How to use the module system to organize code and dependencies in Java 9

How to use the module system to organize code and dependencies in Java 9

Abstract:
With the release of Java 9, the modular system has become an important new feature in the Java language. The modular system provides better code organization structure and dependency management mechanism, helping developers to build scalable, maintainable and testable applications more easily. This article will introduce how to use the module system to organize code and manage dependencies in Java 9, and illustrate various concepts and techniques through sample code.

1. What is a modular system
   In traditional Java development, we organize code into packages and use classpath to manage dependencies. However, as applications become larger and more complex, it becomes increasingly difficult to use the classpath to manage dependencies. The emergence of modular systems is precisely to solve this problem. A modular system allows us to organize code into modules, each with its own dependencies, and manage the code base by explicitly specifying dependencies.
2. Create a module
   To create a module, we need to create a module-info.java file in the root directory of the code base. This file defines information such as the module name, dependencies, and exported packages. The following is an example module-info.java file:

module com.example.app {
    requires java.base;
    requires com.example.library;
    exports com.example.app;
}

In this example, the module is named com.example.app, which depends on the java.base module and the com.example.library module . It also exports the com.example.app package through the exports keyword, which means other modules can access the public classes in this package.

3. Using modules
   In Java 9, we can use the command line parameter --module to specify the module to run. For example, to run the com.example.app module, you can execute the following command:

   java --module-path libs -m com.example.app/com.example.app.Main

where libs is the directory containing all dependent modules. At runtime, the Java virtual machine loads modules based on the dependencies specified in the module-info.java file.

4. Using libraries in modules
   Before Java 9, we usually used classpath to manage dependencies. In a modular system, we can use the requires keyword to specify module dependencies. For example, to use the functionality of the com.example.library module in the com.example.app module, you can add the following content to the module-info.java file:

   module com.example.app {
    requires com.example.library;
   }

In this way, com The .example.app module can use the classes and methods in the com.example.library module.

5. Exporting and encapsulating packages
   In a modular system, we can use the exports and opens keywords to control the visibility of packages. Through the exports keyword, we can expose classes and methods in a package to other modules. Through the opens keyword, we can reflect the classes and methods in a package to other modules.

6. Use the command line tool jdeps to analyze dependencies
   JDK 9 provides a command line tool jdeps that can be used to analyze dependencies between classes and modules. For example, run the following command to view the dependencies of the module com.example.app:

   jdeps --module-path libs --module com.example.app

7. Developing modular applications using IntelliJ IDEA
   If you use IntelliJ IDEA for Java development, it has The modular system provides good support. You can start developing modular applications by creating a new Java 9 project and using module support.

Conclusion:
This article introduced the basic concepts and techniques of using the modular system to organize code and manage dependencies in Java 9. By using a modular system, we can better organize and manage the code base, improving the scalability and maintainability of the application. I hope this article will help you understand and use the Java 9 modular system.

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