The Java Platform Module System

This chapter is from the book

Core Java, Vol. II-Advanced Features, 12th Edition

Learn More Buy

This chapter is from the book

This chapter is from the book 

Core Java, Vol. II-Advanced Features, 12th Edition

Learn More Buy

9.6 Modular JARs

So far, we have simply compiled modules into the directory tree of the source code. Clearly, that is not satisfactory for deployment. Instead, a module can be deployed by placing all its classes in a JAR file, with a module-info.class in the root. Such a JAR file is called a modular JAR.

To create a modular JAR file, use the jar tool in the usual way. If you have multiple packages, it is best to compile with the -d option which places class files into a separate directory. The directory is created if it doesn't already exists. Then use the -C option of the jar command to change to that directory when collecting files.

javac -d modules/com.horstmann.greet $(find com.horstmann.greet -name *.java)
jar -c -v -f com.horstmann.greet.jar -C modules/com.horstmann.greet .

If you use a build tool such as Maven, Ant, or Gradle, just keep building your JAR file as you always do. As long as module-info.class is included, you get a modular JAR.

Then, include the modular JAR in the module path, and the module will be loaded.

As with regular JAR files, you can specify a main class in a modular JAR:

javac -p com.horstmann.greet.jar    -d modules/v2ch09.exportedpkg $(find v2ch09.exportedpkg -name *.java)
jar -c -v -f v2ch09.exportedpkg.jar -e com.horstmann.hello.HelloWorld    -C modules/v2ch09.exportedpkg .

When you launch the program, you specify the module containing the main class:

java -p com.horstmann.greet.jar:v2ch09.exportedpkg.jar -m v2ch09.exportedpkg

When creating a JAR file, you can optionally specify a version number. Use the --module-version parameter, and also add @ and the version number to the JAR file name:

jar -c -v -f com.horstmann.greet@1.0.jar --module-version 1.0 -C com.horstmann.greet .

As already discussed, the version number is not used by the Java Platform Module System for resolving modules, but it can be queried by other tools and frameworks.

Optional<String> version = Greeter.class.getModule().getDescriptor().rawVersion();

jshell --module-path com.horstmann.greet@1.0.jar --add-modules com.horstmann.greet