Modularizing Java Applications. Jigsaw Project in Java 9

Introduction

Java 9 introduced the Jigsaw Project, a major overhaul aimed at modularizing the Java platform and its applications. The Jigsaw Project brings modularity to the Java ecosystem, providing a more structured and scalable approach to building and managing applications. This article explores the Jigsaw Project, its key concepts, and how it helps in modularizing Java applications.

What Is the Jigsaw Project?

The Jigsaw Project, also known as Project Jigsaw, is an initiative to introduce a module system to the Java programming language. It aims to improve the Java platform’s modularity, making it easier to develop, maintain, and scale applications. The module system helps in dividing applications into distinct, self-contained units, known as modules, that have clearly defined dependencies and interfaces.

Key Concepts of the Jigsaw Project

Modules

A module is a collection of related packages and resources with a defined boundary. It encapsulates its internal components and exposes only the necessary parts to other modules. Each module has a module-info.java file that describes its dependencies and exported packages.

Example module-info.java:

module com.example.myapp {
    requires java.sql;
    exports com.example.myapp.service;
}

In this example, the module com.example.myapp requires the java.sql module and exports the com.example.myapp.service package.

Module Declaration

The module-info.java file is used to declare a module and specify its dependencies and exports. This file is placed in the root of the module’s directory.

• ‘requires’: Specifies dependencies on other modules.
• ‘exports’: Makes specific packages available to other modules.
• ‘opens’: Makes packages available for deep reflection.
• ‘uses’: Specifies services that the module uses.
• ‘provides’: Declares services that the module provides.

Automatic Modules

For backward compatibility, Java 9 introduced automatic modules. These are modules that are created from JAR files that do not have a module-info.java file. Automatic modules are used to provide modularity for existing libraries and applications without requiring immediate modification.

Example of using an automatic module:

If you have a JAR file library.jar, it can be added to the module path, and Java will treat it as an automatic module with a module name derived from the JAR file name.

The Module Path

The module path is similar to the classpath but is designed specifically for modules. It helps in locating modules and their dependencies. The module path ensures that module boundaries are respected and prevents conflicting versions of modules.

Example of setting the module path:

javac --module-path mods -d out src/com.example.myapp/module-info.java src/com.example.myapp/com/example/myapp/*.java

In this command, --module-path mods specifies the directory containing modules, and -d out specifies the output directory.

Benefits of Modularization

• Improved Encapsulation
  Modularization improves encapsulation by allowing developers to define clear boundaries between different parts of an application. Modules expose only what is necessary and keep internal details hidden.

• Enhanced Maintainability
  By dividing an application into modules, it becomes easier to manage, understand, and maintain. Changes in one module can be made with minimal impact on other modules.

• Better Dependency Management
  The module system provides explicit dependencies, reducing the risk of version conflicts and classpath issues. It makes it clear which modules depend on each other and which are independent.

• Faster Startup and Smaller Footprint
  Modular applications can be more efficient in terms of startup time and memory usage. Only the required modules are loaded, reducing the overall footprint of the application.

• Improved Security
  Modularization enhances security by providing fine-grained control over what code can be accessed and modified. It limits the exposure of internal APIs and enforces better encapsulation.

Practical Examples

Creating a Module

1. Define a module:

   Create a module-info.java file in the root of your module directory:

   module com.example.myapp {
       requires java.sql;
       exports com.example.myapp.service;
   }
   

2. Compile the module:

   javac -d out --module-source-path src $(find src -name "*.java")
   

3. Run the application:

   java --module-path out -m com.example.myapp/com.example.myapp.Main
   

Using an Automatic Module

1. Add an automatic module to the module path:

   java --module-path path/to/library.jar -m com.example.myapp/com.example.myapp.Main
   

2. Ensure the JAR file has the necessary dependencies:

   Automatic modules are treated as having a module name derived from the JAR file name, allowing existing libraries to be used as modules.

Challenges and Considerations

1. Migration Effort
   Migrating existing applications to use modules may require significant effort, especially for large codebases with complex dependencies.

2. Compatibility
   Not all libraries and frameworks are module-aware, so integrating with existing third-party libraries might pose challenges.

3. Complexity The module system introduces new concepts and terminology that may add complexity to the development process.

Conclusion

The Jigsaw Project in Java 9 introduces a modular system that enhances the way Java applications are built and managed. By allowing developers to create modular applications with clear boundaries and dependencies, Java 9 improves encapsulation, maintainability, and efficiency. While the module system brings many benefits, it also requires careful consideration and planning, especially when integrating with existing code and libraries. Embracing modularization can lead to more scalable and maintainable applications, positioning Java for future growth and adaptability.