

Simple dependency on other module with “requires” and automatic module

• if we want to export all packages and we don’t require any module, then there is no need to define module-info.java

“automatic module” is jar on module path that does not have module-info.java and exports all packages

• automatic module name will be extracted from jar name, e.g. products-0.1-SNAPSHOT.jar becomes products module

• in app’s module-info.java add requires products; and build again

ServiceLoader that “uses” SPI and Service Provider that “provides-SPI-with-IMPL”

Let’s say we want to display products (SPI - Service Provider Interface) from many different suppliers/manufactures (provider/impl). We can load products as service to avoid building the whole application when new supplier will be added.

• in app code load Products SPI via ServiceLoader (note: no need to rebuild products module):

    private static Products getProductsInstance() {
        return ServiceLoader.load(Products.class)
            .findFirst()
            .orElseGet(() -> getDefaultProductsInstance());
    }
  

  note: ServiceLoader#load returns ServiceLoader which is Iterable, so provides other methods like stream(), forEach()

• in app’s module-info.java add uses products.Products;

w/o uses we cannot compile ServiceLoader: java.util.ServiceConfigurationError: products.Products: module app does not declare uses`

another keyword is opens that allows reflection

Remember to end every statement with ; in module-info.java

OK, this was service loader/client part, next add service provider that will provide implementation.

• let’s define module products.pl with class:

  public class PolishProducts implements Products {
  
    @Override
    public List<Products.Product> findAll() {
        return List.of(
            () -> "Pierogi",
            () -> "Żubrówka"
        );
    }
  }
  

• in module-info.java needs to do 2 things:
  • requires products; as we depend on the SPI interface
  • provides products.Products with pl.products.PolishProducts; - declare that Products SPI will be provided with PolishProducts implementation

Migration strategies

• Top down: add all JARs to module path as “automatic” modules, then migrate one module at a time to be “named” module.
• Bottom up: all unmigrated “unnamed” modules stays on class path. Pick next module with the least dependencies and migrate it.

unnamed module is not visible to other modules on the module path, because module on module path cannot refer to classpath.

Tricky review questions

• An unnamed module doesn’t use a module-info.java file.

• module was defined with keyword class!! “class dragon {“

• No modules need to specify requires on the service provider since that is the implementation.

Playground code

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

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