Java 9 Modularity

Chapter 3. Working with Modules

In this chapter, we take the first steps towards modular development using Java 9. Instead of looking at existing modules in the JDK, it’s time to get your hands dirty by writing your first module. To start things off easily, we turn our attention to the simplest possible module. Let’s call it our Modular Hello World. Armed with this experience, we’ll then be ready to take on a more ambitious example with multiple modules. At that point, we’ll introduce the running example to be used throughout this book, called EasyText. It’s designed to gradually grow with you, as you learn more about the module system.

Your First Module

You’ve seen examples of module descriptors in the previous chapter. A module is generally more than just a descriptor, though. The Modular Hello World therefore transcends the level of a single source file: we need to examine it in context. We’ll start by compiling, packaging, and running a single module to get acquainted with the new tooling options for modules.

Anatomy of a Module

Our goal for this first example is to compile the following class into a module and run it (Example 3-1). We start out with a single class in a package, leading to a single module. Modules may contain only types that are inside packages, so a package definition is required.

Example 3-1. HelloWorld.java (➥ chapter3/helloworld)

package com.javamodularity.helloworld;

public class HelloWorld {

   public static void main(String... args) {
      System.out.println ...