14.1.2 Module Path Explained

The module path in Java SE 11 is a crucial concept for managing dependencies and organizing code in modular applications. Understanding the module path is essential for developing and deploying modular Java applications effectively.

Key Concepts

1. Module Path Definition

The module path is a list of directories or JAR files that the Java runtime uses to locate modules. It specifies where the Java runtime should look for module definitions and dependencies. The module path is distinct from the classpath, which is used for non-modular applications.

Example

        java --module-path lib --module com.example.app/com.example.app.Main
    

2. Module Resolution

Module resolution is the process by which the Java runtime locates and loads modules specified on the module path. The runtime resolves modules in the order they appear on the module path, ensuring that dependencies are satisfied before the application runs.

Example

        java --module-path lib:modules --module com.example.app/com.example.app.Main
    

3. Module Path vs. Classpath

The module path and classpath serve different purposes. The module path is used for modular applications, where modules explicitly declare their dependencies. The classpath is used for traditional applications, where dependencies are not explicitly declared and are loaded in a flat namespace.

Example

        java -cp lib/* com.example.app.Main
    

4. Adding Modules to the Module Path

Modules can be added to the module path by specifying directories containing module-info.class files or JAR files that include module definitions. This allows the Java runtime to locate and load the necessary modules for the application.

Example

        java --module-path lib:modules --module com.example.app/com.example.app.Main
    

5. Automatic Modules

Automatic modules are JAR files placed on the module path that do not contain a module-info.class file. The Java runtime automatically treats these JAR files as modules, inferring their module name from the JAR file name. Automatic modules can access all other modules on the module path.

Example

        java --module-path lib/example.jar --module com.example.app/com.example.app.Main
    

Examples and Analogies

Think of the module path as a roadmap that guides the Java runtime to the necessary modules for your application. Just as a roadmap helps you navigate to different locations, the module path helps the runtime locate and load the modules required for your application to run.

For instance, if you are building a modular application that depends on multiple modules, the module path ensures that all dependencies are resolved and loaded in the correct order. This is similar to ensuring that all necessary ingredients are available in the correct sequence when cooking a recipe.

By mastering the module path in Java SE 11, you can create more organized and maintainable modular applications, ensuring that dependencies are managed efficiently and applications run smoothly.