04. Module Types

Modules Types

Classpath vs. Modulepath

Before modules, building a Java project involved placing all project classes and jars on the CLASSPATH. This is the path where the compiler looks for all your class files.

With modules, you place your modules at a different location, called the MODULEPATH. This is the path where the compiler looks for all your modules.

Unnamed Module

All Java applications compiled in Java 9+ use the module system. Even if they contain no modules and do not use a module descriptor, everything placed on the CLASSPATH are placed into what is called the Unnamed Module.

In the below example, some modules are added to the MODULEPATH and some JAR is added to the CLASSPATH.

Automatic Modules

Unnamed Module cannot access content in Modules, because it cannot use the requires statement. Named modules cannot access the Unnamed Module, because the unnamed module cannot be referenced by require.

The solution to this problem is automatic modules. Automatic modules are created by placing non-modular jars on the **modulepath. **If your project has dependencies, they can be placed on the modulepath so that they become automatic modules which you can reference by name in your module descriptor. The default name of automatic modules is the name of the jar, but that name can be overridden by the Automatic-Module-Name property in the JAR Manifest. You can then reference the module using requires.

Module Access

Automatic modules can access everything in the unnamed module. Maven places transitive dependencies on the classpath, so they become part of the unnamed module. The direct dependencies are placed on the modulepath and become automatic modules. This allows your module to access the direct dependencies, but not the transitive dependencies.