Understanding the Java Platform Module System (JPMS)
With the release of Java 9, the Java Platform Module System (JPMS) was introduced to simplify and enhance the organization of Java applications. JPMS, commonly referred to as modules, allows developers to create modular applications, improving code maintainability, and providing a better way to manage dependencies. In this article, we’ll delve into the concept of JPMS, its significance, and how to create and use modules in Java 9 and beyond.
What Are Java Modules?
In Java, a module is a self-contained unit that encapsulates code and data. It defines the dependencies it relies on and the public interfaces it exposes to other modules. Modules can be seen as a way to organize and partition code within a project.
Creating a Java Module
To create a Java module, you need to define a module-info.java file in the module’s root directory. This file specifies the module’s name, its dependencies, and what it exports to other modules.
module com.example.myapp {
requires some.library;
exports com.example.myapp.api;
}
In this example, the module com.example.myapp requires the module some.library and exports the package com.example.myapp.api to other modules.
Benefits of Java Modules
Java modules offer several advantages for developers:
1. Strong Encapsulation
Modules provide strong encapsulation, allowing you to hide internal implementation details from other modules. This reduces the risk of accidental dependencies and minimizes classpath-related issues.
2. Explicit Dependencies
Modules declare their dependencies explicitly. This makes it easier to understand and manage the relationships between modules, reducing the likelihood of unexpected runtime issues.
3. Improved Code Structure
Modules encourage better code organization. By defining clear boundaries between modules, developers can create cleaner and more maintainable codebases.
Using Modules in Java
To use modules in Java, you need to specify module paths when compiling and running your application. Here’s how to compile and run a modular application:
javac --module-path path/to/modules -d mods/com.example.myapp src/com.example.myapp/module-info.java src/com.example.myapp/com/example/myapp/api/MyService.java
java --module-path mods -m com.example.myapp/com.example.myapp.Main
Module Migration from Java 8
If you’re migrating a Java 8 project to Java 9 or later, you can convert your classpath-based project to modules. Begin by creating a module-info.java file for your application and update your build and run configurations accordingly. The module system helps you identify and resolve any missing dependencies.
Java Platform Modules
Java itself is divided into modules. These modules include the core libraries and APIs that were previously part of the monolithic JDK. By modularizing the platform, Oracle and the Java community have created a more flexible and manageable Java ecosystem.
Example of Using Java Modules
Let’s take a practical example of using modules to create a simple Java application with two modules: a main application module and a utility module.
Main Application Module:
module com.example.app {
requires com.example.util;
}
Utility Module:
module com.example.util {
exports com.example.util;
}
In this example, the main application module (com.example.app) requires the utility module (com.example.util) and uses the exported package com.example.util.
Using the Modules in the Main Class:
import com.example.util.Utility;
public class Main {
public static void main(String[] args) {
Utility utility = new Utility();
utility.doSomething();
}
}
The main class imports the Utility class from the utility module and uses it in the application.
Conclusion
The Java Platform Module System (JPMS) introduces modularity to Java, making it easier to develop and manage complex applications. With modules, developers can create more organized and maintainable codebases while minimizing classpath-related issues. By understanding and embracing JPMS, Java developers can take full advantage of the benefits it offers in Java 9 and beyond.