↗️ This README only covers one section of the demo. The master branch contains more information.

The next step is small but important. It changes nothing about the code or its organization but moves it into a Jigsaw module.

To be able to create a module, the project needs a module-info.java:

module org.codefx.demo.advent {
	// no imports or exports
}

Its name (here org.codefx.demo.advent) can be arbitrary but to ensure uniqueness it is recommended to stick to the inverse URL naming schema of packages. So while this is not necessary it will often mean that the module name is a prefix of the packages it contains.

The biggest change is the script¹ to compile and run:

# compile (add module-info.java):
javac -d classes/org.codefx.demo.advent
    ${list of source files}
# package (add module-info.class and specify main class):
jar --create \
	--file mods/org.codefx.demo.advent.jar \
	--main-class org.codefx.demo.advent.Main \
	${compiled class files}
# run (specify a module path and simply name to module to run):
java --module-path mods \
	--module org.codefx.demo.advent

To create a "modular jar" we only need to include a module-info.class in the list of files for jar. We can also specify the main class to run when the module is executed. To run the module we provide a module path to java (via -p) that points to the directory containing all our modules (just one at the moment). For execution it suffices to name the module as the JVM will look for it in the module path and find out that a main class was defined.

¹ the commands must come from the JDK 9 bin directory; compileAndRun.sh appends 9 to them and on my machine the resulting commands (e.g. javac9) are symlinked to the Java 9 variants.