Java 9 simple multi-module project

Sample created to understand the module support introduced by Java 9. Basically is possible define modules inside the application that can:

• import modules
• export specific packages

Each module declare what need to include or what want to share with other modules defining the module-info.java. Below an example:

//bootstrap module require to import a specific package to use
module bootstrap {
  requires andzac.greetings;
}

//greetings module export a single package externally
  module andzac.greetings {
      exports it.andzac.j9;
  }

The module-info should be defined at src folder level.

Example

The demo application is defined with two module:

• greetings that export the package that contains the Greetings class
• bootstrap that require the Greetings class to use its method.

Migration to Java 9 multimodule

As example let's assume that our java application is composed by three component: the main project plus two custom jar modules integrated in the application classpath. The following image summarize the example:

In order to migrate to multi module Java 9 structure there are two different approches:

• bottom-up migration
• top-down migration

Bottom-up migration

The idea is to modularize starting from the leaf of the dependencies tree.

• Java dependencies (java.xml, java.sql, java.base, java.logging) are already modularized
• foo.jar can be modularized in foo module
• bar.jar can be modularzed in bar module
• The Jackson jar cannot be modularized because is 3rd party library.
• project.jar cannot be modularized because rely on not modularized libraries

Moving the jackson dependencies in a unnamed module can address a fully modularized application because in this way also project can be modularized.

Top-down migration

The idea here is to start to modularize the main project, moving everything else in a unnamed automatic module created by the JVM. After that we can go through the big unnamed module and extract from there what can be defined as an indipendent module.